Black Holes in Higher Dimensions
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Abstract
We review blackhole solutions of higherdimensional vacuum gravity and higherdimensional supergravity theories. The discussion of vacuum gravity is pedagogical, with detailed reviews of MyersPerry solutions, black rings, and solutiongenerating techniques. We discuss blackhole solutions of maximal supergravity theories, including black holes in antide Sitter space. General results and open problems are discussed throughout.
Keywords
Black Hole Conformal Field Theory Black Ring Black Brane Black String1 Introduction

String theory contains gravity and requires more than four dimensions. In fact, the first successful statistical counting of blackhole entropy in string theory was performed for a fivedimensional black hole [229]. This example provides the best laboratory for the microscopic string theory of black holes.

The AdS/CFT correspondence relates the dynamics of a ddimensional black hole with those of a quantum field theory in d − 1 dimensions [187] (for a review see [1]).

The production of higherdimensional black holes in future colliders becomes a conceivable possibility in scenarios involving large extra dimensions and TeVscale gravity [30, 155].

As mathematical objects, blackhole spacetimes are among the most important Lorentzian Ricciflat manifolds in any dimension.
These, however, refer to applications of the subject — important though they are — but we believe that higherdimensional gravity is also of intrinsic interest. Just as the study of quantum field theories, with a field content very different than any conceivable extension of the Standard Model, has been a very useful endeavor, throwing light on general features of quantum fields, we believe that endowing general relativity with a tunable parameter — namely the spacetime dimensionality d — should also lead to valuable insights into the nature of the theory, in particular into its most basic objects: black holes. For instance, fourdimensional black holes are known to have a number of remarkable features, such as uniqueness, spherical topology, dynamical stability, and to satisfy a set of simple laws — the laws of black hole mechanics. One would like to know which of these are peculiar to fourdimensions, and which hold more generally. At the very least, this study will lead to a deeper understanding of classical black holes and of what spacetime can do at its most extreme.
There is a growing awareness that the physics of higherdimensional black holes can be markedly different, and much richer, than in four dimensions. Arguably, two advances are largely responsible for this perception: the discovery of dynamical instabilities in extended blackhole horizons [118] and the discovery of blackhole solutions with horizons of nonspherical topology that are not fully characterized by their conserved charges [83].
At the risk of anticipating results and concepts that will be developed only later in this review, in the following we try to give simple answers to two frequently asked questions: 1) why should one expect any interesting new dynamics in higherdimensional general relativity, and 2) what are the main obstacles to a direct generalization of the fourdimensional techniques and results. A straightforward answer to both questions is to simply say that as the number of dimensions grows, the number of degrees of freedom of the gravitational field also increases, but more specific, yet intuitive, answers are possible.
1.1 Why gravity is richer in d > 4
The novel features of higherdimensional black holes that have been identified so far can be understood in physical terms as due to the combination of two main ingredients: different rotation dynamics and the appearance of extended black objects.
The other novel ingredient that appears in d > 4 but is absent in lower dimensions (at least in vacuum gravity) is the presence of black objects with extended horizons, i.e., black strings and, in general, black pbranes. Although these are not asymptoticallyflat solutions, they provide the basic intuition for understanding novel kinds of asymptoticallyflat black holes.
Let us begin with the simple observation that, given a blackhole solution of the vacuum Einstein equations in d dimensions with horizon geometry Σ_{ H }, we can immediately construct a vacuum solution in d + 1 dimensions by simply adding a flat spatial direction^{1}. The new horizon geometry is then a black string with horizon Σ_{ H } × ℝ. Since the Schwarzschild solution is easily generalized to any d ≥ 4, it follows that black strings exist in any d ≥ 5. In general, adding p flat directions we find that black pbranes with horizon S^{ q } × ℝ^{ p } (with q ≥ 2) exist in any d ≥ 6 + p − q.
How are these related to new kinds of asymptoticallyflat black holes? Heuristically, take a piece of black string with S^{ q } × ℝ horizon, and curve it to form a black ring with horizon topology S^{ q } × S^{1}. Since the black string has a tension, the S^{1}, being contractible, will tend to collapse. But we may try to set the ring into rotation and in this way provide a centrifugal repulsion that balances the tension. This turns out to be possible in any d ≥ 5, so we expect that nonspherical horizon topologies are a generic feature of higherdimensional general relativity.
It is also natural to try to apply this heuristic construction to black pbranes with p > 1, namely, to bend the worldvolume spatial directions into a compact manifold and balance the tension by introducing suitable rotations. The possibilities are still under investigation, but it is clear that an increasing variety of black holes should be expected as d grows. Observe again that the underlying reason is a combination of extended horizons with rotation.
Horizon topologies other than spherical are forbidden in d = 4 by wellknown theorems [132]. These are rigorous, but also rather technical and formal results. Can we find a simple, intuitive explanation for the absence of vacuum black rings in d = 4? The previous argument would trace this fact back to the absence of asymptoticallyflat vacuum black holes in d = 3. This is often attributed to the absence of propagating degrees of freedom for the threedimensional graviton (or one of its paraphrases: 2+1gravity is topological, the Weyl tensor vanishes identically, etc), but here we shall use the simple observation that the quantity GM is dimensionless in d =3. Hence, given any amount of mass, there is no length scale to tell us where the blackhole horizon should be^{2}. So we attribute the absence of black strings in d = 4 to the lack of such a scale. This observation goes some way towards understanding the absence of vacuum black rings with horizon topology S^{1} × S^{1} in four dimensions; it implies that there cannot exist blackring solutions with different scales for each of the two circles, and, in particular one can not make one radius arbitrarily larger than the other. This argument, though, could still allow for black rings, where the radii of the two S^{1} are set by the same scale, i.e., the black rings should be plump. The horizontopology theorems then tell us that plump black rings do not exist; they would actually be within a spherical horizon.
Extended horizons also introduce a feature absent in d = 4: dynamical horizon instabilities [118]. Again, this is to some extent an issue of scales. Black brane horizons can be much larger in some of their directions than in others, and so perturbations with wavelengths on the order of the ‘short’ horizon length can fit several times along the ‘long’ extended directions. Since the horizon area tends to increase by dividing up the extended horizon into black holes of roughly the same size in all its dimensions, this provides grounds to expect an instability of the extended horizon (however, when other scales are present, as in charged solutions, the situation can become quite a bit more complicated). It turns out that higherdimensional rotation can extend the horizon much more in some directions than in others, which is expected to trigger this kind of instability [81]. At the threshold of the instability, a zeromode deformation of the horizon has been conjectured to lead to new ‘pinched’ black holes that do not have fourdimensional counterparts.
Finally, an important question raised in higher dimensions refers to the rigidity of the horizon. In four dimensions, stationarity implies the existence of a U(1) rotational isometry [132]. In higher dimensions stationarity has been proven to imply one rigid rotation symmetry too [138], but not (yet?) more than one. However, all known higherdimensional black holes have multiple rotational symmetries. Are there stationary black holes with less symmetry, for example just the single U(1) isometry guaranteed in general? Or are black holes always as rigid as can be? This is, in our opinion, the main unsolved problem on the way to a complete classification of fivedimensional black holes and an important issue in understanding the possibilities for black holes in higher dimensions.
1.2 Why gravity is more difficult in d > 4
Again, the simple answer to this question is the larger number of degrees of freedom. However, this cannot be an entirely satisfactory reply, since one often restricts oneself to solutions with a large degree of symmetry for which the number of actual degrees of freedom may not depend on the dimensionality of spacetime. A more satisfying answer should explain why the methods that are so successful in d = 4 become harder, less useful, or even inapplicable, in higher dimensions.
Still, the larger number of metric components, and of equations determining them, is the main reason for the failure so far to find a useful extension of the NewmanPenrose (NP) formalism to d > 4. This formalism, in which all the Einstein equations and Bianchi identities are written out explicitly, was instrumental in deriving the Kerr solution and analyzing its perturbations. The formalism is tailored to deal with algebraicallyspecial solutions, but even if algebraic classifications have been developed for higher dimensions [51] and applied to known blackhole solutions, no practical extension of the NP formalism has appeared yet that can be used to derive the solutions, nor to study their perturbations.
Then, it seems natural to restrict oneself to solutions with a high degree of symmetry. Spherical symmetry yields easily by force of Birkhoff’s theorem. The next simplest possibility is to impose stationarity and axial symmetry. In four dimensions this implies the existence of two commuting Abelian isometries, time translation and axial rotation, which are extremely powerful; by integrating out the two isometries from the theory, we obtain an integrable twodimensional GL(2, ℝ) sigmamodel. The literature on these theories is enormous and many solutiongenerating techniques are available, which provide a variety of derivations of the Kerr solution.
There are two natural ways of extending axial symmetry to higher dimensions. We may look for solutions invariant under the group O(d − 2) of spatial rotations around a given line axis, where the orbits of O(d − 2) are (d − 3)spheres. However, in more than four dimensions these orbits have nonzero curvature. As a consequence, after dimensional reduction of these orbits, the sigma model acquires terms (of exponential type) that prevent a straightforward integration of the equations (see [34, 35] for an investigation of these equations).
This suggests that one should look for a different higherdimensional extension of the fourdimensional axial symmetry. Instead of rotations around a line, consider rotations around (spatial) codimension2 hypersurfaces. These are U(1) symmetries. If we assume d − 3 commuting U(1) symmetries, so that we have a spatial U(1)^{ d }^{−3} symmetry in addition to the timelike symmetry ℝ, then the vacuum Einstein equations again reduce to an integrable twodimensional GL(d − 2, ℝ) sigma model with powerful solutiongenerating techniques.
Finally, the classification of possible horizon topologies becomes increasingly complicated in higher dimensions [98]. In four spacetime dimensions the (spatial section of the) horizon is a twodimensional surface, so the possible topologies can be easily characterized and restricted. Much less restriction is possible as d is increased.
All these aspects will be discussed in more detail below.
2 Scope and Organization of this Article
2.1 Scope
The emphasis of this article is on classical properties of exact higherdimensional blackhole solutions. We devote most space to a rather pedagogical discussion of vacuum solutions. Since this includes black rings, there is some overlap with our earlier review [84]. The present review discusses material that has appeared since [84], in particular the “doubly spinning” blackring solution of [212]. However, we shall not discuss several aspects of blackring physics that were dealt with at length in [84], for example, blackring microphysics. On the other hand, we present some new material: Figures 3, 6, 7, 13, and 14 describe the physical parameter ranges (phase space) of higherdimensional black holes, and Figure 4 for the area of 5D MyersPerry solutions, have not been presented earlier. Some of our discussion of the properties of the solutions is also new.
Our discussion of nonvacuum black holes is less pedagogical than that of the vacuum solutions. It is essentially a survey of the literature. In going beyond vacuum solutions, we had to decide what kinds of matter fields to consider. Since much of the motivation for the study of extra dimensions comes from string theory, we have restricted ourselves to considering blackhole solutions of supergravity theories known to arise as consistent truncations of d =10, 11 supergravity. We consider both asymptotically flat and asymptotically antide Sitter black holes.
In the asymptotically flat case, we consider only solutions of maximal supergravity theories arising from the toroidal reduction of d = 10, 11 supergravity to five or more dimensions. In particular, this implies that in five dimensions we demand the presence of a ChernSimons term for the gauge field, with a precise coefficient. A review of charged rotating black holes with other values for the ChernSimons coupling can be found in [161].
In the asymptotically AdS case, we consider solutions of gauged supergravity theories arising from the dimensional reduction of d = 10, 11 supergravity on spheres, in particular the maximalgauged supergravity theories in d = 4, 5, 7. Obviously d = 4 does not fall within our “higherdimensional” remit but asymptotically AdS d = 4 black holes are not as familiar as their asymptoticallyflat cousins so it seems worthwhile reviewing them here. In the AdS case, several different asymptotic boundary conditions are of physical interest. We consider only black holes obeying standard “normalizable” boundary conditions [1]. Note that all known blackhole solutions satisfying these restrictions involve only Abelian gauge fields.
Important related subjects that we do not discuss include: black holes in braneworld scenarios (reviewed in [186]); black holes in spacetimes with KaluzaKlein asymptotics (reviewed in [129]), and in general black holes with different asymptotics than flat or AdS; black holes in higherderivative theories [199, 33]; blackhole formation at the LHC or in cosmic rays, and the spectrum of their radiation (reviewed in [30, 155]).
2.2 Organization
Sections 3 to 6 are devoted to asymptoticallyflat vacuum solutions: Section 3 introduces basic notions and solutions, in particular the SchwarzschildTangherlini black hole. Section 4 presents the MyersPerry solutions, first with a single angular momentum, then with arbitrary rotation. Section 5 reviews the great recent progress in fivedimensional vacuum black holes: first we discuss black rings, with one and two angular momenta; then we introduce the general analysis of solutions with two rotational isometries (or d − 3, in general). In Section 6 we briefly describe a first attempt at understanding d ≥ 6 vacuum black holes beyond the MP solutions.
Section 7 reviews asymptoticallyflat black holes with gauge fields (within the restricted class mentioned above). Section 8 concludes our overview of asymptoticallyflat solutions (vacuum and charged) with a discussion of general results and some open problems. Finally, Section 9 reviews asymptotically AdS blackhole solutions of gauged supergravity theories.
3 Basic Concepts and Solutions
In this section we present the basic framework for general relativity in higher dimensions, beginning with the definition of conserved charges in vacuum, i.e., mass and angular momentum, and the introduction of a set of dimensionless variables that are convenient for describing the phase space and phase diagrams of higherdimensional rotating black holes. Then we introduce the Tangherlini solutions that generalize the fourdimensional Schwarzschild solution. The analysis that proves their classical stability is then reviewed. Black strings and black pbranes, and their GregoryLaflamme instability, are briefly discussed for their relevance to novel kinds of rotating black holes.
