Dynamical Boson Stars
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Abstract
The idea of stable, localized bundles of energy has strong appeal as a model for particles. In the 1950s, John Wheeler envisioned such bundles as smooth configurations of electromagnetic energy that he called geons, but none were found. Instead, particlelike solutions were found in the late 1960s with the addition of a scalar field, and these were given the name boson stars. Since then, boson stars find use in a wide variety of models as sources of dark matter, as black hole mimickers, in simple models of binary systems, and as a tool in finding black holes in higher dimensions with only a single Killing vector. We discuss important varieties of boson stars, their dynamic properties, and some of their uses, concentrating on recent efforts.
Keywords
Black Hole Dark Matter Scalar Field Neutron Star Newtonian Limit1 Introduction
Particlelike objects have a very long and broad history in science, arising long before Newton’s corpuscles of light, and spanning the range from fundamental to astronomical. In the mid1950s, John Wheeler sought to construct stable, particlelike solutions from only the smooth, classical fields of electromagnetism coupled to general relativity [220, 182]. Such solutions would represent something of a “gravitational atom”, but the solutions Wheeler found, which he called geons, were unstable. However, in the following decade, Kaup replaced electromagnetism with a complex scalar field [126], and found KleinGordon geons that, in all their guises, have become wellknown as today’s boson stars (see Section II of [194] for a discussion of the naming history of boson stars).
As compact, stationary configurations of scalar field bound by gravity, boson stars are called upon to fill a number of different roles. Most obviously, could such solutions actually represent astrophysical objects, either observed directly or indirectly through its gravity? Instead, if constructed larger than a galaxy, could a boson star serve as the dark matter halo that explains the flat rotation curve observed for most galaxies?
The equations describing boson stars are relatively simple, and so even if they do not exist in nature, they still serve as a simple and important model for compact objects, ranging from particles to stars and galaxies. In all these cases, boson stars represent a balance between the dispersive nature of the scalar field and the attraction of gravity holding it together.
This review is organized as follows. The rest of this section describes some general features about boson stars. The system of equations describing the evolution of the scalar field and gravity (i.e., the EinsteinKleinGordon equations) are presented in Section 2. These equations are restricted to the spherical symmetric case (with a harmonic ansatz for the complex scalar field and a simple massive potential) to obtain a bosonstar family of solutions. To accommodate all their possible uses, a large variety of bosonstar types have come into existence, many of which are described in more detail in Section 3. For example, one can vary the form of the scalar field potential to achieve a large range of masses and compactness than with just a mass term in the potential. Certain types of potential admit solitonlike solutions even in the absence of gravity, leading to socalled Qstars. One can adopt Newtonian gravity instead of general relativity, or construct solutions from a real scalar field instead of a complex one. It is also possible to find solutions coupled to an electromagnetic field or a perfect fluid, leading respectively to charged boson stars and fermionboson stars. Rotating boson stars are found to have an angular momentum which is not arbitrary, but instead quantized. Multistate boson stars with more than one complex scalar field are also considered.
We discuss the dynamics of boson stars in Section 4. Arguably, the most important property of bosonstar dynamics concerns their stability. A stability analysis of the solutions can be performed either by studying linear perturbations, catastrophe theory, or numerical nonlinear evolutions. The latter option allows for the study of the final state of perturbed stars. Possible endstates include dispersion to infinity of the scalar field, migration from unstable to stable configurations, and collapse to a black hole. There is also the question of formation of boson stars. Full numerical evolutions in 3D allow for the merger of binary boson stars, which display a large range of different behaviors as well producing distinct gravitationalwave signatures.
Finally, we review the impact of boson stars in astronomy in Section 5 (as an astrophysical object, black hole mimickers and origin of dark matter) and in mathematics in Section 6 (studies of critical behavior, the Hoop conjecture and higher dimensions). We conclude with some remarks and future directions.
1.1 The nature of a boson star
Such a field possesses energy because of its spatial gradients and time derivatives and this energy gravitates holding the star together. Less clear is what supports the star against the force of gravity. Its constituent scalar field obeys a KleinGordon wave equation which tends to disperse fields. This is the same dispersion which underlies the Heisenberg uncertainty principle. Indeed, Kaup’s original work [126] found energy eigenstates for a semiclassical, complex scalar field, discovering that gravitational collapse was not inevitable. Ruffini and Bonazzola [188] followed up on this work by quantizing a real scalar field representing some number of bosons and they found the same field equations.
There are, of course, many other soliton and solitonlike solutions in three dimensions finding a variety of ways to evade Derrick’s theorem. For example, the fieldtheory monopole of ’t Hooft and Polyakov is a localized solution of a properly gauged triplet scalar field. Such a solution is a topological soliton because the monopole possesses false vacuum energy which is topologically trapped. The monopole is one among a number of different topological defects that requires an infinite amount of energy to “unwind” the potential energy trapped within (see [218] for a general introduction to defects and the introduction of [189] for a discussion of relevant classical field theory concepts).
In Section 2, we present the underlying equations and mathematical solutions, but here we are concerned with the physical nature of these boson stars. When searching for an actual boson star, we look not for a quantized wave function, or even a semiclassical one. Instead, we search for a fundamental scalar, say the longsought Higgs boson. The Large Hadron Collider (LHC) hopes to determine the existence and nature of the Higgs, with evidence at the time of writing suggesting a Higgs boson with mass ≈ 125 GeV/c^{2} [184]. If the Higgs does not ultimately appear, there are other candidates such as an axion particle. Boson stars are then either a collection of stable fundamental bosonic particles bound by gravity, or else a collection of unstable particles that, with the gravitational binding, have an inverse process efficient enough to reach an equilibrium. They can thus be considered a BoseEinstein condensate (BEC), although boson stars can also exist in an excited state as well.
1.2 Other reviews
A number of other reviews of boson stars have appeared. Most recently, Schunck and Mielke [194] concentrate on the possibility of detecting BS, extending their previous reviews [162, 163]. In 1992, a number of reviews appeared: Jetzer [122] concentrates on the astrophysical relevance of BS (in particular their relevance for explaining dark matter) while Liddle and Madsen [152] focus on their formation. Other reviews include [208, 149].
2 Solving for Boson Stars
In this section, we present the equations governing bosonstar solutions, namely the Einstein equations for the geometry description and the KleinGordon equation to represent the (complex) scalar field. We refer to this coupled system as the EinsteinKleinGordon (EKG) equations.
The covariant equations describing boson stars are presented in Section 2.2, which is followed by choosing particular coordinates consistent with a 3+1 decomposition in Section 2.3. A form for the potential of the scalar field is then chosen and solutions are presented in Section 2.4.
2.1 Conventions
Throughout this review, Roman letters from the beginning of the alphabet a, b, c, … denote spacetime indices ranging from 0 to 3, while letters near the middle i, j, k, … range from 1 to 3, denoting spatial indices. Unless otherwise stated, we use units such that ħ = c = 1 so that the Planck mass becomes M_{planck} = G^{−1/2}. We also use the signature convention (−, +, +, +) for the metric.
2.2 The Lagrangian, evolution equations and conserved quantities
2.3 The 3+1 decomposition of the spacetime

specify the choice of coordinates. The spacetime is foliated by a family of spacelike hypersurfaces, which are crossed by a congruence of time lines that will determine our observers (i.e., coordinates). This congruence is described by the vector field t^{ a } = αn^{ a } + β^{ a }, where α is the lapse function which measures the proper time of the observers, is the shift vector that measures the displacement of the observers between consecutive hypersurfaces and n^{ a } is the timelike unit vector normal to the spacelike hypersurfaces.
 decompose every 4D object into its 3+1 components. The choice of coordinates allows for the definition of a projection tensor \({\gamma ^a}_b \equiv \delta _b^a + {n^a}{n_b}\). Any fourdimensional tensor can be decomposed into 3+1 pieces using the spatial projector to obtain the spatial components, or contracting with n^{ a } for the time components. For instance, the line element can be written in a general form asThe stressenergy tensor can then be decomposed into its various components as$$d{s^2} =  {\alpha ^2}d{t^2} + {\gamma _{ij}}(d{x^i} + {\beta ^i}dt)(d{x^j} + {\beta ^j}dt).$$(16)$$\tau \equiv {T^{ab}}{n_a}{n_b},\quad {S_i} \equiv {T_{ab}}{n^a}{\gamma ^a}_i,\quad \,{S_{ij}} \equiv {T_{ab}}{\gamma ^a}_i{\gamma ^b}_j.$$(17)
 write down the field equations in terms of the 3+1 components. Within the framework outlined here, the induced (or equivalently, the spatial 3D) metric and the scalar field ϕ are as yet still unknown (remember that the lapse and the shift just describe our choice of coordinates). In the original 3+1 decomposition (ADM formulation [9]) an additional geometrical tensor \({K_{ij}} \equiv  (1/2){{\mathcal L}_{\rm{n}}}{\gamma _{ij}} =  1/(2\alpha)({\partial _t}  {{\mathcal L}_\beta}){\gamma _{ij}}\) is introduced to describe the change of the induced metric along the congruence of observers. Loosely speaking, one can view the determination of γ_{ ij } and K_{ ij } as akin to the specification of a position and velocity for projectile motion. In terms of the extrinsic curvature and its trace, \({\rm{trK}} \equiv {K_i}^i\), the Einstein equations can be written as$${R_i}^i+ {({\rm{trK}})^2}  K_i^jK_j^i = 16\pi G\tau$$(18)$${\nabla _j}({K_i}^j  {\rm{trK}}\;\delta _i^j) = 8\pi G{S_i}$$(19)In a similar fashion, one can introduce a quantity \(Q \equiv  {{\mathcal L}_{\rm{n}}}\phi\) for the KleinGordon equation which reduces it to an equation first order in time, second order in space$$({\partial _t}  {{\mathcal L}_\beta}){K_{ij}} =  {\nabla _i}{\nabla _j}\alpha + \alpha \left({{R_{ij}}  2K_i^k{K_{jk}} + {\rm{trK}}\,{K_{ij}}  8\pi G\left[ {{S_{ij}}  {{{\gamma _{ij}}} \over 2}({\rm{trS }}\tau)} \right]} \right)$$(20)$${\partial _t}(\sqrt \gamma Q)  {\partial _i}({\beta ^i}\sqrt \gamma Q) + {\partial _i}\left({\alpha \sqrt \gamma {\gamma ^{ij}}{\partial _j}\phi} \right) = \alpha \sqrt \gamma {{dV} \over {d{{\left\vert \phi \right\vert}^2}}}\phi.$$(21)