3.1 Conserved charges
3.2 The SchwarzschildTangherlini solution and black pbranes
These black brane spacetimes are not (globally) asymptotically flat, so we only introduce them insofar as they are relevant for understanding the physics of asymptoticallyflat black holes.
3.3 Stability of the static black hole
The stability of the d > 4 Schwarzschild solution against linearized gravitational perturbations can be analyzed by decomposing such perturbations into scalar, vector and tensor types according to how they transform under the rotationalsymmetry group SO(d − 1) [105, 163, 151]. Assuming a time dependence e^{−iωt} and expanding in spherical harmonics on S^{d−2}, the equations governing each type of perturbation reduce to a single ODE governing the radial dependence. This equation can be written in the form of a timeindependent Schrödinger equation with “energy” eigenvalue ω^{2}.
In investigating stability, we consider perturbations that are regular on the future horizon and outgoing at infinity. An instability would correspond to a mode with Im ω > 0. For such modes, the boundary conditions at the horizon and infinity imply that the lefthand side (LHS) of the Schrödinger equation is selfadjoint, and hence ω^{2} is real. Therefore, an unstable mode must have negative imaginary ω. For tensor modes, the potential in the Schrödinger equation is manifestly positive, hence ω^{2} > 0 and there is no instability [105]. For vectors and scalars, the potential is not everywhere positive. Nevertheless, it can be shown that the operator appearing on the LHS of the Schrödinger equation is positive, hence ω^{2} > 0 and there is no instability [151]. In conclusion, the d > 4 Schwarzschild solution is stable against linearized gravitational perturbations.
3.4 GregoryLaflamme instability
The instabilities of black strings and black branes [118, 119] have been reviewed in [167, 128], so we shall be brief in this section and only mention the features that are most relevant to our subject. We shall only discuss neutral black holes and black branes; when charges are present, the problem becomes more complex.
This instability is the prototype for situations in which the size of the horizon is much larger in some directions than in others. Consider, as a simple, extreme case of this, the black string obtained by adding a flat direction z to the Schwarzschild solution. One can decompose linearized gravitational perturbations into scalar, vector and tensor types according to how they transform with respect to transformations of the Schwarzschild coordinates. Scalar and vector perturbations of this solution are stable [117]. Tensor perturbations that are homogeneous along the zdirection are also stable, since they are the same as tensor perturbations of the Schwarzschild black hole. However, there appears to be an instability for longwavelength tensor perturbations with nontrivial dependence on z; the frequency ω of perturbations ∼ e^{−i(ωt−kz)} acquires a positive imaginary part when k < k_{ GL } ∼ 1/r_{0}, where r_{0} is the Schwarzschild horizon radius. Thus, if the string is compactified on a circle of length L > 2π/k_{ GL } ∼ r_{0}, it becomes unstable. Of the unstable modes, the fastest one (with the largest imaginary frequency) occurs for k roughly one half of k_{ GL }. The instability creates inhomogeneities along the direction of the string. Their evolution beyond the linear approximation has been followed numerically in [42]. It is unclear yet what the endpoint is; the inhomogeneities may well grow until a sphere pinches down to a singularity.^{3} In this case, the Planck scale will be reached along the evolution, and fragmentation of the black string into black holes, with a consistent increase in the total horizon entropy, may occur.
Another important feature of this phenomenon is the appearance of a zeromode (i.e., static) perturbation with k = k_{ GL }. Perturbing the black string with this mode yields a new static solution with inhomogeneities along the string direction [117, 120]. Following numerically these static perturbations beyond the linear approximation has given a new class of inhomogeneous black strings [248].
These results easily generalize to black pbranes; for a wavevector k along the p directions tangent to the brane, the perturbations ∼ exp (−iωt+ik · z)) with k ≤ k_{ GL } are unstable. The value of k_{ GL } depends on the codimension of the black brane, but not on p.
4 MyersPerry Solutions
In this section, we review these solutions and their properties, beginning from black holes with a single rotation, and then extending them to arbitrary rotation. The existence of ultraspinning regimes in d ≥ 6 is emphasized. The symmetries and stability of the MP solutions are also discussed.
4.1 Rotation in a single plane
Let us begin with solutions that rotate in a single plane. These are not only simpler, but also exhibit more clearly the qualitatively new physics afforded by the additional dimensions.
As in Tangherlini’s solution, this metric seems to follow from a rather straightforward extension of the Kerr solution, which is recovered when d = 4. The first line in Equation (32) looks indeed like the Kerr solution, with the 1/r falloff replaced, in appropriate places, by 1/r^{ d }^{−3}. The second line contains the line element on a (d−4)sphere, which accounts for the additional spatial dimensions. It might, therefore, seem that, again, the properties of these black holes should not differ much from their fourdimensional counterparts.
For d > 6, Δ(r) is always positive at large values of r, but the term −μ/r^{ d }^{−5} makes it negative at small r (we are assuming positive mass). Therefore Δ always has a (single) positive real root independent of the value of a. Hence, regular blackhole solutions exist with arbitrarily large a. Solutions with large angular momentum per unit mass are referred to as “ultraspinning”.
4.2 General solution
4.2.1 Phase space
The determination of r_{0} involves an equation of degree 2N, which in general is difficult, if not impossible, to solve algebraically. So the presence of horizons for generic parameters in Equation (42) and (43) is difficult to ascertain. Nevertheless, a number of features, in particular the ultraspinning regimes that are important in the determination of the allowed parameter range, can be analyzed.
Following Equation (21), we can fix the mass and define dimensionless quantities j_{ i } for each of the angular momenta. Up to a normalization constant, the rotation parameters a_{ i } at fixed mass are equivalent to the j_{ i }. We take (j_{1},…, j_{ N }) as the coordinates in the phase space of solutions. We aim to determine the region in this space that corresponds to actual blackhole solutions.
Consider first the case in which all spin parameters are nonzero. Then an upper extremality bound on a combination of the spins arises. If it is exceeded, naked singularities appear, as in the d = 4 Kerr black hole [200]. So we can expect that, as long as all spin parameters take values not too dissimilar, j_{1} ∼ j_{2} ∼ … ∼ j_{ N }, all spins must remain parametrically O(1), i.e., there is no ultraspinning regime in which all j_{ i } ≫ 1.
Next, observe that for odd d, a sufficient (but not necessary) condition for the existence of a horizon is that any two of the spin parameters vanish, i.e., if two a_{ i } vanish, a horizon will always exist, irrespective of how large the remaining spin parameters are. For even d, the existence of a horizon is guaranteed if any one of the spins vanishes. Thus, arbitrarily large (i.e., ultraspinning) values can be achieved for all but two (one) of the j_{ i } in odd (even) dimensions.
Assume, then, an ultraspinning regime in which n rotation parameters are comparable among themselves, and much larger than the remaining N − n ones. A limit then exists to a black 2nbrane of limiting horizon topology S^{d−2} → ℝ^{2n} × S^{d−2(n+1)}. The limiting geometry is in fact the direct product of ℝ^{2n} and a (d − 2n)dimensional MyersPerry black hole [81]. Thus, in an ultraspinning regime the allowed phase space of ddimensional black holes can be inferred from that of (d − 2n)dimensional black holes. Let us then begin from d = 5, 6 and proceed to higher d.
A similar ‘reduction’ to a phase space in two fewer dimensions along ultraspinning directions appears in the phase space of d = 8 MP black holes; see Figure 3(b); a section at constant large j_{3} becomes asymptotically of the same shape as the sixdimensional diagram (49), rescaled by a j_{3}dependent factor.
These examples illustrate how we can infer the qualitative form of the phase space in dimension d if we know it in d − 2, e.g., in d = 9, 10, with four angular momenta, the sections of the phase space at large j_{4} approach the shapes in Figure 3(a) and (b), respectively.
4.2.2 Global structure
Let us now discuss briefly the global structure of these solutions, following [200]. The global topology of the solutions outside the event horizon is essentially the same as for the Kerr solution. However, there are cases in which there can be only one nondegenerate horizon: even d with at least one spin vanishing; odd d with at least two spins vanishing; odd d with one a_{ i } = 0 and \(\mu > \sum\nolimits_i {\prod\nolimits_{j \neq i} {a_j^2}}\) There is also the possibility, for odd d and all nonvanishing spin parameters, of solutions with event horizons with negative μ. However, they contain naked closed causal curves.
The MP solutions have singularities where μr/ΠF → ∞ for even d, μr^{2}/ΠF → ∞ for odd d. For even d and all spin parameters nonvanishing, the solution has a curvature singularity where F = 0, which is the boundary of a (d − 2)ball at r = 0, thus generalizing the ring singularity of the Kerr solution; as in the latter, the solution can be extended to negative r. If one of the a_{ i } = 0, then r = 0 itself is singular. For odd d and all a_{ i } ≠ 0, there is no curvature singularity at any r^{2} ≥ 0. The extension to r^{2} < 0 contains singularities, though. If one spin parameter vanishes, say a_{1} = 0, then there is a curvature singularity at the edge of a (d − 3)ball at r = 0, μ_{1} = 0; however, in this case, the ball itself is the locus of a conical singularity. If more than one spin parameter vanishes then r = 0 is singular. The causal nature of these singularities varies according to the number of horizons that the solution possesses; see [200] for further details.
4.3 Symmetries
The MyersPerry solutions are manifestly invariant under time translations, as well as under the rotations generated by the N Killing vector fields ∂/∂ϕ_{ i }. These symmetries form a ℝ × U(1)^{ N } isometry group. In general, this is the full isometry group (up to discrete factors). However, the solutions exhibit symmetry enhancement for special values of the angular momentum. For example, the solution rotating in a single plane (32) has a manifest ℝ × U(1) × SO(d − 3) symmetry. If n angular momenta are equal and nonvanishing then the U(1)^{ n } associated with the corresponding 2planes is enhanced to a nonAbelian U(n) symmetry. This reflects the freedom to rotate these 2planes into each other. If n angular momenta vanish then the symmetry enhancement is from U(1)^{ n } to an orthogonal group SO(2n) or SO(2n+1) for d odd or even respectively [243]. Enhancement of symmetry is reflected in the metric depending on fewer coordinates. For example, in the most extreme case of N equal angular momenta in 2N+1 dimensions, the solution has isometry group ℝ × U(N) and is cohomogeneity1, i.e., it depends on a single (radial) coordinate [111, 112].
In addition to isometries, the Kerr solution possesses a “hidden” symmetry associated with the existence of a secondrank Killing tensor, i.e., a symmetric tensor K_{ μν } obeying K(_{μν;ρ}) = 0 [245]. This gives rise to an extra constant of motion along geodesics, rendering the geodesic equation integrable. It turns out that the general MyersPerry solution also possesses hidden symmetries [174, 91] (this was first realized for the special case of d = 5 [93, 94]). In fact, it has sufficiently many hidden symmetries to render the geodesic equation integrable [207, 173]. In addition, the KleinGordon equation governing a free massive scalar field is separable in the MyersPerry background [90]. These developments have been reviewed in [88].
4.4 Stability
The classical linearized stability of these black holes remains largely an open problem. As just mentioned, it is possible to separate variables in the equation governing scalarfield perturbations [147, 26, 195]. However, little progress has been made with the study of linearized gravitational perturbations. For Kerr, the study of gravitational perturbations is analytically tractable because of a seeminglymiraculous decoupling of the components of the equation governing such perturbations, allowing it to be reduced to a single scalar equation [234, 235]. An analogous decoupling has not been achieved for MyersPerry black holes, except in a particular case that we discuss below.
Nevertheless, it has been possible to infer the appearance of an instability in the ultraspinning regime of black holes in d ≥ 6 [81]. We have seen that in this regime, when n rotation parameters a_{ i } become much larger than the mass parameter μ and the rest of the a_{ i }, the geometry of the blackhole horizon flattens out along the fastrotation planes and approaches a black 2nbrane. As discussed in Section 3.4, black pbranes are unstable against developing ripples along their spatial worldvolume directions. Therefore, in the limit of infinite rotation, the MP black holes evolve into unstable configurations. It is then natural to conjecture that the instability already sets in at finite values of the rotation parameters. In fact, the rotation may not need to be too large in order for the instability to appear. The GL instability of a neutral black brane horizon ℝ^{ p } × S^{ q } appears when the size L of the horizon along the brane directions is larger than the size r_{0} of the S^{ q }. We have seen that the sizes of the horizon along directions parallel and transverse to the rotation plane are ∼ a_{ i } and ∼ r_{0}, respectively. This branelike behavior of MP black holes begins when a_{ i } ≳ r_{0}, which suggests that the instability will appear shortly after crossing thresholds like (39). This idea is supported by the study of the possible fragmentation of the rotating MP black hole: the total horizon area can increase by splitting into smaller black holes whenever a_{ i } ≳ r_{0} [81]. The analysis of [81] indicates that the instability should be triggered by gravitational perturbations. It is, therefore, not surprising that scalarfield perturbations appear to remain stable even in the ultraspinning regime [26, 195].
This instability has also played a central role in proposals for connecting MP black holes to new blackhole phases in d ≥ 6. We discuss this in Section 6.
The one case in which progress has been made with the analytical study of linearized gravitational perturbations is the case of odd dimensionality, d = 2N+1, with equal angular momenta [177, 197]. As discussed above, this MyersPerry solution is cohomogeneity1, which implies that the equations governing perturbations of this background are just ODEs. There are two different approaches to this problem, one for N > 1 [177] and one for N = 1 (i.e., d = 5) [197].
For d = 5, the spatial geometry of the horizon is described by a homogeneous metric on S^{3}, with SU(2) × U(1) isometry group. Since S^{3} ∼ SU(2), one can define a basis of SU(2)invariant 1forms and expand the components of the metric perturbation using this basis [197]. The equations governing gravitational perturbations will then reduce to a set of coupled scalar ODEs. These equations have not yet been derived for the MyersPerry solution (however, this method has been applied to study perturbations of a static KaluzaKlein black hole with SU(2) × U(1) symmetry [158]).
For N > 1, gravitational perturbations can be classified into scalar, vector and tensor types according to how they transform with respect to the U(N) isometry group. The different types of perturbation decouple from each other. Tensor perturbations are governed by a single ODE that is almost identical to that governing a massless scalar field. Numerical studies of this ODE give no sign of any instability [177]. Vector and scalar type perturbations appear to give coupled ODEs; the analysis of these has not yet been completed.
It seems likely that other MP solutions with enhanced symmetry will also lead to more tractable equations for gravitational perturbations. For example, it would be interesting to consider the cases of equal angular momenta in even dimensions (which resemble the Kerr solution in many physical properties), and MP solutions with a single nonzero angular momentum (whose geometry (32) contains a fourdimensional factor, at a constant angle in the S^{d−4}, mathematically similar to the Kerr metric; in fact this fourdimensional geometry is type D). The latter case would allow one to test whether the ultraspinning instability is present.
5 Vacuum Solutions in Five Dimensions
In Section 4 we discussed the MP solutions, which can be regarded as higherdimensional versions of the Kerr solution. However, in recent years it has been realized that higher dimensions allow for a much richer landscape of blackhole solutions that do not have fourdimensional counterparts. In particular, there has been great progress in our understanding of fivedimensional vacuum black holes, insofar as we consider stationary solutions with two rotational Killing vectors. The reason is that this sector of the theory is completely integrable, and solutiongenerating techniques are available. We begin by analyzing in Section 5.1 a qualitatively new class of solutions with connected horizons: black rings with one and two angular momenta. Then, in Section 5.2, we present a general study of stationary solutions with two rotational symmetries; actually, we can discuss the general case of d − 3 commuting U(1) spatial isometries. The simplest of these are the generalized Weyl solutions (Section 5.2.1). The general case is addressed in Section 5.2.2, and we discuss the characterization of solutions by their rod structures. The powerful solutiongenerating technique of Belinsky and Zakharov, based on inversescattering methods, is then introduced: the emphasis is on its practical application in generating old and new blackhole solutions. Section 5.3 discusses multipleblackhole solutions (black Saturns, dirings and bicycling black rings) obtained in this way. Work towards determining the stability properties of black rings and multiple black holes is reviewed in Section 5.4.
5.1 Black rings
5.1.1 One angular momentum
The physical parameters of the solution (mass, angular momentum, area, angular velocity, surface gravity) in terms of ν and R can be found in [84]. It can be seen that while R provides a measure of the radius of the ring’s S^{1}, the parameter can be interpreted as a ‘thickness’ parameter characterizing its shape, corresponding roughly to the ratio between the S^{2} and the S^{1} radii.
This curve is easily seen to have a cusp at ν = 1/2, which corresponds to a minimum value of \(j = \sqrt {27/32}\) and a maximum a_{ H } = 1. Branching off from this cusp, the thin blackring solutions (0 < ν < 1/2) extend to j →∞ as ν → 0, with asymptotic a_{ H } → 0. The fat blackring branch (1/2 ≤ ν < 1) has lower area and extends only to j → 1, ending at ν→ 1 at the same zeroarea singularity as the MP solution. This implies that in the range \(\sqrt {27/32} \leq j < 1\) there exist three different solutions (thin and fat black rings and MP black holes) with the same value of j. The notion of blackhole uniqueness that was proven to hold in four dimensions does not extend to five dimensions.
[76] and [89] contain detailed analyses of the geometrical features of blackring horizons. Some geodesics of the blackring metric have been studied with a view towards different applications: [205] studies them in the context of the Penrose process, and [76] considers them for tests of stability. [143] is a more complete analysis of geodesics.
5.1.2 Two angular momenta
Rotation in the second independent plane corresponds to rotation of the S^{2} of the ring. In the limit of infinite S^{1} radius, a section along the length of the ring gives an S^{2} that is essentially like that of a fourdimensional black hole: setting it into rotation is thus similar to having a Kerrlike black hole. Thus, an upper, extremal bound on the rotation of the S^{2} is expected (actually, the motion of the ring along its S^{1} yields a momentum that can be viewed as an electric KaluzaKlein charge, so instead of a Kerr solution, the R→∞limit yields a rotating electric KK black hole).
 1.Extremal black rings, with maximal j_{2} for given j_{1}, along the curve(thin solid curve in Figure 6). This curve extends between j_{1} = 3/4, j_{2} = 1/4 (as ν→ 1) to j_{1}→ ∞, j_{2} → 0 (as ν → 0). (See [217] for more discussion of extremal rings.)$${j_1} = {{1 + 4\sqrt \nu + \nu} \over {4{\nu ^{1/4}}(1 + \sqrt \nu)}},\quad {j_2} = {{{\nu ^{1/4}}} \over {2(1 + \sqrt \nu)}},\quad 0 \leq \nu \leq 1$$(62)
 2.Nonextremal minimally spinning black rings, with minimal j_{1} for given j_{2}, along the curve(thick solid curve in Figure 6). This curve extends between j_{1} = 4/5, j_{2} = 1/5 (as ν → 1) and \({j_1} = \sqrt {27/32}\), j_{2} = 0 (as ν → 0).$$\begin{array}{*{20}c} {{j_1} = {{{{\left({3(1 + {\nu ^2}) + (1 + \nu)\sqrt {(9 + \nu)(1 + 9\nu)}  26\nu} \right)}^{3/2}}} \over {8{{(1  \nu)}^2}\sqrt {\left({\sqrt {(9 + \nu)(1 + 9\nu)}  1  \nu} \right)\left({5(1 + \nu)  \sqrt {(9 + \nu)(1 + 9\nu)}} \right)}}},} \\ {{j_2} = {{{{\sqrt {\nu \left({3 + \sqrt {(9 + \nu)(1 + 9\nu)} + \nu \left({18\sqrt {(9 + \nu)(1 + 9\nu)}  103 + \nu \left({3\nu + \sqrt {(9 + \nu)(1 + 9v)}  103} \right)} \right)} \right)}} \over {\sqrt {(9 + \nu)(1 + 9\nu)  1  \nu}}}} \over {2\sqrt 2 {{(1  \nu)}^2}}}} \\ \end{array}$$(63)
 3.
Limiting extremal MP black holes, with j+j_{2} = 1 within the range j_{1} ∈ [3/4, 4/5] (dashed line in Figure 6). There is a discontinuous increase in the area when the black rings reach the extremal MP line.
[78] contains sectional plots of the surface a_{ H }(j_{1}, j_{2}) for black rings at constant j_{2}, for j_{1} > j_{2} ≥ 0, from which it is possible to obtain an idea of the shape of the surface. In the complete range of j_{1} and j_{2} the phase space of fivedimensional black holes (with connected event horizons) consists of the ‘dome’ of MP black holes (Figure 4 replicated on all four quadrants), with ‘romanesque vaults’ of black rings protruding from its corners, and additional substructure in the region of nonuniqueness — our knowledge of architecture is insufficient to describe it in words.
On the other hand, the temperature of the black ring — which for thin rings with a single spin is bounded below and diverges as the ring becomes infinitely long and thin (at fixed mass) — decreases to zero when the second spin is taken to the extremal limit, so there exist ‘cold’ thin black rings.
Some consequences of these features to properties of multiplering solutions will be discussed in Section 5.3.
5.2 Stationary axisymmetric solutions with d − 3 rotational symmetries
A sector of fivedimensional vacuum general relativity in which a complete classification of blackhole solutions may soon be achieved is the class of stationary solutions with two angular Killing vectors. Integration of the three Killing directions yields a twodimensional nonlinear sigma model that is completely integrable. Solutions can be characterized in terms of their rod structure along multiple directions, introduced in [82] and extended in [127]. It has been proven that these data (whose relation to physical parameters is unfortunately not quite direct), in addition to the total mass and angular momenta, uniquely characterize asymptoticallyflat solutions [139].
Equations (68) and (69) are the equations for the principal chiral field model, a nonlinear sigma model with group GL(d − 2, ℝ), which is a completely integrable system. In the present case, it is also subject to the constraint (67). This introduces additional features, some of which will be discussed below.
In order to understand the structure of the solutions of this system, it is convenient to first analyze a simple particular case [82].
5.2.1 Weyl solutions
5.2.2 General axisymmetric class
In the general case where the Killing vectors are not orthogonal to each other, the simple construction in terms of solutions of the linear Laplace equation does not apply. Nevertheless, the equations can still be completely integrated, and the characterization of solutions in terms of rod structure can be generalized.
5.2.2.1 Rod structure and regularity
Let us begin by extending the characterization of rods [127]. In general, a rod is an interval along the zaxis, where the action of a Killing vector υ has fixed points (for a spacelike vector) or it becomes null (for a timelike vector). In both cases υ^{2} → 0 at the rod. In general, υ will be a linear combination υ = υ^{ a }ξ_{(a)} in a given Killing basis ξ_{(a)}, a = 0,…, d − 3; usually, this basis is chosen so that it becomes a coordinate basis of orthogonal vectors at asymptotic infinity. In the orthogonal case of the previous section 5.2.1 one could assign a basis vector (and only one, for rod density 1/2) ξ_{(a)} to each rod, but this is not possible, in general, if the vectors are not orthogonal. For instance, at a rod corresponding to a rotating horizon, the Killing vector that vanishes is typically of the form \({\partial _t} + {\Omega _a}{\partial _{{\phi _a}}}\).
The rod is referred to as timelike or spacelike according to the character of the rescaled norm g_{ ab }(r, z)υ^{ a }υ^{ b }/r^{2} at the rod r → 0, z → [a_{k−1}, a_{ k }]. For a timelike rod normalized such that the generator of asymptotic time translations enters with coefficient equal to one, the rest of the coefficients correspond to the angular velocities of the horizon (if this satisfies all other regularity requirements).
Further analysis of Equations (68), (70) and their source terms can be found in [130].
5.2.2.2 The method of Belinsky and Zakharov
As shown by Belinsky and Zakharov (BZ), the system of nonlinear equations (67) and (68) is completely integrable [11, 10]. It admits a Lax pair of linear equations (spectral equations) whose compatibility conditions coincide with the original nonlinear system. This allows one to generate an infinite number of solutions, starting from known ones, following a purely algebraic procedure. Since, as we have seen, we can very easily generate diagonal solutions, this method allows us to construct a vast class of axisymmetric solutions. It seems likely, but to our knowledge has not been proven, that all axisymmetric solutions can be generated in this way.
A possible way out of this problem is to restrict oneself to transformations that act only on a 2 × 2 block of the seed g_{0} [165]. In this case it is possible to apply the above renormalization to only this part of the metric — effectively, the same as in four dimensions — and thus obtain a solution with the correct, physical rod densities. However, it is clear that, if we start from diagonal seeds, this method cannot deal with solutions with offdiagonal terms in more than one 2 × 2 block, e.g., with a single rotation. It cannot be applied to obtain solutions with rotation in several planes. Still, [7] applies this method to obtain solutions with arbitrary number of rods, with rotation in a single plane.
Fortunately, a clever and very practical way out of this problem has been proposed by Pomeransky [211], that can deal with the general case in any number of dimensions. The key idea is the observation that Equation (97) is independent of the ‘realignment’ vectors \({m_{0b}}^{(k)}\). One may then start from a solution with physical rod densities, ‘remove’ a number of solitons from it (i.e., add solitons or antisolitons with negative densities −1/2), and then readd these same solitons, but now with different vectors \({m_{0b}}^{(k)}\), so the rods affected by these solitons acquire, in general, new directions. If the original seed solution satisfied the determinant constraint (67), then by construction so will the metric obtained after readding the solitons (including, in particular, the sign). And more importantly, if the densities of the initial rods are all ±1/2 and negative densities do not appear in the end result, the final metric will only contain regular 1/2 densities.
If the vectors \(m_{0a}^{(k)}\) for the readded solitons mix the time and spatial Killing directions, then this procedure may yield a stationary (rotating) version of the initial static solution. The method requires the determination of the function Ψ_{0}(λ, r, z) that solves Equation (88) for the seed. This is straightforward to determine for diagonal seeds (see the examples below), so for these the method is completely algebraic. Although even a twosoliton transformation of a multiplerod metric can easily result in long expressions for the metric coefficients, the method can be readily implemented in a computer program for symbolic manipulation. The procedure can also be applied, although it becomes quite more complicated, to nondiagonal seeds. In this case, the function Ψ_{0}(λ, r, z) for the nondiagonal seed is most simply determined if this solution itself is constructed starting from a diagonal seed. The doublyspinning black ring of [212] was obtained in this manner.
5.2.2.3 BZ construction of the Kerr, MyersPerry and blackring solutions
We sketch here the method to obtain all known blackhole solutions with connected horizons in four and five dimensions. These solutions illustrate how to add rotation to S^{2}, S^{3} and S^{1} × S^{2} components of multipleblackhole horizons.
The doublyspinning ring has resisted all attempts at deriving it directly from a diagonal, static seed. Instead, [212] obtained it in a twostep process. Rotation of the S^{2} of the ring is similar to the rotation of the Kerr solution. In fact, the blackring solutions with rotation only along the S^{2} can be obtained by applying to the static black ring the same kind of twosoliton transformations that yielded Kerr from a Schwarzschild seed (101) [239]. Hence, if we begin from the black ring rotating along the S^{1} and perform similar soliton and antisoliton transformations at the endpoints of the horizon rod, we can expect to find a doublyspinning ring. The main technical difficulty is in constructing the function Ψ_{0}(λ, r, z) for the singlespin blackring solution (50). However, if we construct this solution via a onesoliton transformation as we have just explained, this function is directly obtained from Equation (90). In this manner, solution (57) was derived.