 enforce any assumed symmetries. Although the boson star is found by a harmonic ansatz for the time dependence, here we choose to retain the full timedependence. However, a considerably simplification is provided by assuming that the spacetime is spherically symmetric. Following [141], the most general metric in this case can be written in terms of spherical coordinates aswhere α(t, r) is the lapse function, β(t, r) is the radial component of the shift vector and a(t, r), b(t, r) represent components of the spatial metric, with dΩ^{2} the metric of a unit twosphere. With this metric, the extrinsic curvature only has two independent components \(K_j^i = {\rm{diag(}}{K^r}_r,{K^\theta}_\theta {K^\theta}_\theta)\). The constraint equations, Eqs. (18) and (19), can now be written as$$d{s^2} = ( {\alpha ^2} + {a^2}{\beta ^2})d{t^2} + 2{a^2}\beta dt\;dr + {a^2}d{r^2} + {r^2}{b^2}d{\Omega ^2},$$(22)$$ {2 \over {arb}}\left\{{{\partial _r}\left[ {{{{\partial _r}(rb)} \over a}} \right] + {1 \over {rb}}\left[ {{\partial _r}\left({{{rb} \over a}{\partial _r}(rb)} \right)  a} \right]} \right\} + 4{K^r}_r{K^\theta}_\theta + 2{K^\theta}_\theta {K^\theta}_\theta$$(23)$$= {{8\pi G} \over {{a^2}}}\left[ {{{\left\vert \Phi \right\vert}^2} + {{\left\vert \Pi \right\vert}^2} + {a^2}V({{\left\vert \phi \right\vert}^2})} \right]$$(24)where we have defined the auxiliary scalarfield variables$${\partial _r}{K^\theta}_\theta + {{{\partial _r}(rb)} \over {rb}}({K^\theta}_\theta  {K^r}_r) = {{2\pi G} \over a}(\overline \Pi \Phi + \Pi \overline \Phi),$$(25)The evolution equations for the metric and extrinsic curvature components reduce to$$\Phi \equiv {\partial _r}\phi, \,\quad \Pi \equiv {a \over \alpha}({\partial _t}\phi  \beta {\partial _r}\phi).$$(26)$${\partial _t}a = {\partial _r}(a\beta)  \alpha a{K^r}_r$$(27)$${\partial _t}b = {\beta \over r}{\partial _r}(rb)  \alpha b{K^\theta}_\theta$$(28)Similarly, the reduction of the KleinGordon equation to first order in time and space leads to the following set of evolution equations$$\begin{array}{*{20}c} {{\partial _t}{K^r}_r  \beta {\partial _r}{K^r}_r =  {1 \over a}{\partial _r}\left({{{{\partial _r}\alpha} \over a}} \right)} \quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\quad\\ {\quad\quad\quad\quad\quad + \alpha \left\{{ {2 \over {arb}}{\partial _r}\left[ {{{{\partial _r}(rb)} \over a}} \right] + {\rm{trK}}{K^r}_r  {{4\pi G} \over {{a^2}}}[2{{\left\vert \Phi \right\vert}^2} + {a^2}V({{\left\vert \phi \right\vert}^2})]} \right\}} \\ {{\partial _t}{K^\theta}_\theta  \beta {\partial _r}{K^\theta}_\theta = {\alpha \over {{{(rb)}^2}}}  {1 \over {a{{(rb)}^2}}}{\partial _r}\left[ {{{\alpha rb} \over a}{\partial _r}(rb)} \right] + \alpha [{\rm{trK}}{K^\theta}_\theta  4\pi GV({{\left\vert \phi \right\vert}^2})].} \quad\\ \end{array}$$(29)$${\partial _t}\phi = \beta \Phi + {\alpha \over a}\Pi$$(30)$${\partial _t}\Phi = {\partial _r}\left({\beta \Phi + {\alpha \over a}\Pi} \right)$$(31)This set of equations, Eqs. (23)–(32), describes general, timedependent, spherically symmetric solutions of a gravitationallycoupled complex scalar field. In the next section, we proceed to solve for the specific case of a boson star.$${\partial _t}\Pi = {1 \over {{{(rb)}^2}}}{\partial _r}\left[ {{{(rb)}^2}\left({\beta \Pi + {\alpha \over a}\Phi} \right)} \right] + 2\left[ {\alpha {K^\theta}_\theta  \beta {{{\partial _r}(rb)} \over {rb}}} \right]\Pi  \alpha a{{dV} \over {d{{\left\vert \phi \right\vert}^2}}}\phi.$$(32)
2.4 Miniboson stars
As above, boson stars are spherically symmetric solutions of the Eqs. (38–40) with asymptotic behavior given by Eqs. (41–45). For a given value of the central amplitude of the scalar field ϕ_{0}(r = 0) = ϕ_{ c }, there exist configurations with some effective radius and a given mass satisfying the previous conditions for a different set of n discrete eigenvalues ω^{(n)}. As n increases, one obtains solutions with an increasing number of nodes in ϕ_{0}. The configuration without nodes is the ground state, while all those with any nodes are excited states. As the number of nodes increases, the distribution of the mass as a function of the radius becomes more homogeneous.
As the amplitude ϕ_{ c } increases, the stable configuration has a larger mass while its effective radius decreases. This trend indicates that the compactness of the boson star increases. However, at some point the mass instead decreases with increasing central amplitude. Similar to models of neutron stars (see Section 4 of [59]), this turnaround implies a maximum allowed mass for a boson star in the ground state, which numerically was found to be \({M_{\max}} = 0.633M_{{\rm{Planck}}}^2/m\). The existence of a maximum mass for boson stars is a relativistic effect, which is not present in the Newtonian limit, while the maximum of baryonic stars is an intrinsic property.