This method has also been applied to construct solutions with disconnected components of the horizon, which we shall discuss next. The previous examples provide several ‘rules of thumb’ for constructing such solutions. However, there is no precise recipe for the most efficient way of generating the sought solution. Quite often, unexpected pathologies show up, of both local and global type, so a careful analysis of the solutions generated through this method is always necessary.
Finally, observe that there are certain arbitrary choices in this method; it is possible to choose different solitons and antisolitons, with different orientations, and still get essentially the same final physical solution. Also, the intermediate rescaling, and the form for \({{\tilde \Psi}_0}\), admit different choices. All this may lead to differentlooking forms of the final solution, some of them possibly simpler than others. Occasionally, spurious singularities may be introduced through bad choices.
5.2.2.4 Other methods
In four dimensions there exists another technique, akin to the Bäcklund transformation that adds solitons to a seed solution, to integrate the equations for the stationary axisymmetric class of vacuum solutions [203, 137, 29]. [192, 149] have extended this to higher dimensions. Unfortunately, though this method can produce simpler expressions than the BZ technique, it cannot deal with more than two offdiagonal terms, and hence, no more than a single angular momentum. [192] applied this method to derive a black ring with rotation in the S^{2} (but not along the S^{1}). The same authors used this technique to provide the first systematic derivation (i.e., through explicit integration of Einstein’s equations, instead of guesswork) of the black ring with rotation along S^{1} [148]. The connection between the Bäcklund transformation method and the BZ technique has been studied in [238], where it is argued that all the solutions obtained by a twosoliton Bäcklund transformation on an arbitrary diagonal seed are contained among those that the BZ method generates from the same seed. This is not too surprising in view of similar, and more general, results in four dimensions [53, 54]. It may be interesting to investigate the application of related but more efficient, axisymmetric solutiongenerating methods [226, 189] to higher dimensions.
[114] develops a different algebraic method to obtain stationary axisymmetric solutions in five dimensions from a given seed. An SO(2, 1) subgroup of the “hidden” SL(3, ℝ) symmetry of solutions with at least one spatial Killing vector (the presence of a second one is assumed later) is identified that preserves the asymptotic boundary conditions, and whose action on a static solution generates a oneparameter family of stationary solutions with angular momentum, e.g., one can obtain the MyersPerry solution from a SchwarzschildTangherlini seed. It is conjectured that all vacuum stationary axisymmetric solutions can be obtained by repeated application of these transformations on static seeds.
5.3 Multipleblackhole solutions
In d = 4 it is believed that there are no stationary multipleblackhole solutions of vacuum gravity. However, such solutions do exist in d = 5. ‘Black Saturn’ solutions, in which a central MPtype of black hole is surrounded by a concentric rotating black ring, have been constructed in [77]. They exhibit a number of interesting features, such as rotational dragging of one black object by the other, as well as both co and counterrotation. For instance, we may start from a static seed and act with the kind of onesoliton BZ transformation that turns on the rotation of the black ring. This gives angular momentum (measured by a Komar integral on the horizon) to the black ring but not to the central black hole. However, the horizon rod of this black hole is reoriented and acquires a nonzero angular velocity: the black hole is dragged along by the blackring rotation. It is also possible (this needs an additional onesoliton transformation that turns on the rotation of the MP black hole) to have a central black hole with a static horizon that nevertheless has nonvanishing angular momentum; the ‘proper’ inner rotation of the black hole is cancelled at its horizon by the blackring drag force.
The explicit solutions are rather complicated, but an intuitive discussion of their properties is presented in [73]. The existence of black Saturns is hardly surprising; since black rings can have arbitrarily large radius, it is clear that we can put a small black hole at the center of a very long black ring, and the interaction between the two objects will be negligible. In fact, since a black ring can be made arbitrarily thin and light for any fixed value of its angular momentum, for any nonzero values of the total mass and angular momentum, we can obtain a configuration with larger total area than any MP black hole or black ring; put almost all the mass in a central, almoststatic black hole, and the angular momentum in a very thin and long black ring. Such configurations can be argued to attain the maximal area (i.e., entropy) for given values of M and J. Observe also that for fixed total (M, J) we can vary, say, the mass and spin of the black ring, while adjusting the mass and spin of the central black hole to add up to the total M and J. These configurations, then, exhibit doublycontinuous nonuniqueness.
We can similarly consider multiplering solutions. Dirings, with two concentric black rings rotating on the same plane, were first constructed in [150]; [85] rederived them using the BZ approach. Each new ring adds two parameters to the continuous degeneracy of solutions with given total M and J.
It is also possible to have two black rings lying and rotating on orthogonal, independent planes. Such bicycling black rings have been constructed using the BZ method [152, 78], and provide a way of obtaining configurations with arbitrarily large values of both angular momenta for fixed mass — which cannot be achieved simultaneously for both spins, either by the MP black holes or by doublyspinning black rings. The solutions in [152, 78] are obtained by applying to each of the two rings the kind of transformations that generate the singlyspinning black ring. Thus each black ring possesses angular momentum only on its plane, along the S^{1}, but not in the orthogonal plane, on the S^{2} — nevertheless, they drag each other so that the two horizon angular velocities are both nonzero on each of the two horizons. The solutions contain four free parameters, corresponding to, e.g., the masses of each of the rings and their two angular momenta. It is clear that a more general, sixparameter solution must exist in which each black ring has both angular momenta turned on.
It can be argued, extending the arguments in [73], that multipleblackhole solutions allow one to cover the entire (j, j_{2}) phase plane of fivedimensional solutions. It would be interesting to determine for which parameter values these multiple black holes have the same surface gravity and angular velocities on all disconnected components of the horizon. With this constraint, multipleblackhole solutions still allow one to cover a larger region of the (j_{1}, j_{2}) plane than already covered by solutions with a connected horizon;see Figure 7. For instance, it can be argued that some bicycling black rings (within the sixparameter family mentioned above for which the four angular velocities of the entire system can be varied independently) should satisfy these ‘thermal equilibrium’ conditions; as we have seen, a doublyspinning black ring can have Ω_{1} = Ω_{2}. Thus, if we consider two identical doublyspinning thin black rings, one on each of the two planes, then we can make them have S^{1}angular velocity equal to the S^{2}angular velocity of the other ring in the orthogonal plane. These solutions then lie along the lines j_{1} = j_{2}, reaching arbitrarily large j_{1,2}, which is not covered by the singleblackhole phases in Figure 7. Clearly, there will also exist configurations extending continuously away from this line.
Black Saturns with a single black ring that satisfy ‘thermal equilibrium’ conditions should also exist. In fact, the possibility of varying the temperature of the ring by tuning the rotation in the S^{2} might allow one to cover portions of the (j_{1}, j_{2}) plane beyond Figure 7. If so, this would be unlike the situation with a single rotation, where thermalequilibrium Saturns lie within the range of j of black rings, Figure 10.
The Weyl ansatz of Section 5.2.1 enables one to easily generate solutions in d = 5 with multiple black holes of horizon topology S^{3}, which are asymptotically flat [82, 231]. However, all these solutions possess conical singularities reflecting the attraction between the different black holes. It seems unlikely that the extension to include offdiagonal metric components (rotation and twists) could eliminate these singularities and yield balanced solutions.
5.4 Stability
The linearized perturbations of the blackring metric (50) have not yielded to analytical study. The apparent absence of a Killing tensor prevents the separation of variables even for massless scalarfield perturbations. In addition, the problem of decoupling the equations to find a master equation for linearized gravitational perturbations, already present for the MyersPerry solutions, is, if anything, exacerbated for black rings.
Studies of the classical stability of black rings have, therefore, been mostly heuristic. Already the original paper [83] pointed out that very thin black rings locally look like boosted black strings (this was made precise in [71]), which were expected to suffer from GLtype instabilities. The instability of boosted black strings was indeed confirmed in [144]. Thus, thin black rings are expected to be unstable to the formation of ripples along their S^{1} direction. This issue was examined in further detail in [76], which found that thin black rings seem to be able to accommodate unstable GL modes down to values j ∼ O(1). Thus, it is conceivable that a large fraction of black rings in the thin branch, and possibly all of this branch, suffer from this instability. The ripples rotate with the black ring and then should emit gravitational radiation. However, the timescale for this emission is much longer than the timescale of the fastest GL mode, so the pinchdown created by this instability will dominate the evolution, at least initially. The final fate of this instability of black rings depends on the endpoint of the GL instability, but it is conceivable, and compatible with an increment of the total area, that the blackring fragments separate into smaller black holes that then fly away.
Another kind of instability was discussed in [76]. By considering offshell deformations of the black ring (namely, allowing for conical singularities), it is possible to compute an effective potential for radial deformations of the black ring. Fat black rings sit at maxima of this potential, while thin black rings sit at minima. Thus, fat black rings are expected to be unstable to variations of their radius, and presumably collapse to form MP black holes. The analysis in [76] is in fact consistent with a previous, more abstract analysis of local stability in [2]. This is based on the ‘turningpoint’ method of Poincaré, which studies equilibrium curves for phases near bifurcation points. For the case of black rings, one focuses on the cusp, where the two branches meet. One then assumes that these curves correspond to extrema of some potential, e.g., an entropy, that can be defined everywhere on the plane (j, a_{ H }). The cusp then corresponds to an inflection point of this potential at which a branch of maxima and a branch of minima meet. By continuity, the branch with the higher entropy will be the most stable branch, and the one with lower entropy will be unstable. Thus, for black rings an unstable mode is added when going from the upper (thin) to the lower (fat) branch. This is precisely as found in [76] from the mechanical potential for radial deformations.
Thus, a large fraction of all singlespin neutral black rings are expected to be classically unstable, and it remains an open problem whether a window of stability exists for thin black rings with j ∼ O(1). The stability, however, can improve greatly with the addition of charges and dipoles.
Doublyspinning black rings are expected to suffer from similar instabilities. Insofar as a fat ring branch that meets at a cusp with a thin ring branch exists, the fat rings are expected to be unstable. Very thin rings are also expected to be unstable to GLperturbations that form ripples. The angular momentum on the S^{2} may be redistributed nonuniformly along the ring, with the larger blobs concentrating more spin. In addition, although it has been suggested that superradiant ergoregion instabilities associated to rotation of the S^{2} might exist [68], a proper account of the asymptotic behavior of superradiant modes needs to be made before concluding that the instability is actually present.
Much of what we can say about the classical stability of black Saturns and multiple rings follows from what we have said above for each of its components, e.g., if their rings are thin enough, they are expected to be GLunstable. We know essentially nothing about what happens when the gravitational interaction among the black objects involved is strong. For instance, we do not know if the GL instability is still present when a thin black ring lassoes at very close range a much larger, central, MP black hole.
Massive geodesics on the plane of a black ring (see [143]) show that a particle at the center of the S^{1} is unstable to migrating away towards the black ring. This suggests that a black Saturn with a small black hole at the center of a larger black ring should be unstable. One possibility for a different instability of black Saturns appears from the analysis of counterrotating configurations in [77]. For large enough counterrotation, the Komarmass of the central black hole vanishes and then becomes negative. By itself, this does not imply any pathology, as long as the total ADM mass is positive and the horizon remains regular, which it does. However, it suggests that the counterrotation in this regime becomes so extreme that the black hole might tend to be expelled off the plane of rotation.
Clearly, the classical stability of all, old and new, rotating blackhole solutions of fivedimensional general relativity remains largely an open problem, where much work remains to be done.
6 Vacuum Solutions in More Than Five Dimensions
With no available techniques to construct asymptoticallyflat exact solutions beyond those found by Myers and Perry, the situation in d ≥ 6 is much less developed than in d =5. Recall that the symmetry requirements for the class of solutions (66) are incompatible with the asymptotic symmetries of globally flat space in d > 5; metrics of type (66) necessarily involve directions with nonflat asymptotics (e.g., infinitelyextended black strings and branes) and/or asymptotic compact KaluzaKlein circles. In order to preserve asymptotic flatness one would instead ask for rotational symmetry around ‘axes’ that are hyperplanes of a codimension larger than two, but the integrability of the resulting equations [34, 35] remains essentially an unsolved problem.
However, despite the paucity of exact solutions, there are strong indications that the variety of black holes that populate general relativity in d ≥ 6 is vastly larger than in d = 4, 5. A first indication came from the conjecture in [81] of the existence of black holes with spherical horizon topology but with axiallysymmetric ‘ripples’ (or ‘pinches’). The plausible existence of black rings in any d ≥ 5 was argued in [144, 76]. More recently, [80] has constructed approximate solutions for black rings in any d ≥ 5 and then exploited the conjecture of [81] to try to draw a phase diagram with connections and mergers between the different expected phases. In the following we summarize these results.
6.1 Approximate solutions from curved thin branes
In the absence of exact techniques, [80] resorts to approximate constructions, in particular to the method of matched asymptotic expansions previously used in the context of black holes localized in KaluzaKlein circles in [126, 115, 156, 116]^{7}. The basic idea is to find two widely separated scales in the problem, call them R_{1} and R_{2}, with R_{2} ≪ R_{1}, and then try to solve the equations in two limits; first, as a perturbative expansion for small R_{2}, and then in an expansion in 1/R_{1}. The former solves the equations in the far region r ≫ R_{2} in which the boundary condition, e.g., asymptotic flatness, fixes the integration constants. The second expansion is valid in the near region r ≪ R_{1}. In order to fix the integration constants in this case, one matches the two expansions in the overlap region R_{2} ≪ r ≪ R_{1} in which both approximations are valid. The process can then be iterated to higher orders in the expansion; see [115] for an explanation of the systematics involved.
In order to construct a black ring with horizon topology S^{1} × S^{d−3}, we take the scales R_{1}, R_{2} to be the radii of the S^{1} and S^{d−3}, respectively^{8}. To implement the above procedure, we take R_{2} = r_{0}, the horizon radius of the S^{d−3} of a straight boosted black string, and R_{1} = R the large circle radius of a very thin circular string. Thus, in effect, to first order in the expansion what one does is: (i) find the solution within the linearized approximation, i.e., for small r_{0}/r, around a Minkowski background for an infinitelythin circular string with momentum along the circle; (ii) perturb a straight boosted black string so as to bend it into an arc of very large radius 1/R. Step (ii) not only requires matching to the previous solution in order to provide boundary conditions for the homogeneous differential equations; one also needs to check that the perturbations can be made compatible with regularity of the horizon.