3 Varieties of Boson Stars
Quite a number of different flavours of boson stars are present in the literature. They can have charge, a fermionic component, or rotation. They can be constructed with various potentials for the scalar field. The form of gravity which holds them together can even be modified to, say, Newtonian gravity or even no gravity at all (Qballs). To a certain extent, such modifications are akin to varying the equation of state of a normal, fermionic star. Here we briefly review some of these variations, paying particular attention to recent work.
3.1 Selfinteraction potentials
Originally, boson stars were constructed with a freefield potential without any kind of selfinteraction, obtaining a maximum mass with a dependence \(M \approx M_{{\rm{Planck}}}^2/m\). This mass, for typical masses of bosonic particle candidates, is much smaller than the Chandrasekhar mass \({M_{{\rm{Ch}}}} \approx M_{{\rm{Planck}}}^3/{m^2}\) obtained for fermionic stars, and so they were known as miniboson stars. In order to extend this limit and reach astrophysical masses comparable to the Chandrasekhar mass, the potential was generalized to include a selfinteraction term that provided an extra pressure against gravitational collapse.
Many subsequent papers further analyze the EKG solutions with polynomial, or even more general nonpolynomial, potentials. One work in particular [195] studied the properties of the galactic dark matter halos modeled with these boson stars. They found that a necessary condition to obtain stable, compact solutions with an exponential decrease of the scalar field, the series expansion of these potentials must contain the usual mass term m^{2}ϕ^{2}.
More exotic ideas similarly try to include a pressure to increase the mass of BSs. Ref. [2] considers a form of repulsive selfinteraction mediated by vector mesons within the meanfield approximation. However, the authors leave the solution of the fully nonlinear system of the KleinGordon and Proca equations to future work. Ref. [18] models stars made from the condensation of axions, using the semirelativistic approach with two different potentials. Mathematically this approach involves averages such that the equations are equivalent to assuming the axion is constituted by a complex scalar field with harmonic time dependence.
There is another type of nontopological soliton star, called Qstars [155], which also admits soliton solutions in the absence of gravity (i.e., Qballs [56, 149]). The potential, besides being also a function of ϕ^{2}, must satisfy the following conditions: it must behave like ≈ ϕ^{2} near ϕ = 0, it has to be bounded < ϕ^{2} in an intermediate region and must be larger > ϕ^{2} for ϕ → ∞. The Qstars also have three regions; an interior solution of radius \(R \approx {M_{{\rm{Planck}}}}/\phi _0^2\), (i.e., ϕ_{0} ≈ m is the free particle inverse Compton wavelength) a very thin surface region of thickness 1/ϕ_{0}, and finally the exterior solution without matter, which reduces to Schwarzschild in spherical symmetry. The mass of these Qstars scales now as \(M_{{\rm{Planck}}}^3/\phi _0^2\). The stability of these Qstars has been studied recently using catastrophe theory, such as [209, 135]. Rotating, axisymmetric Qballs were constructed in [133, 134]. Related, rotating solutions in 2+1 with the signumGordon equation instead of the KG equation are found in [10].
Other interesting works have studied the formation of Qballs by the AffleckDine mechanism [125], their dynamics in one, two and three spatial dimensions [22], and their viability as a selfinteracting dark matter candidate [139].
Ref. [29] considers a chemical potential to construct BSs, arguing that the effect of the chemical potential is to reduce the parameter space of stable solutions. Related work modifies the kinetic term of the action instead of the potential. Ref. [1] studies the resulting BSs for a class of K field theories, finding solutions of two types: (i) compact balls possessing a naked singularity at their center and (ii) compact shells with a singular inner boundary which resemble black holes. Ref. [3] considers coherent states of a scalar field instead of a BS within kessence in the context of explaining dark matter. Ref. [72] modifies the kinetic term with just a minus sign to convert the scalar field to a phantom field. Although, a regular real scalar field has no spherically symmetric, local static solutions, they find such solutions with a real phantom scalar field.
3.2 Newtonian boson stars
The possibility of including selfinteraction terms in the potential was considered in [106], studying also the gravitational cooling (i.e., the relaxation and virialization through the emission of scalar field bursts) of spherical perturbations. Nonspherical perturbations were further studied in [27], showing that the final state is a spherically symmetric configuration. Single Newtonian boson stars were studied in [98], either when they are boosted with/without an external central potential. Rotating stars were first successfully constructed in [203] within the Newtonian approach. Numerical evolutions of binary boson stars in Newtonian gravity are discussed in Section 4.2.
Recent work by Chavanis with Newtonian gravity solves the GrossPitaevskii equation, a variant of Eq. (58) which involves a pseudopotential for a BoseEinstein condensate, to model either dark matter or compact alternatives to neutron stars [46, 45, 47].
Much recent work considers boson stars from a quantum perspective as a BoseEinstein condensate involving some number, N, of scalar fields. Ref. [160] studies the collapse of boson stars mathematically in the mean field limit in which N → ∞. Ref. [130] argues for the existence of bosonic atoms instead of stars. Ref. [16] uses numerical methods to study the mean field dynamics of BSs.
3.3 Charged boson stars
We look for a time independent metric by first assuming a harmonically varying scalar field as in Eq. (33). We work in spherical coordinates and assume spherical symmetry. With a proper gauge choice, the vector potential takes a particularly simple form with only a single, nontrivial component A_{ a } = (A_{0}(r), 0, 0, 0). This choice implies an everywhere vanishing magnetic field so that the electromagnetic field is purely electric. The boundary conditions for the vector potential are obtained by requiring the electric field to vanish at the origin because of regularity, ∂_{ r } A_{0}(r = 0) = 0. Because the electromagnetic field depends only on derivatives of the potential, we can use this freedom to set A_{0}(∞) = 0 [123].