It is worth noting that the form of the solution thus found exhibits a considerable increase in complexity when going from d = 5, where an exact solution is available, to d > 5; simple linear functions of r in d = 5 change to hypergeometric functions in d > 5. We take this as an indication that exact closed analytical forms for these solutions may not exist in d > 5.
We will not dwell here on the details of the perturbative construction of the solution (see [80] for this), but instead we shall emphasize that adopting the view that a black object is approximated by a certain very thin black brane curved into a given shape can easily yield nontrivial information about new kinds of black holes. Eventually, of course, the assumption that the horizon remains regular after curving needs to be checked.
Consider then a stationary black brane, possibly with some momentum along its worldvolume, with horizon topology ℝ^{ p }^{+1} × S^{ q }, with q = d − p − 2. When viewed at distances much larger than the size r_{0} of the S^{ q }, we can approximate the metric of the blackbrane spacetime by the gravitational field created by an ‘equivalent source’ with distributional energy tensor \(T_\mu ^\nu \propto r_0^{q  1}{\delta ^{(q + 1)}}(r)\), with nonzero components only along directions tangent to the worldvolume, and where r = 0 corresponds to the location of the brane. Now we want to put this same source on a curved, compact pdimensional spatial surface in a given background spacetime (e.g., Minkowski, but possibly (anti)de Sitter or others, too). In principle we can obtain the mass M and angular momenta J_{ i } of the new object by integrating \(T_t^t\) and \(T_t^i\) over the entire spatial section of the brane worldvolume. Moreover, the total area \({{\mathcal A}_H}\) is similarly obtained by replacing the volume of ℝ^{ p } with the volume of the new surface. Thus, it appears that we can easily obtain the relation \({{\mathcal A}_H}(M,{J_i})\) in this manner.
6.2 Phase diagram
Using the dimensionless area and spin variables (21), Equation (113) allows one to compute the asymptotic form of the curve a_{ H }(j) in the phase diagram at large j for black rings. However, when j is of order one, the approximations in the matched asymptotic expansion break down, and the gravitational interaction of the ring with itself becomes important. At present we have no analytical tools to deal with this regime for generic solutions. In most cases, numerical analysis may be needed to obtain precise information.
Nevertheless, [80] contains advanced heuristic arguments, which propose a completion of the curves that is qualitatively consistent with all the information available at present. A basic ingredient is the observation in [81], discussed in Section 4.1, that in the ultraspinning regime in d ≥ 6, MP black holes approach the geometry of a black membrane ≈ ℝ^{2} × S^{d−4} spread out along the plane of rotation.
We have already discussed how using this analogy, [81] argues that ultraspinning MP black holes should exhibit a GregoryLaflammetype of instability. Since the threshold mode of the GL instability gives rise to a new branch of static nonuniform black strings and branes [117, 120, 248], [81] argues that it is natural to conjecture the existence of new branches of axisymmetric ‘lumpy’ (or ‘pinched’) black holes, branching off from the MP solutions along the stationary axisymmetric zeromode perturbation of the GLlike instability.
Indirect evidence for the existence of black holes with pinched horizons is provided by the results of [180], which finds ‘pinched plasmaball’ solutions of fluid dynamics that are CFT duals of pinched black holes in sixdimensional AdS space. The approximations involved in the construction require that the horizon size of the dual black holes be larger than the AdS curvature radius, and thus do not admit a limit to flat space. Nevertheless, their existence provides an example, if indirect, that pinched horizons make an appearance in d = 6 (and not in d = 5).
6.3 Stability

GLtype instabilities in ultraspinning regimes. Both MP black holes and black rings approach, in this regime, string or membranelike configurations that are expected to be unstable to growing nonuniformities along the ‘long’ directions on the horizon. Such instabilities give rise, in the cases where the nonuniformities can remain stationary, to zero modes that branch off into new solutions with broken symmetry.

Turningpoint instabilities. Given the phase diagram with different curves a_{ H }(j) in it, we expect that near a point where different phases meet, the branch with lesser area should have one more unstable mode than those with greater area. This happens along blackring curves at the turning points where j reaches a minimum. The instability is presumably related to radial deformations of the ring. It also happens at mergers between phases.
We end this section emphasizing that, presumably, new concepts and tools are required for the characterization of black holes in d ≥ 6, let alone for their explicit construction.
The general problem of the dynamical linearized stability of MP black holes, in particular in the case with a single rotation, becomes especially acute for the determination of possible blackhole phases in d ≥ 6. The arguments in favor of an ultraspinning instability seem difficult to evade, so a most pressing problem is to locate the point (i.e., the value of j) at which this instability appears as a stationary mode, and then perturb the solution along this mode to determine the direction in which the new branch of solutions evolves.
7 Solutions with a Gauge Field
7.1 Introduction
Black holes with nontrivial gauge fields play an important role in string theory, not least because the types of charge that they carry help identify their microscopic constituents (e.g., branes). In this section we shall review briefly such solutions for d ≥ 5. We shall concentrate exclusively on solutions of maximal supergravity theories arising as consistent toroidal reductions of d = 10, 11 supergravity.
The bosonic sector of maximal supergravity theories contains massless scalars taking values in some coset space G/H where G is noncompact and H is compact. Since the scalars are massless, their asymptotic values (for an asymptoticallyflat solution) are arbitrary, i.e., they are moduli. G is a global symmetry of the supergravity theory (broken to a discrete Uduality symmetry in string/Mtheory). By acting with G we can choose the moduli to be anything convenient. Given a particular choice for the moduli, the global symmetry group is broken to H. See, e.g., [210] for a review of this.
The only type of conserved gauge charge that an asymptotically flat solution can carry is electric charge with respect to a 2form field strength (or magnetic charge with respect to a d − 2 form field strength, but this can always be dualized into a 2form). The group H acts nontrivially on the charges of a solution. Hence, the strategy in looking for blackhole solutions is to identify a “seed” solution with a small number of parameters, from which a solution with the most general charge assignments can be obtained by acting with H. For example, dimensional reduction of d = 11 supergravity on T^{6} gives the maximal \({\mathcal N} = 4,d = 5\), d =5 supergravity, which has 27 Abelian vectors and 42 scalars parameterizing the coset E_{6(6)}/US_{ p }(8). It turns out that the a solution with 27 independent charges can be obtained by acting with US_{ p }(8) on a seed solution with just 3 charges [58].
7.2 Topologicallyspherical black holes
7.2.1 Nonextremal solutions
The construction of stationary, charged, topologicallyspherical, blackhole solutions (“charged MyersPerry”) in maximal supergravity theories was described in [58]. It turns out that the seed solutions are the same as for toroidal compactifications of heterotic string theory. The latter seed solutions were constructed in [63] for d = 5 and [64, 184] for 6 ≤ d ≤ 9. In addition to their mass and angular momenta, they are parameterized by three electric charges in d = 5 and two electric charges for 6 ≤ d ≤ 9. For d = 10 (type IIA theory), the solution describing a rotating black hole charged with respect to the RamondRamond 2form field strength (i.e., D0brane charge) can be obtained from the general rotating brane solution given in [55].
In the limit of vanishing angular momenta, all of these solutions reduce to d > 4 generalizations of the ReissnerNordström solution. This static case is the only case for which linearized stability has been investigated. The spherical symmetry of static solutions permits a scalar/vector/tensor decomposition of perturbations. For ReissnerNordström solutions of d > 4 EinsteinMaxwell theory, decoupled equations governing each type have been obtained [164]. These have been used to prove analytically the stability of tensor and vector perturbations for d ≥ 5 and scalar perturbations for d = 5 [164]. Numerical studies have revealed that scalar perturbations are also stable for 6 ≤ d ≤ 11 [169].
7.2.2 Supersymmetric solutions: the BreckenridgeMyersPeetVafa (BMPV) black hole
Black holes saturating a BogomolnyiPrasadSommerfield (BPS) inequality play an important role in string theory. The canonical example is the first blackhole entropy calculation [229], in which the BPS condition provided the justification for relating the BekensteinHawking entropy of a classical blackhole solution to the statistical entropy of a microscopic brane configuration.
The only known asymptoticallyflat BPS blackhole solutions occur for d = 4, 5. For d = 5, one can obtain BPS blackhole solutions as a limit of the nonextremal charged rotating solutions just discussed. Starting from the sixparameter seed solution of [63] one can obtain a fourparameter BPS black hole: the BMPV black hole [17, 242]. The solution has equal angular momenta J_{1} = J_{2} = J and the four parameters are J and the three electric charges. The mass is fixed by the BPS relation. Note that one loses two parameters in the BPS limit; this is because the BPS limit and the extremal limit are distinct for rotating black holes — a BPS black hole is necessarily extremal, but the converse is untrue. As for the MyersPerry solution, the equality of the angular momenta gives rise to a nonAbelian isometry group ℝ × U(1) × SU(2). Classical properties of the BMPV solution have been discussed in detail in [102, 106].
7.3 Black rings with gauge fields
7.3.1 Dipole rings
Black holes can only carry electric charge with respect to a twoform. However, higherrank pforms may also be excited by a black hole, even though there is no net charge associated with them. This occurs naturally for black rings. Consider a black string in d dimensions. A string carries electric charge associated with a threeform field strength H. The electric charge is proportional to the integral of _{*}H over a (d − 3)sphere that links the string. Now consider a black ring with topology S^{1} × S^{d−3} formed by bending the string into a loop and giving it angular momentum around the S^{1}. This would be asymptotically flat and, hence, the charge associated with H would be zero. Nevertheless, H would be nonzero. Its strength can be measured by the flux of _{*}H through a S^{d−3} linking the ring, which is no longer a conserved quantity but rather a nonconserved electric dipole. Such a ring would have three parameters, but would only have two conserved charges (mass and angular momentum), hence it would exhibit continuous nonuniqueness.
Exact solutions describing such “dipole rings” have been constructed in d = 5 [79]. In d = 5, one can dualize a 3form H to a twoform, so these solutions carry magnetic dipoles with respect to twoform field strengths. The dipole rings of [79] are solutions of \({\mathcal N} = 1,d = 5\) supergravity coupled to two vector multiplets, which is a theory with U(1)^{3} gauge group that can be obtained by consistent truncation of the maximal d = 5 supergravity theory. These rings are characterized by their mass M, angular momentum J_{1} around the S^{1} of the horizon, and three dipoles q_{ i }. The angular momentum J_{2} on the S^{2} of the horizon vanishes. They have the same ℝ × U(1)^{2} symmetry as vacuum black rings. These solutions are seed solutions for the construction of solutions of maximal d = 5 supergravity with 27 independent dipoles obtained by acting with USp(8) as described above. One would expect the existence of more general dipolering solutions with two independent nonzero angular momenta, but these are yet to be discovered.
7.3.2 Charged black rings, supersymmetric black rings
Black rings can also carry conserved electric charges with respect to a 2form field strength (the first regular example was found in [70], which can be regarded as having two charges and one dipole in a U(1)^{3} supergravity theory; see also [71]). Hence there is the possibility that they can saturate a BPS inequality. The first example of a supersymmetric blackring solution was obtained for minimal d = 5 supergravity in [74] using a canonical form for supersymmetric solutions of this theory [101]. This was then generalized to a supersymmetric blackring solution of the U(1)^{3} supergravity in [12, 75, 100]. The latter solution has 7 independent parameters, which can be taken to be the 3 charges, 3 dipoles and J_{1}. The mass is fixed by the BPS relation and J_{2} is determined by the charges and dipoles. See [84] for more detailed discussion of these solutions.
The most general, stationary, blackring solution of the U(1)^{3} supergravity theory is expected to have nine parameters, since one would expect the three charges, three dipoles, two angular momenta and the mass to be independent. This solution has not yet been constructed. Note that the general nonBPS solution should have two more parameters than the general BPS solution, just as for topologicallyspherical rotating black holes. The most general, known, nonextremal solution [72] has seven parameters, and was obtained by applying solutiongenerating transformations to the dipole ring solutions of [79]. This solution does not have a regular BPS limit, and there is no limit in which it reduces to a vacuum black ring with two angular momenta.
It has been argued that a nine parameter blackring solution could not be a seed solution for the most general blackring solution of maximal d = 5 supergravity [181]. By acting with USp(8), one can construct a solution with 27 independent charges from a seed with 3 independent charges or one can construct a solution with 27 independent dipoles from a seed with three independent dipoles. However, one cannot do both at once. If one wants to construct a solution with 27 independent charges and 27 independent dipoles from a seed solution with three independent charges, then this seed must have 15 independent dipoles, and hence (including the mass and angular momenta) 21 parameters in total. The seed solution for the most general supersymmetric black ring in maximal d = 5 supergravity is expected to have 19 parameters [181].
7.4 Solutiongenerating techniques
[16] develops solutiongenerating techniques in minimal d = 5 supergravity, based on Uduality properties of the latter. By applying one such transformation to the neutral doublyspinning black ring of Section 5.1.2, they obtain a new charged ring solution of fivedimensional supergravity. However, this solution suffers from the same problem of DiracMisner singularities that [71] described when a neutral singlespin ring seed is used. It appears that the problem could be solved, like in [72], by including an additional parameter that is then tuned to cancel the pathologies. It also seems possible that, like in [71], the neutral doublyspinning black ring is a good seed for black rings with two charges, one dipole, and two independent angular momenta in the U(1)^{3} supergravity theory.
Besides the solutiongenerating techniques based on string theory and supergravity (sequences of boosts and dualities) there have been a number of analyses of the EinsteinMaxwell(dilaton) equations leading to other techniques for generating stationary solutions. [146] studies general properties of the EinsteinMaxwell equations in ddimensions with d − 3 commuting Killingvector fields. [233] shows how fourdimensional, vacuum, stationary, axisymmetric solutions can be used to obtain static, axisymmetric solutions of fivedimensional dilaton gravity coupled to a twoform gauge field. Also, within the class of fivedimensional stationary solutions of the EinsteinMaxwell(dilaton) equations with two rotational symmetries, solutiongenerating techniques have been developed in [175] and [251, 253]. Note that many of these papers do not take into account the ChernSimons term present in d = 5 supergravity. This term is relevant when both electric and magnetic components of the gauge field are present. Thus, it can be ignored for electricallycharged static solutions, which do not give rise to magnetic dipoles. It does not play a role, either, for the dipole rings of [79], which has allowed a systematic derivation of these solutions [252, 254].
7.5 Multipleblackhole solutions
The inclusion of electric charge makes it considerably easier to construct multipleblackhole solutions than in the vacuum case discussed above. In d = 4, there exist wellknown static solutions describing multiple extremal ReissnerNordström black holes held in equilibrium by a cancellation of electric repulsion and gravitational attraction [131]. Similar static solutions can be constructed in d > 4 [198]. However, although the d = 4 solutions have smooth horizons [131], the d > 4 solutions have horizons of low differentiability [246, 22].
In d = 4, 5, multipleblackhole solutions can be supersymmetric. Supersymmetry makes it easy to write down stationary solutions corresponding to multiple BMPV black holes [102]. However, the regularity of these solutions has not been investigated. Presumably they are no smoother than the static solutions just mentioned. Although electromagnetic and gravitational forces cancel, one might expect spinspin interactions to play a role in these solutions, perhaps leading to even lower smoothness, unless the spins are aligned. Note that, in general, superposition of these black holes breaks all symmetries of a single central BMPV solution except for timetranslation invariance.
Supersymmetric solutions describing stationary superpositions of multiple concentric black rings have also been constructed [99, 100]. The rings have a common center, and can either lie in the same plane, or in orthogonal planes. The superposition does not break any symmetries. This may be the reason that these solutions have smooth horizons.
Turning to nonextremal solutions, one would certainly expect generalizations of the solutions of section 5.3 with nontrivial gauge fields. A solution describing a MyersPerry black hole with a concentric dipole ring is presented in [255].
8 General Results and Open Problems
8.1 Introduction
In four dimensions, the blackhole uniqueness theorem states that there is at most one stationary, asymptoticallyflat, vacuum blackhole solution with given mass and angular momentum: the Kerr black hole. The coexistence of MyersPerry black holes and black rings shows explicitly that blackhole uniqueness is violated in five dimensions. By now there is strong evidence that this is even more dramatically true in more than five dimensions. However, even if higherdimensional black holes are not uniquely characterized by their conserved charges, we can still hope to classify them. A major goal of research in higherdimensional general relativity is to solve the classification problem: determine all stationary, asymptoticallyflat blackhole solutions of the higherdimensional vacuum Einstein equation (or Einstein equation coupled to appropriate matter). We are still a long way from this goal, but partial progress has been made, as we shall review below.
8.2 Blackhole topology
Logically, the first step in the proof of the d = 4 blackhole uniqueness theorem is Hawking’s blackhole topology theorem [132], which states that (a spatial crosssection of) the event horizon must be topologically S^{2}. The strongest version of this theorem makes use of topological censorship. Loosely, this is the statement that, in an asymptoticallyflat and globallyhyperbolic spacetime obeying the nullenergy condition, every causal curve beginning and ending at infinity can be homotopically deformed to infinity. This can be used to prove the topology theorem for stationary [48], and even nonstationary [153, 95], black holes.
The existence of black rings demonstrates that topologically nonspherical horizons are possible for d > 4. But there are still restrictions on the topology of the event horizon. It has been shown [98, 96] that a stationary, asymptoticallyflat, blackhole spacetime, obeying the dominant energy condition, must have a horizon that is “positive Yamabe”, i.e., it must admit a metric of positive Ricci scalar. This restricts the allowed topologies. After d = 4, the strongest restriction is for d = 5, in which case the topology must be a connected sum of spherical spaces (3spheres with identifications) and S^{1} × S^{2}’s.
Another topological restriction arises from cobordism theory. Consider surrounding the black hole with a large sphere. Let Σ denote a spacelike hypersurface that runs from this sphere down to the event horizon, which it intersects in some compact surface H. Then Σ is a cobordism between H and a sphere. The existence of such a cobordism imposes topological restrictions on H. These have been discussed in [136]. The results are much less restrictive than those just mentioned.
It may be that horizon topology is not very useful for classifying black holes in d ≥ 6. As we discussed in the introduction 1, it is the combination of extended horizons with the novel possibilities for rotation that give higherdimensional black holes much of their richness. For a black ring with horizon topology S^{1} × S^{2}, the two factors differ in that the S^{1}, being contractible, needs rotation to be stabilized, whereas the S^{2} is already a minimal surface. In d ≥ 6 we can envisage even more complicated situations arising from the bending into different surfaces of the worldvolume directions of a variety of black pbranes. Not only horizon topology, but also, and more importantly, extrinsic geometry and dynamical considerations seem to be relevant to the existence of these black holes.
8.3 Uniqueness of static black holes
In d = 4 dimensions, the Schwarzschild solution is the unique, static, asymptoticallyflat, vacuum blackhole solution. The strongest version of this theorem allows for a possiblydisconnected event horizon, and the proof uses the positive energy theorem [19]. This proof can be extended to d > 4dimensions to establish uniqueness of the d > 4dimensional Schwarzschild solution amongst static vacuum solutions [110]. The method can also be generalized to prove a uniqueness theorem for static, asymptoticallyflat, blackhole solutions of d > 4dimensional EinsteinMaxwelldilaton theory; such black holes are uniquely characterized by their mass and charge and are described by generalized ReissnerNordström solutions [109].
These theorems assume that there are no degenerate components of the horizon. This assumption can be eliminated for d = 4 vacuum gravity [44, 46]. In EinsteinMaxwell theory, one can show that the only solutions with degenerate horizons are the MajumdarPapapetrou multiReissnerNordström solutions [47]. These results have been generalized to d > 4 EinsteinMaxwell theory [218, 220].
In conclusion, the classification problem for static black holes has been solved, at least for the class of theories mentioned.
It must be noted, though, that the assumption of staticity is stronger than requiring vanishing total angular momentum. The existence of black Saturns (Section 5.3) shows that there exists an infinite number of solutions (with disconnected event horizons) characterized by a given mass and vanishing angular momentum.
8.4 Stationary black holes
In d = 4, the uniqueness theorem for stationary black holes relies on Hawking’s result that a stationary black hole must be axisymmetric [132]. This result has been generalized to higher dimensions [138]. More precisely, it can be shown that a stationary, nonextremal, asymptoticallyflat, rotating, blackhole solution of d > 4 dimensional EinsteinMaxwell theory must admit a spacelike Killing vector field that generates rotations. Here “rotating” means that the Killing field that generates time translations is not null on the event horizon, i.e., the angular velocity is nonvanishing. However, it can be shown that a nonrotating black hole must be static for EinsteinMaxwell theory in d = 4 [230] and d > 4 [219], so this assumption can be eliminated. The result of [138] also applies in the presence of a cosmological constant (e.g., asymptotically antide Sitter black holes).
This theorem guarantees the existence of a single rotational symmetry. However, the known higherdimensional blackhole solutions (i.e., MyersPerry and black rings) admit multiple rotational symmetries; in d dimensions there are ⌊(d − 1)/2⌋ rotational Killing fields. Is there some underlying reason that this must be true, or is this simply a reflection of the fact that we are only able to find solutions with a lot of symmetry?
If there do exist solutions with less symmetry then they must be nonstatic (because of the uniqueness theorem for d > 4 static black holes). One could look for evidence that such solutions exist by considering perturbations of known solutions [216]. For example, the existence of nonuniform black strings was first conjectured on the evidence that uniform strings exhibit a static zeromode that breaks translational symmetry. If a MyersPerry black hole had a stationary zeromode that breaks some of its rotational symmetry then that would be evidence in favor of the existence of a new branch of solutions with less symmetry [216]. Alternatively, bifurcations could occur without breaking any rotational symmetry. As we have discussed, [80, 81] have conjectured that such bifurcations will happen in d ≥ 6; see Figure 12. Either case could lead to nonuniqueness within solutions of spherical topology.
The formalism of [151] allows one to show that the only regular stationary perturbations of a Schwarzschild black hole lead into the MP family of black holes [162]. Thus, the MP solutions are the only stationary black holes that have a regular static limit.
The issue of how much symmetry a general stationary black hole must possess is probably the main impediment to progress with the classification problem. At present, the only uniqueness results for stationary higherdimensional black holes assume the existence of multiple rotational symmetries. These results are for d = 5; if one assumes the existence of two rotational symmetries then it can be shown that the MyersPerry solution is the unique, stationary, nonextremal, asymptoticallyflat, vacuum blackhole solution of spherical topology [194]. More generally, it has been shown that stationary, nonextremal, asymptoticallyflat, vacuum blackhole solutions with two rotational symmetries are uniquely characterized by their mass, angular momenta, and rod structure (see Section 5.2.2.1) [139]. The remaining step in a full classification of all d = 5 stationary vacuum black holes with two rotational symmetries is to prove that the only rod structures giving rise to regular blackhole solutions are those associated with the known (MyersPerry and black ring) solutions.
The situation in d ≥ 6 is much further away from a complete classification, even for the class of solutions with the maximal number of rotational symmetries. For instance, the tools to classify the (yet to be found) infinite number of families of solutions with ‘pinched horizons’ proposed in [80, 81] are still to be developed.
It is clear that the notion of blackhole uniqueness that holds in four dimensions, namely that conserved charges serve to fully specify a black hole, does not admit any simple extension to higher dimensions. This leaves us with two open questions: (a) what is the simplest and most convenient set of parameters that fully specify a black hole; (b) how many blackhole solutions with given conserved charges are relevant in a given physical situation.
Concerning the first question, we note that while the rod structure may provide the additional data to determine fivedimensional vacuum black holes, one may still desire a characterization in terms of physical parameters. In other words, since the dimensionless angular momenta j_{ a } are insufficient to specify the solutions, an adequate physical parameterization of the phase space of higherdimensional black holes is still missing. [225, 241] have studied whether higher multipole moments may serve this purpose, but the results appear to be inconclusive.
The second question is more vague, as it hinges not only on the answer to the previous question, but also on the specification of the problem one is studying. It has been speculated that the conserved charges may still suffice to select a unique classical stationary configuration, if supplied with additional conditions, such as the specification of horizon topology or requiring classical stability [166]. In five dimensions we already know that horizon topology alone is not enough, since there are both thin and fat black rings with the same j. We have seen that, very likely, an even larger nonuniqueness occurs in all d ≥ 6. Classical dynamical stability to linearized perturbations, which is not at all an issue in the fourdimensional classification, is presumably a much more restrictive condition, but even in five dimensions it is unclear whether it always picks out only one solution for a given (j_{1}, j_{2}). One must also bear in mind that the classical instability of a black hole does not per se rule it out as a physically relevant solution; the timescale of the instability must be compared to the time scale of the situation at hand. Some classical instabilities (e.g., ergoregion instabilities [25]) may be very slow.
It seems possible, although so far we are nowhere near having any compelling argument, that the requirement of classical, linearized stability, plus, possibly, horizon topology, suffices to fully specify a unique vacuum blackhole solution with given conserved charges. However, in the presence of gauge fields, this seems less likely, since dipoles not only introduce larger degeneracies but also tend to enhance the classical stability of the solutions.
8.5 Supersymmetric black holes
As mentioned earlier, the study of BPS black holes is of special importance in string theory and it is natural to ask whether one can classify BPS black holes. Asymptoticallyflat BPS blackhole solutions are known only for d = 4, 5.
In addition to rendering microscopic computations tractable, the additional ingredient of supersymmetry makes the classification of black holes easier. A supersymmetric solution admits a globallydefined supercovariantly constant spinor (see, e.g., [107]). This is such a strong constraint that it is often possible to determine the general solution with this property. This was first done for minimal d = 4, N = 2 supergravity, whose bosonic sector is EinsteinMaxwell theory. It can be shown [237] that any supersymmetric solution of this theory must be either a ppwave or a member of the wellknown IsraelWilsonPerjes family of solutions (see, e.g., [228]). The only blackhole solutions in the IWP family are believed to be the multiReissnerNordström solutions [131], and this can be proved subject to one assumption^{9} [45]. Hence, this is a uniqueness theorem for supersymmetric blackhole solutions of minimal d = 4, N = 2 supergravity.
This success has been partially extended to minimal d = 5 supergravity. It can be shown that any supersymmetric black hole must have nearhorizon geometry locally isometric to either (i) the nearhorizon geometry of the BMPV black hole, (ii) AdS_{3} × S^{2}, or (iii) flat space [216]. Case (iii) would give a black hole with T^{3} horizon, which seems unlikely in view of the blackhole topology theorem discussed above (although this does not cover supersymmetric black holes, since they are necessarily extremal). An explicit form for supersymmetric solutions of this theory is known [101] and can be exploited, together with an additional assumption^{10}, to show that the only black hole of type (i) is the BMPV black hole itself. The supersymmetric black ring of [74] belongs to class (ii). The remaining step required to complete the classification would be to prove that this is the only solution in this class. These results can be extended to minimal d = 5 supergravity coupled to vector multiplets [123].