With these conditions, it is possible to find numerical solutions in equilibrium as described in Ref. [123]. Solutions are found for \({{\tilde e}^2} \equiv {e^2}M_{{\rm{Planck}}}^2/(8\pi {m^2}) < 1/2\). For \({{\tilde e}^2} > 1/2\) the repulsive Coulomb force is bigger than the gravitational attraction and no solutions are found. This bound on the BS charge in terms of its mass ensures that one cannot construct an overcharged BS, in analogy to the overcharged monopoles of Ref. [154]. An overcharged monopole is one with more charge than mass and is, therefore, susceptible to gravitational collapse by accreting sufficient (neutral) mass. However, because its charge is higher than its mass, such collapse might lead to an extremal ReissnerNordström BH, but BSs do not appear to allow for this possibility. Interestingly, Ref. [190] finds that if one removes gravity, the obtained Qballs may have no limit on their charge.
Recent work with charged BSs includes the publication of Maple [157] routines to study boson nebulae charge [63, 169, 168]
Other work generalizes the Qballs and Qshells found with a certain potential, which leads to the signumGordon equation for the scalar field [131, 132]. In particular, shell solutions can be found with a black hole in its interior, which has implications for black hole scalar hair (for a review of black hole uniqueness see [113]).
One can also consider Qballs coupled to an electromagnetic field, a regime appropriate for particle physics. Within such a context, Ref. [76] studies the chiral magnetic effect arising from a Qball. Other work in Ref. [36] studies charged, spinning Qballs.
3.4 Oscillatons
As mentioned earlier, it is not possible to find timeindependent, spacetime solutions for a real scalar field. However, there are nonsingular, timedependent nearequilibrium configurations of selfgravitating real scalar fields, which are known as oscillatons [197]. These solutions are similar to boson stars, with the exception that the spacetime must also have a time dependence in order to avoid singularities.
A careful analysis of the high frequency components of this construction reveals difficulties in avoiding infinite total energy while maintaining the asymptotically flat boundary condition [172]. Therefore, the truncated solutions constructed above are not exactly time periodic. Indeed, very accurate numerical work has shown that the oscillatons radiate scalar field on extremely long time scales while their frequency increases [84, 97]. This work finds a mass loss rate of just one part in 10^{12} per oscillation period, much too small for most numerical simulations to observe. The solutions are, therefore, only nearequilibrium solutions and can be extremely longlived.
Closely related, are oscillons that exist in flatspace and that were first mentioned as “pulsons” in 1977 [33]. There is an extensive literature on such solutions, many of which appear in [79]. A series of papers establishes that oscillons similarly radiate on very long time scales [79, 80, 81, 82]. An interesting numerical approach to evolving oscillons adopts coordinates that blueshift and damp outgoing radiation of the massive scalar field [115, 117]. A detailed look at the long term dynamics of these solutions suggests the existence of a fractal boundary in parameter space between oscillatons that lead to expansion of a truevacuum bubble and those that disperse [116].
3.5 Rotating boson stars
Recently, their stability properties were found to be similar to nonrotating stars [135]. Rotating boson stars have been shown to develop a strong ergoregion instability when rapidly spinning on short characteristic timescales (i.e., 0.1 seconds −1 week for objects with mass M = 1−10^{6} M_{⊙} and angular momentum J > 0.4 GM^{2}), indicating that very compact objects with large rotation are probably black holes [44].
More discussion concerning the numerical methods and limitations of some of these approaches can be found in Lai’s PhD thesis [141].
3.6 Fermionicbosonic stars
The possibility of compact stellar objects made with a mixture of bosonic and fermionic matter was studied in Refs. [111, 112]. In the simplest case, the boson component interacts with the fermionic component only via the gravitational field, although different couplings were suggested in [112] and have been further explored in [66, 180]. Such a simple interaction is, at the very least, consistent with models of a bosonic dark matter coupling only gravitationally with visible matter, and the idea that such a bosonic component would become gravitationally bound within fermionic stars is arguably a natural expectation.
It was shown that the stability arguments made with boson stars can also be applied to these mixed objects [121]. The existence of slowly rotating fermionboson stars was shown in [65], although no solutions were found in previous attempts [136]. Also see [73] for unstable solutions consisting of a real scalar field coupled to a perfect fluid with a polytropic equation of state.
3.7 Multistate boson stars
It turns out that excited BSs, as dark matter halo candidates, provide for flatter, and hence more realistic, galactic rotation curves than ground state BSs. The problem is that they are generally unstable to decay to their ground state. Combining excited states with the ground states in what are called multistate BSs is one way around this.
Similar results were found in the Newtonian limit [215], however, with a slightly higher stability limit N^{(1)} ≥ 1.13 N^{(2)}. This work stresses that combining several excited states makes it possible to obtain flatter rotation curves than only with ground state, producing better models for galactic dark matter halos (see also discussion of boson stars as an explanation of dark matter in Section 5.3).
Ref. [110] considers two scalar fields describing boson stars that are phase shifted in time with respect to each other, studying the dynamics numerically. In particular, one can consider multiple scalar fields with an explicit interaction (beyond just gravity) between them, say V (ϕ^{(1)} (ϕ^{(2)}. Refs. [37, 38] construct such solutions, considering the individual particlelike configurations for each complex field as interacting with each other.