These results show that much more is known about d = 5 supersymmetric black holes than about more general stationary d = 5 black holes. Note that no assumptions about the topology of the horizon, or the number of rotational symmetries are required to obtain these results; they are outputs, not inputs. One might interpret this as weak evidence that, for general d = 5 black holes, topologies other than S^{3} and S^{1} × S^{2} cannot occur and that the assumption of two rotational symmetries is reasonable. However, one should be cautious in generalizing from BPS to nonBPS black holes, since it is known that many properties of the former (e.g., stability) do not always generalize to the latter.
8.6 Algebraic classification
In d = 4 dimensions, spacetimes can be classified according to the algebraic type of the Weyl tensor. Associated with the Weyl tensor are four “principal null vectors” [244]. In general these are distinct, but two or more of them coincide in an algebraically special spacetime. For example, the KerrNewman spacetime is type D, which means that it has two pairs of identical principal null vectors.
Given that known d = 4 blackhole solutions are algebraically special, it is natural to investigate whether the same is true in d > 4 dimensions. Before doing this, it is necessary to classify possible algebraic types of the Weyl tensor in higher dimensions. This can be done using a spinorial approach for the special case of d = 5 [65]. The formalism for all dimensions d ≥ 4 has been developed in [50] (and reviewed in [51]). It is based on “aligned null directions”, which generalize the concept of principal null vectors in d = 4. A general d > 4 spacetime admits no aligned null directions. The Weyl tensor is said to be algebraically special if one or more aligned null directions exist.
The algebraic types of some higherdimensional blackhole solutions have been determined. The MyersPerry black hole belongs to the higherdimensional generalization of the d = 4 type D class [66, 213, 215]. The black ring is also known to be algebraically special, although not as special as the MyersPerry black hole [213, 215]. Further analysis of the Weyl tensor and principal null congruences in higher dimensions can be found in [214].
In d = 4 dimensions, interesting new solutions (e.g., the spinning Cmetric) were discovered by solving the Einstein equations to determine all solutions of type D [159]. Perhaps the same strategy would be fruitful in higher dimensions. A particular class of spacetimes, namely RobinsonTrautman, admitting a hypersurfaceorthogonal, nonshearing and expanding geodesic null congruence, has been studied in [209]. However, unlike in four dimensions, this class does not contain the analogue of the Cmetric.
8.7 Laws of blackhole mechanics
8.8 Hawking radiation and blackhole thermodynamics
The extension of Hawking’s original calculation to most of the black holes that we have discussed in this review presents several difficulties, but we regard this as mostly a technical issue. In our opinion, there is no physically reasonable objection to the expectation that Hawking radiation is essentially unmodified in higher dimensions; a black hole emits radiation that, up to greybody corrections, has a Planckian spectrum of temperature T = κ/2π and chemical potentials Ω, Φ, etc.
Some of the technical difficulties relate to the problem of wave propagation in the blackhole background; this can only be dealt with analytically for MyersPerry black holes, since only in this case have the variables been separated (and then only for scalars and vectors in the general rotating case). There is in fact a considerable body of literature on the problem of radiation from MP black holes, largely motivated by their possible detection in scenarios with large extra dimensions. As mentioned in Section 2, this is outside the scope of this paper and we refer to [30, 155] for reviews.
There are also peculiarities with wave propagation that depend on the parity of the number of dimensions [206, 24], but these are unlikely to modify in any essential way the Planckian spectrum of radiation. This is, in fact, confirmed by detailed microscopic derivations of Hawking radiation in five dimensions based on string theory [188]. Other approaches to Hawking radiation that do not require one to analyze wave propagation have been applied to black rings [193], confirming the expected results. An early result was the analysis of vacuum polarization in higherdimensional black holes in [92]. More recently, [204] claims that the evolution of a fivedimensional rotating black hole emitting scalar Hawking radiation leads, for arbitrary initial values of the two rotation parameters a and b, to a fixed asymptotic value of a/M^{2} = b/M^{2} = const ≠ 0.
The spectrum of radiation from a multipleblackhole configuration will contain several components with parameters (T^{(i)},Ω^{(i)}, Φ^{(i)}, …), so it will not really be a thermal distribution unless all the black holes have equal intensive parameters. This is, of course, the conventional condition for thermal equilibrium.
The Euclidean formulation of blackhole thermodynamics remains largely the same as in four dimensions. For rotating solutions, it is more convenient not to continue analytically the angular velocities and instead to work with complex sections that have real actions. In fact, black rings do not admit nonsingular real Euclidean sections [76]. Multipleblackhole configurations with disconnected components of the horizon with different surface gravities, angular velocities, and electric potentials clearly do not admit regular Euclidean sections. Still, the Euclidean periodicity of the horizon generator formally yields the horizon temperature in the usual fashion.
8.9 Apparent and isolated horizons and critical phenomena
A number of other interesting studies of higherdimensional black holes have been made. The properties of higherdimensional apparent horizons have been analyzed in [223], which provides simple criteria to determine them. The isoperimetric inequalities and the hoop conjecture, concerning bounds on the sizes of apparent horizons through the mass they enclose, involve new features in higher dimensions. For instance, the fourdimensional hoop conjecture posits that a necessary and sufficient condition for the formation of a black hole is that a mass M gets compacted into a region whose circumference in every direction is \({\mathcal C} \leq 4\pi GM\) [236]. A generalization of this conjecture to d > 4 using a hoop of spatial dimension 1, in the form \({\mathcal C} \leq \# {(GM)^{1/(d  3)}}\), is unfeasible; the existence of black objects whose horizons have arbitrary extent in some directions (e.g., black strings, black rings, and ultraspinning black holes) shows that this condition is not necessary. It seems possible, however, that plausible, necessary, and sufficient conditions exist using the area of hoops of spatial codimension d − 3 in the form \({{\mathcal C}_{d  3}} \leq \# GM\), although black rings may require hoops with nonspherical topology [145, 8]. There is also some evidence that the isoperimetric inequalities, which bound the spatial area of the apparent horizon by the mass that it encloses [208], may be extended in the form \({\mathcal A} \leq \# {(GM)^{{{d  2} \over {d  3}}}}\) [145, 8]. See [257, 256, 224] for further work on these subjects.
The study of possible topologies for blackhole event horizons may be helped by the study of possible apparent horizons in initial data sets. [222] shows that it is possible to construct timesymmetric initialdata sets for black holes with apparenthorizon topology with the form of a product of spheres. Time symmetry, however, implies that these apparent horizons cannot correspond to rotating black holes and it is likely that their evolution in time leads to collapse into a spherical horizon.
The formalism of isolated horizons and the laws of blackhole mechanics for them, have been extended to higher dimensions in [182, 170], and then to fivedimensional EinsteinMaxwell theory with the ChernSimons term [183] and antide Sitter asymptotics [5].
Finally, the critical phenomena in the collapse of a massless scalar at the onset of blackhole formation, discovered by Choptuik [41], have been studied in [227].
9 Solutions with a Cosmological Constant
9.1 Motivation
The motivation for considering higherdimensional black holes with a cosmological constant arises from the AdS/CFT correspondence [1]. This is an equivalence between string theory on spacetimes asymptotic to AdS_{ d } × X, where X is a compact manifold, and a conformal field theory (CFT) defined on the Einstein universe R × S^{d−2}, which is the conformal boundary of AdS_{ d }. The best understood example is the case of type IIB string theory on spacetimes asymptotic to AdS_{ 5 } × S^{5}, which is dual to \({\mathcal N} = 4SU(N)\) superYangMills theory on R × S^{3}. Type IIB string theory can be replaced by IIB supergravity in the limit of large N and strong’ t Hooft coupling in the YangMills theory.
Most studies of black holes in the AdS/CFT correspondence involve dimensional reduction on X to obtain a ddimensional gauged supergravity theory with a negative cosmological constant. For example, one can reduce type IIB supergravity on S^{5} to obtain \(d = 5,{\mathcal N} = 4SO(6)\) gauged supergravity. One then seeks asymptotically antide Sitter blackhole solutions of the gauged supergravity theory. This is certainly easier than trying to find solutions in ten or eleven dimensions. However, one should bear in mind that there may exist asymptotically AdS_{ d } × X blackhole solutions that cannot be dimensionally reduced to d dimensions. Such solutions would not be discovered using gauged supergravity.
In this section we shall discuss asymptotically AdS_{ d } blackhole solutions of the d = 4, 5, 6, 7 gauged supergravity theories arising from the reduction of d = 10 or d = 11 supergravity on spheres. The emphasis will be on classical properties of the solutions rather than their implications for CFT. In AdS, linearized supergravity perturbations can be classified as normalizable or nonnormalizable according to how they behave near infinity [1]. By “asymptotically AdS” we mean that we are restricting ourselves to considering solutions that approach a normalizable deformation of global AdS near infinity. A nonnormalizable perturbation would correspond to a deformation of the CFT, for instance, making it nonconformal. Blackhole solutions with such asymptotics have been constructed, but space prevents us from considering them here.
9.2 SchwarzschildAdS
It is expected that the SchwarzschildAdS black hole is the unique, static, asymptotically AdS, blackhole solution of vacuum gravity with a negative cosmological constant, but this has not been proven.
The thermodynamics of SchwarzschildAdS were discussed by Hawking and Page for d = 4 [134] and Witten for d > 4 [250]. Let r_{+} denote the horizon radius of the solution. For a small black hole, r_{+} ≪ ℓ the thermodynamic properties are qualitatively similar to those of an asymptoticallyflat Schwarzschild black hole, i.e., the temperature decreases with increasing r_{+} so the heat capacity of the hole is negative (as r_{+} is a monotonic function of μ). However, there is an intermediate value of r_{+} ∼ ℓ at which the temperature reaches a global minimum T_{min} and then becomes an increasing function of r_{+}. Hence the heat capacity of large black holes is positive. This implies that the black hole can reach a stable equilibrium with its own radiation (which is confined near the hole by the gravitational potential ∼ r^{2}/ℓ^{2} at large r). Note that for T > T_{min} there are two blackhole solutions with the same temperature: a large one with positive specific heat and a small one with negative specific heat.
These properties lead to an interesting phase structure for gravity in AdS [134]. At low temperature, T < T_{min}, there is no blackhole solution and the preferred phase is thermal radiation in AdS. At T ∼ T_{min}, black holes exists but have greater free energy than thermal radiation. However, there is a critical temperature T_{HP} > T_{min} beyond which the large black hole has lower free energy than thermal radiation and the small black hole. The interpretation is that the canonical ensemble for gravity in AdS exhibits a (firstorder) phase transition at T = T_{min}.
In the AdS/CFT context, this HawkingPage phase transition is interpreted as the gravitational description of a thermal phase transition of the (strongly coupled) CFT on the Einstein universe [249, 250].
When oxidized, to ten or eleven dimensions, the radius r_{+} of a small SchwarzschildAdS black hole will be much less than the radius of curvature (∼ ℓ) of the internal space X. This suggests that the black hole will suffer from a classical GregoryLaflammetype instability. The probable endpoint of the instability would be a small black hole localized on X, and therefore would not admit a description in gauged supergravity. Since the radius of curvature of X is typically ℓ and the black hole is much smaller than ℓ, the geometry near the hole should be well approximated by the ten or elevendimensional Schwarzschild solution (see e.g., [141]). However, an exact solution of this form is not known.
9.3 Stationary vacuum solutions
If we consider pure gravity with a negative cosmological constant then the most general known family of asymptoticallyAdS blackhole solutions is the generalization of the KerrMyersPerry solutions to include a cosmological constant. It seems likely that black rings would exist in asymptoticallyAdS spacetimes, but no exact solutions are known.^{12}
KerrMyersPerryAdS solutions have the same symmetries as their asymptoticallyflat cousins, and exhibit similar enhancement of symmetry in special cases. The integrability of the geodesic equation and separability of the KleinGordon equation also extends to this case [207, 173, 90].
These solutions reduce to the SchwarzschildAdS solution in the limit of zero angular momentum. It has been shown that the only regular stationary perturbations of the SchwarzschildAdS solution are those that correspond to taking infinitesimal angular momenta in these rotating solutions [162]. Hence, if other stationary vacuum blackhole solutions exist (e.g., black rings) then they are not continuously connected to the SchwarzschildAdS solution.
The dual CFT interpretation is of CFT matter in thermal equilibrium rotating around the Einstein universe [133]. There is an interesting phase structure, generalizing that found for SchwarzschildAdS [133, 13, 20, 135]. For sufficiently large black holes, one can study the dual CFT using a fluid mechanics approximation, which gives quantitative agreement with blackhole thermodynamics [14].
What happens if Ω_{ i }ℓ > 1? Such black holes are believed to be classically unstable. It was observed in [135] that rotating black holes in AdS may suffer from a superradiant instability, in which energy and angular momentum are extracted from the black hole by superradiant modes. However, it was proven that this cannot occur if Ω_{ i }ℓ ≤ 1. But if Ωiℓ > 1 then an instability may be present. This makes sense from a dual CFT perspective; configurations with Ωiℓ > 1 would correspond to CFT matter rotating faster than light in the Einstein universe [133]. The existence of an instability was first demonstrated for small d = 4 KerrAdS black holes in [23]. A general analysis of odddimensional black holes with equal angular momenta reveals that the threshold of instability is at Ω_{ i }ℓ = 1 [177], i.e., precisely where the stability argument of [135] fails. The endpoint of this classical bulk instability is not known.
In d = 4, Figure 13 reveals (using Ω = dM/dJ) that all extremal KerrAdS black holes have Ωℓ > 1 and are, therefore, expected to be unstable. We have checked that d = 5 extremal MyersPerryAdS black holes also have Ω_{ i }ℓ > 1 and so they too should be classically unstable. However, the instability should be very slow when the blackhole size is much smaller than the AdS radius ℓ, and one expects it to disappear as ℓ → ∞: it takes an increasingly long time for the superradiant modes to bounce back off the AdS boundary.