3.8 Alternative theories of gravity
Instead of modifying the scalar field potential, one can instead consider alternative theories of gravity. Constraints on such theories are already significant given the great success of general relativity [221]. However, the fast advance of electromagnetic observations and the anticipated gravitationalwave observations promise much more in this area, in particular in the context of compact objects that probe strongfield gravity.
An ambitious effort is begun in Ref. [176], which studies a very general gravitational Lagrangian (“extended scalartensor theories”) with both fluid stars and boson stars. The goal is for observations of compact stars to constrain such theories of gravity.
It has been found that scalar tensor theories allow for spontaneous scalarization in which the scalar component of the gravity theory transitions to a nontrivial configuration analogously to ferromagnetism with neutron stars [62]. Such scalarization is also found to occur in the context of bosonstar evolution [6].
Boson stars also occur within conformal gravity and with scalartensor extensions to it [40, 41].
One motivation for alternative theories is to explain the apparent existence of dark matter without resorting to some unknown dark matter component. Perhaps the most well known of these is MOND (modified Newtonian dynamics) in which gravity is modified only at large distances [164, 165] (for a review see [77]). Boson stars are studied within TeVeS (TensorVectorScalar), a relativistic generalization of MOND [58]. In particular, their evolutions of boson stars develop caustic singularities, and the authors propose modifications of the theory to avoid such problems.
3.9 Gauged boson stars
In 1988, Bartnik and McKinnon published quite unexpected results showing the existence of particlelike solutions within SU(2) YangMills coupled to gravity [20]. These solutions, although unstable, were unexpected because no particlelike solutions are found in either the YangMills or gravity sectors in isolation. Recall also that no particlelike solutions were found with gravity coupled to electromagnetism in early efforts to find Wheeler’s geon (however, see Section 6.3 for discussion of Ref. [70], which finds geons within AdS).
Bartnik and McKinnon generalize from the Abelian U(1) gauge group to the nonAbelian SU(2) group and thereby find these unexpected particlelike solutions. One can consider, as does Ref. [194] (see Section IIp), these globally regular solutions (and their generalizations to SU(n) for n > 2) as gauged boson stars even though these contain no scalar field. One can instead explicitly include a scalar field doublet coupled to the YangMills gauge field [39] as perhaps a more direct generalization of the (U(1)) charged boson stars discussed in Section 3.3.
Ref. [74] studies BSs formed from a gauge condensate of an SU(3) gauge field, and Ref. [41] extends the BartnikMcKinnon solutions to conformal gravity with a Higgs field [41].
4 Dynamics of Boson Stars
In this section, the formation, stability and dynamical evolution of boson stars are discussed. One approach to the question of stability considers small perturbations around an equilibrium configuration, so that the system remains in the linearized regime. Growing modes indicate instability. However, a solution can be linearly stable and yet have a nonlinear instability. One example is Minkowski space, which, under small perturbations, relaxes back to flat, but, for sufficiently large perturbations, leads to blackhole formation, decidedly not Minkowski. To study nonlinear stability, other methods are needed. In particular, full numerical evolutions of the EinsteinKleinGordon (EKG) equations are quite useful for understanding the dynamics of boson stars.
4.1 Gravitational stability
A linear stability analysis consists of studying the time evolution of infinitesimal perturbations about an equilibrium configuration, usually with the additional constraint that the total number of particles must be conserved. In the case of spherically symmetric, fermionic stars described by perfect fluid, it is possible to find an eigenvalue equation for the perturbations that determines the normal modes and frequencies of the radial oscillations (see, for example, Ref. [86]). Stability theorems also allow for a direct characterization of the stability branches of the equilibrium solutions [91, 60]. Analogously, one can write a similar eigenvalue equation for boson stars and show the validity of similar stability theorems. In addition to these methods, the stability of boson stars has also been studied using two other, independent methods: by applying catastrophe theory and by solving numerically the time dependent EinsteinKleinGordon equations. All these methods agree with the results obtained in the linear stability analysis.
4.1.1 Linear stability analysis
The stability of the star depends crucially on the sign of the smallest eigenvalue. Because of time reversal symmetry, only σ^{2} enters the equations [148], and we label the smallest eigenvalue \(\sigma _0^2\). If it is negative, the eigenmode grows exponentially with time and the star is unstable. On the other hand, for positive eigenvalues the configuration has no unstable modes and is therefore stable. The critical point at which the stability transitions from stable to unstable, therefore, occurs when the smallest eigenvalue vanishes, σ_{0} = 0.
Equilibrium solutions can be parametrized with a single variable, such as the central value of the scalar field ϕ_{ c }, and so we can write M = M(ϕ_{ c }) and N = N (ϕ_{ c }). Stability theorems then indicate that transitions between stable and unstable configurations occur only at critical points of the parameterization (M′(ϕ_{ c }) = N′(ϕ_{ c }) = 0) [60, 91, 107, 207]. Linear perturbation analysis provides a more detailed picture such as the growth rates and the eigenmodes of the perturbations.
Ref. [94] carries out such an analysis for perturbations that conserve mass and charge. They find the first three perturbative modes and their growth rates, and they identify at which precise values of ϕ_{ c } these modes become unstable. Starting from small values, they find that ground state BSs are stable up to the critical point of maximum mass. Further increases in the central value subsequently encounter additional unstable modes. This same type of analysis applied to excited state BSs showed that the same stability criterion applies for perturbations that conserve the total particle number [120]. For more general perturbations that do not conserve particle number, excited states are generally unstable to decaying to the ground state.