9.4 Gauged supergravity theories
In order to discuss charged antide Sitter black holes we will need to specify which gauged supergravity theories we are interested in. The bestunderstood examples arise from the dimensional reduction of d = 10 or d = 11 dimensional supergravity theories on spheres to give theories with maximal supersymmetry and nonAbelian gauge groups. However, most work on constructing explicit black hole solutions has dealt with consistent truncations of these theories, with reduced supersymmetry, in which the nonAbelian gauge group is replaced by its maximal Abelian subgroup. Indeed, there is no known blackhole solution with a nontrivial nonAbelian gauge field obeying normalizable boundary conditions.
There is a consistent dimensional reduction of d = 11 supergravity on S^{7} to give \(d = 4,\, {\mathcal N} = 8,\, SO(8)\) gauged supergravity [67]. This nonAbelian theory can be consistently truncated to give \(d = 4,{\mathcal N} = 2,\,U{(1)^4}\) gauged supergravity, whose bosonic sector is Einstein gravity coupled to four Maxwell fields and three complex scalars [55]. The scalar potential is negative at its global maximum. The AdS_{4} ground state of the theory has the scalars taking constant values at this maximum. One can truncate this theory further by taking the scalars to sit at the top of their potential, and setting the Maxwell fields equal to each other. This gives minimal \(d = 4,{\mathcal N} = 2\) gauged supergravity, whose bosonic sector is EinsteinMaxwell theory with a cosmological constant. The embedding of minimal \(d = 4,{\mathcal N} = 2\) gauged supergravity theories into d = 11 supergravity can be given explicitly [31], and is much simpler than the embedding of the nonAbelian \({\mathcal N} = 8\) theory.
The d = 11 supergravity theory can also be dimensionally reduced on S^{4} to give \(d = 7,{\mathcal N} = 2,\,SO(5)\) gauged supergravity [201, 202].
The d = 10 massive IIA supergravity can be dimensionally reduced on S^{4} to give \(d = 6,{\mathcal N} = 2,\,SU(2)\) gauged supergravity [59]. This theory has halfmaximal supersymmetry.
It is believed that the d = 10 type IIB supergravity theory can be consistently reduced on S^{5} to give \(d = 5,{\mathcal N} = 4,\,SO(6)\) gauged supergravity, although this has been established only for a subsector of the full theory [62]. This theory can be truncated further to give \(d = 5,{\mathcal N} = 1,U{(1)^3}\) gauged supergravity with three vectors and two scalars. Again, setting the scalars to constants and making the vectors equal gives the minimal d = 5 gauged supergravity, whose bosonic sector is EinsteinMaxwell theory with a negative cosmological constant and a ChernSimons coupling. The explicit embeddings of these Abelian theories into d = 10 type IIB supergravity are known [31, 55].
It is sometimes possible to obtain a given lowerdimensional supergravity theory from several different compactifications of a higherdimensional theory. For example, minimal d = 5 gauged supergravity can be obtained by compactifying type IIB supergravity on any SasakiEinstein space [18]. More generally, if there is a supersymmetric solution of type IIB supergravity that is a warped product of AdS_{5} with some compact manifold X_{5}, then type IIB supergravity can be dimensionally reduced on X_{5} to give minimal d = 5 gauged supergravity [104]. An analogous statement holds for compactifications of d = 11 supergravity to give minimal \({\mathcal N} = 1,\,d = 4\) gauged supergravity or minimal d = 5 gauged supergravity [103, 104].
9.5 Static charged solutions
The d = 4 ReissnerNordströmAdS black hole is a solution of minimal \({\mathcal N} = 2\) gauged supergravity. It is parameterized by its mass M and electric and magnetic charges Q, P. This solution is stable against linearized perturbations within this (EinsteinMaxwell) theory [164]. Compared with its asymptoticallyflat counterpart, perhaps the most surprising feature of this solution is that it never saturates a BPS bound. If the mass of the black hole is lowered, it will eventually become extremal, but the extremal solution is not BPS. If one imposes the BPS condition on the solution, then one obtains a naked singularity rather than a black hole [221, 185].
Static, sphericallysymmetric, charged, blackhole solutions of the \({\mathcal N} = 2,\,d = 4,\,U{(1)^4}\) gauged supergravity theory were obtained in [69]. The solutions carry only electric charges and are parameterized by their mass M and electric charges Q_{ i }. Alternatively they can be dualized to give purely magnetic solutions. Once again, they never saturate a BPS bound. One would expect the existence of dyonic solutions of this theory, but such solutions have not been constructed.
Static, sphericallysymmetric, charged, blackhole solutions of d = 5, U(1)^{3} gauged supergravity were obtained in [9]. They are parameterized by their mass M and electric charges Q_{ i }. If the charges are set equal to each other then one recovers the d = 5 ReissnerNordström solution of minimal d = 5 gauged supergravity. The solutions never saturate a BPS bound.
A static, sphericallysymmetric, charged blackhole solution of d = 6, SU(2) gauged supergravity was given in [59]. Only a single Abelian component of the gauge field is excited, and the solution is parameterized by its charge and mass.
Static, sphericallysymmetric, charged, blackhole solutions of d = 7, SO(5) gauged supergravity are known [55]. They can be embedded into a truncated version of the full theory in which there are two Abelian vectors and two scalars. They are parameterized by their mass and electric charges.
Electricallycharged, asymptoticallyAdS, blackhole solutions exhibit a HawkingPage like phase transition in the bulk, which entails a corresponding phase transition for the dual CFT at finite temperature in the presence of chemical potentials for the Rcharge. This has been studied in [31, 56, 57, 32].
These black holes exhibit an interesting instability [121, 122]. This is best understood for a black hole so large (compared to the AdS radius) that the curvature of its horizon can be neglected, i.e., it can be approximated by a black brane. The dual CFT interpretation is as a finite temperature configuration in flat space with finite charge density. For certain regions of parameter space, it turns out that the entropy increases if the charge density becomes nonuniform (with the total charge and energy held fixed). Hence, the thermal CFT state exhibits an instability. Using the AdS/CFT dictionary, this maps to a classical instability in the bulk in which the horizon becomes translationally nonuniform, i.e., a GregoryLaflamme instability. The remarkable feature of this argument is that it reveals that a classical GregoryLaflamme instability should be present precisely when the black brane becomes locally thermodynamically unstable. Here, local thermodynamic stability means having an entropy, which is concave down as a function of the energy and other conserved charges (if the only conserved charge is the energy, then this is equivalent to positivity of the heat capacity). The GubserMitra (or “correlated stability”) conjecture asserts that this correspondence should apply to any black brane, not just asymptoticallyAdS solutions. See [128] for more discussion of this correspondence.
For finiteradius black holes, the argument is not so clear cut because the dual CFT lives in the Einstein universe rather than flat spacetime, so finite size effects will affect the CFT argument and the GubserMitra conjecture does not apply. Nevertheless, it should be a good approximation for sufficiently large black holes and hence there will be a certain range of parameters for which large charged black holes are classically unstable.^{13}
9.6 Stationary charged solutions
The most general, known, stationary, blackhole solution of minimal \(d = 4,{\mathcal N} = 2\) gauged supergravity is the KerrNewmanAdS solution, which is uniquely parameterized by its mass M, angular momentum J and electric and magnetic charges (Q, P). The thermodynamic properties of this solution, and implications for the dual CFT were investigated in [20]. An important property of this solution is that it can preserve some supersymmetry. This occurs for a oneparameter subfamily specified by the electric charge: M = M(Q), J(Q), P = 0 [171, 21]. Hence supersymmetric black holes can exist in AdS but they exhibit an important qualitative difference from the asymptotically flat case; they must rotate.
Charged rotating blackhole solutions of more general d = 4 gauged supergravity theories, e.g., \({\mathcal N} = 2,U{(1)^4}\) gauged supergravity, should also exist. Electrically charged, rotating solutions of the U(1)^{4} theory, with the four charges set pairwise equal, were constructed in [36].
Charged, rotating blackhole solutions of d = 7, SO(5) gauged supergravity have been constructed by truncating to a U(1)^{2} theory [40, 43]. In this theory, one expects the existence of a topologicallyspherical blackhole solution parameterized by its mass, three angular momenta, and two electric charges. This general solution is not yet known. However, solutions with three equal angular momenta but unequal charges have been constructed [40], as have solutions with equal charges but unequal angular momenta [43]. Both types of solution admit BPS limits.
Charged, rotating blackhole solutions of d = 6 gauged supergravity have not yet been constructed.
The construction of charged rotating blackhole solutions of d = 5 gauged supergravity has attracted more attention [125, 124, 60, 61, 37, 38, 178, 39, 191]. The most general known blackhole solution of the minimal theory is that of [38]. This solution is parameterized by the conserved charges of the theory, i.e., the mass M, electric charge Q and two angular momenta J_{1}, J_{2}. Intuition based on results proved for asymptoticallyflat solutions suggests that, for this theory, this is the most general topologicallyspherical stationary black hole with two rotational symmetries. In the BPS limit, these solutions reduce to a twoparameter family of supersymmetric black holes. In other words, one loses two parameters in the BPS limit (just as for nonstatic asymptoticallyflat black holes in d = 5, e.g., the BMPV black hole or BPS black rings).
Analogous solutions of d = 5, U(1)^{3} gauged supergravity are expected to be parameterized by the six conserved quantities M, J_{1}, J_{2}, Q_{1}, Q_{2}, Q_{3}. However, a sixparameter solution is not yet known. The most general known solutions are the fourparameter BPS solution of [178], and the fiveparameter nonextremal solution of [191], which has two of the charges Q_{ i } equal. The former is expected to be the general BPS limit of the yet to be discovered sixparameter blackhole solution (as one expects to lose two parameters in the BPS limit). The latter solution should be obtained from the general sixparameter solution by setting two of the charges equal.
Supersymmetric AdS black holes have Ω_{ i }ℓ = 1, which implies that they rotate at the speed of light with respect to the conformal boundary [125]. More precisely, the corotating Killing field becomes null on the conformal boundary. Hence, the CFT interpretation of these black holes involves matter rotating at the speed of light in the Einstein universe. The main motivation for studying supersymmetric AdS black holes is the expectation that it should be possible to perform a microscopic CFT calculation of their entropy. The idea is to count states in weakly coupled CFT and then extrapolate to strong coupling. In doing this, one has to count only states in short BPS multiplets that do not combine into long multiplets as the coupling is increased. One way of trying to do this is to work with an index that receives vanishing contributions from states in multiplets that can combine into long multiplets. Unfortunately, such indices do not give agreement with blackhole entropy [160]. This is not a contradiction; although certain multiplets may have the right quantum numbers to combine into a long multiplet, the dynamics of the theory may prevent this from occurring, so the index undercounts BPS states.
The fact that these supersymmetric black holes have only four independent parameters is puzzling from the CFT perspective, since BPS states in the CFT carry five independent charges. Maybe there are more general blackhole solutions. It seems unlikely that one could generalize the solutions of [178] to include an extra parameter since then one would also have an extra parameter, in the corresponding nonBPS solutions, which would therefore form a seven parameter family in a theory with only six conserved charges. This seems unlikely for topologicallyspherical black holes. But we know that black rings can carry nonconserved charges, so maybe this points to the existence of supersymmetric AdS black rings. However, it has been shown that such solutions do not exist in minimal d = 5 gauged supergravity [179]. The proof involves classifying supersymmetric nearhorizon geometries (with two rotational symmetries), and showing that S^{1} × S^{2} topology horizons always suffer from a conical singularity, except in the limit in which the cosmological constant vanishes. Analogous results for the U(1)^{3} theory have also been obtained [176]. So if AdS black rings exist then they cannot be “balanced” in the BPS limit.
Maybe the resolution of the puzzle involves 10d black holes with no 5D interpretation, or 5D black holes involving nonAbelian gauge fields, or 5D black holes with only one rotational symmetry. Alternatively, perhaps we already know all the BPS blackhole solutions and the puzzle arises from a lack of understanding of the CFT. For example, maybe, at strong coupling, only a four charge subspace of BPS CFT states carries enough entropy to correspond to a macroscopic black hole.
Footnotes
 1.
This is no longer true if the field equations involve not only the Ricci tensor but also the Weyl tensor, such as in Lovelock theories.
 2.
It follows that the introduction of a length scale, for instance in the form of a (negative) cosmological constant, is a necessary condition for the existence of a black hole in 2+1 dimensions. But gravity may still remain topological.
 3.
 4.
This choice corresponds to rotation in the positive sense (i.e., increasing Φ). The solution presented in [200] is obtained by Φ → −Φ, which gives rotation in a negative sense.
 5.
 6.
An equivalent system, but with a cosmological interpretation under a Wick rotation of the coordinates \((t,r,z) \rightarrow (x,r,\tilde z)\), is discussed in [86], along with some simple solutiongenerating techniques.
 7.
 8.
The S^{d−3} is not round for known solutions, but one can define an effective scale R_{2} as the radius of a round S^{d−3} with the same area.
 9.
Supersymmetric solutions admit a globally defined Killing vector field that is timelike or null. The assumption is that it is nonnull everywhere outside the horizon.
 10.
The same assumption as for the d = 4, N = 2 case just discussed.
 11.
The “topological black holes” with \(U(r) = k  {\mu \over {{r^{d  3}}}} + {{{r^2}} \over {{\ell ^2}}},k = 0,  1\) and toroidal or hyperbolic horizons [15] are excluded from our review by their asymptotics.
 12.
Note that topological censorship can be used to exclude the existence of topologically nonspherical black holes in AdS_{4} [97].
 13.
Note that this does not disagree with the stability result of [164] for d = 4 ReissnerNordströmAdS since the instability involves scalar fields and hence cannot be seen within minimal \({\mathcal N} = 2\) gauged supergravity.
Notes
Acknowledgments
RE is supported in part by DURSI 2005 SGR 00082, CICYT FPA 200404582C0202 and FPA 200766665C0202, and the European Community FP6 program MRTNCT2004005104. HSR is a Royal Society University Research Fellow.
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