A more involved analysis by [148] uses a Hamiltonian formalism to study BS stability. Considering first order perturbations that conserve mass and charge (δN = 0), their results agree with those of [94, 120]. However, they extend their approach to consider more general perturbations, which do not conserve the total number of particles (i.e., δN ≠ 0). To do so, they must work with the second order quantities. They found complex eigenvalues for the excited states that indicate that excited state boson stars are unstable. More detail and discussion on the different stability analysis can be found in Ref. [122].
Catastrophe theory is part of the study of dynamical systems that began in the 1960s and studies large changes in systems resulting from small changes to certain important parameters (for a physicsoriented review see [204]). Its use in the context of boson stars is to evaluate stability, and to do so one constructs a series of solutions in terms of a limited and appropriate set of parameters. Under certain conditions, such a series generates a curve smooth everywhere except for certain points. Within a given smooth expanse between such singular points, the solutions share the same stability properties. In other words, bifurcations occur at the singular points so that solutions after the singularity gain an additional, unstable mode. Much of the recent work in this area confirms the previous conclusions from linear perturbation analysis [209, 210, 211, 212] and from earlier work with catastrophe theory [140].
Another recent work using catastrophe theory finds that rotating stars share a similar stability picture as nonrotating solutions [135]. However, only fast spinning stars are subject to an ergoregion instability [44].
4.1.2 Nonlinear stability of single boson stars
The dynamical evolution of spherically symmetric perturbations of boson stars has also been studied by solving numerically the EinsteinKleinGordon equations (Section 2.3), or its Newtonian limit (Section 3.2), the SchrödingerPoisson system. The first such work was Ref. [196] in which the stability of the ground state was studied by considering finite perturbations, which may change the total mass and the particle number (i.e., δN ≠ 0 and δM ≠ 0). The results corroborated the linear stability analysis in the sense that they found a stable and an unstable branch with a transition between them at a critical value, ϕ_{crit}, of the central scalar field corresponding to the maximal BS mass \({M_{\max}} = 0.633M_{{\rm{Planck}}}^2/m\).
The perturbed unstable configurations will either collapse to a black hole or migrate to a stable configuration, depending on the nature of the initial perturbation. If the density of the star is increased, it will collapse to a black hole. On the other hand, if it is decreased, the star explodes, expanding quickly as it approaches the stable branch. Along with the expansion, energy in the form of scalar field is radiated away, leaving a very perturbed stable star, less massive than the original unstable one.
More recently, the stability of the ground state was revisited with 3D simulations using a Cartesian grid [102]. The Einstein equations were written in terms of the BSSN formulation [200, 23], which is one of the most commonly used formulations in numerical relativity. Intrinsic numerical error from discretization served to perturb the ground state for both stable and unstable stars. It was found that unstable stars with negative binding energy would collapse and form a black hole, while ones with positive binding energy would suffer an excess of kinetic energy and disperse to infinity.
That these unstable stars would disperse, instead of simply expanding into some less compact stable solution, disagrees with the previous results of Ref. [196], and was subsequently further analyzed in [104] in spherical symmetry with an explicit perturbation (i.e., a Gaussian shell of particles, which increases the mass of the star around 0.1%). The spherically symmetric results corroborated the previous 3D calculations, suggesting that the slightly perturbed configurations of the unstable branch have three possible endstates: (i) collapse to BH, (ii) migration to a less dense stable solution, or (iii) dispersal to infinity, dependent on the sign of the binding energy.
Much less is known about rotating BSs, which are more difficult to construct and to evolve because they are, at most, axisymmetric, not spherically symmetric. However, as mentioned in Section 3.5, they appear to have both stable and unstable branches [135] and are subject to an ergoregion instability at high rotation rates [44]. To our knowledge, no one has evolved rotating BS initial data. However, as discussed in the next section, simulations of BS binaries [167, 173] have found rotating boson stars as a result of merger.
4.2 Dynamics of binary boson stars
The dynamics of binary boson stars is sufficiently complicated that it generally requires numerical solutions. The necessary lack of symmetry and the resolution requirement dictated by the harmonic time dependence of the scalar field combine so that significant computational resources must be expended for such a study. However, boson stars serve as simple proxies for compact objects without the difficulties (shocks and surfaces) associated with perfect fluid stars, and, as such, binary BS systems have been studied in the twobody problem of general relativity. When sufficiently distant from each other, the precise structure of the star should be irrelevant as suggested by Damour’s “effacement theorem” [61].
The first simulations of boson stars with full general relativity were reported in [12], where the gravitational waves were computed for a headon collision. The general behavior is similar to the one displayed for the Newtonian limit; the stars attract each other through gravitational interaction and then merge to produce a largely perturbed boson star. However, in this case the merger of the binary was promptly followed by collapse to a black hole, an outcome not possible when working within Newtonian gravity instead of general relativity. Unfortunately, very little detail was given on the dynamics.
Much more elucidating was work in axisymmetry [141], in which headon collisions of identical boson stars were studied in the context of critical collapse (discussed in Section 6.1) with general relativity. Stars with identical masses of M = 0.47 ≈ 0.75 M_{max} were chosen, and so it is not surprising that for small initial momenta the stars merged together to form an unstable single star (i.e., its mass was larger than the maximum allowed mass, M_{max}). The unstable hypermassive star subsequently collapsed to a black hole. However, for large initial momentum the stars passed through each other, displaying a form of solitonic behavior since the individual identities were recovered after the interaction. The stars showed a particular interference pattern during the overlap, much like that displayed in Figures 1 and 13.
Another study considered the very high speed, headon collision of BSs [55]. Beginning with two identical boson stars boosted with Lorentz factors ranging as high as 4, the stars generally demonstrate solitonic behavior upon collision, as shown in the insets of Figure 18. This work is further discussed in Section 6.2.
The orbital case was later studied in [173]. This case is much more involved both from the computational point of view (i.e., there is less symmetry in the problem) and from the theoretical point of view, since for the final object to settle into a stationary, rotating boson star it must satisfy the additional quantization condition for the angular momentum of Eq. (71).
One simulation consisted of an identical pair each with individual mass M = 0.5, with small orbital angular momentum such that J ≤ N. In this case, the binary merges forming a rotating bar that oscillates for some time before ultimately splitting apart. This can be considered as a scattered interaction, which could not settle down to a stable boson star unless all the angular momentum was radiated.
Other simulations of orbiting, identical binaries have been performed within the conformally flat approximation instead of full GR, which neglects gravitational waves (GW) [167]. Three different qualitative behaviors were found. For high angular momentum, the stars orbit for comparatively long times around each other. For intermediate values, the stars merged and formed a pulsating and rotating boson star. For low angular momentum, the merger produces a black hole. No evidence was found of the stars splitting apart after the merger.
5 Boson Stars in Astronomy
Scalar fields are often employed by astronomers and cosmologists in their efforts to model the Universe. Most models of inflation adopt a scalar field as the inflaton field, the vacuum energy of which drives the exponential inflation of the Universe. Dark energy also motivates many scalar field models, such as kessence and phantom energy models. It is therefore not surprising that boson stars, as compact configurations of scalar field, are called upon to provide consequences similar to those observed.
5.1 As astrophysical stellar objects
We have already discussed a number of similarities between boson stars and models of neutron stars. Just as one can parameterize models of neutron stars by their central densities, one can consider a 1parameter family of boson stars according to the central magnitude of the scalar field. Considering the mass as a function of this parameter, one finds the existence of a local maximum across which solutions transition from stable to unstable, just as is the case for neutron stars. Models of neutron stars can be constructed with different equations of state, whereas boson stars are constructed with differing scalar field potentials.
One difference of consequence concerns the stellar surface. Neutron stars of course have a surface at which the fluid density is discontinuous, as discussed for example in [99, 101]. In contrast, the scalar field that constitutes the boson star is smooth everywhere and lacks a particular surface. In its place, one generally defines a radius that encompasses some percentage (e.g. 99%) of the stellar mass. Such a difference could have observational consequences when matter accretes onto either type of star.
It is still an open question whether some of the stars already observed and interpreted as neutron stars could instead be astrophysical boson stars. In a similar fashion, it is not known whether many, if not all, of the stars we observe already have a bosonic component that has settled into the gravitational well of the star (see Section 3.6 for a discussion of fermionicbosonic stars). The bosonic contribution may arise from exotic matter, which could appear at high densities inside the neutron star or from some sort of dark matter accretion. This possibility has gained popularity over the last years and there have been several attempts to constrain the properties of weakly interacting dark matter particles (WIMPs) by examining signatures related to their accretion and/or annihilation inside stars (for instance, see [137] and works cited in the introduction).
Recently, it was suggested that, due to the stronger gravitational field of neutron stars compared to other stars such as white dwarfs and main sequence stars, WIMPs will accrete more efficiently, leading to two different possibilities. If the dark matter is its own antiparticle, it will selfannihilate and heat the neutron star. This temperature increase could be observable in old stars, especially if they are close to the galactic center [137, 64]. If WIMPs do not selfannihilate, they will settle in the center of the star forming a sort of fermionicbosonic star. The accretion of dark matter would then increase the star’s compactness until the star collapses [64].
Because of the similarities between boson stars and neutron stars, one finds that boson stars are often used in place of the other. This is especially so within numerical work because boson stars are easier to evolve than neutron star models. One can, for example compare the gravitationalwave signature of a bosonstar merger with that of more conventional compact object binaries consisting of BHs and NSs (for a review of BHNS binaries see [201]). At early times, the precise structure of the stars is irrelevant and the signatures are largely the same. However, for the late stages of merger, the relative phase of the boson stars determines the GW signature [174, 173].
Ref. [171] follows such work by considering the result of a collision between a BH and a boson star. In particular, they consider the problem as a perturbation of a black hole via scalar accretion and analyze the resulting gravitationalwave output. The hope is that observations of gravitational waves that are expected in a few years from aLIGO/Virgo will be able to distinguish the type of matter accreting onto a BH.
With the continued advancement in observation, both in the electromagnetic and gravitational spectra, perhaps soon we will have evidence for these questions. At the same time, further study of boson stars can help identify possible distinguishing observational effects in these bands. One example where knowledge is lacking is the interaction between boson stars with a magnetic field. Whereas a neutron star can source its own magnetic field and a neutral star can obtain an induced charge when moving with respect to a magnetic field, we are aware of no studies of the interaction of boson stars with a magnetic field.
5.2 Compact alternatives to black holes
As a localized scalar field configuration, a boson star can be constructed as a noninteracting compact object, as long as one does not include any explicit coupling to any electromagnetic or other fields. In that respect, it resembles a BH, although it lacks a horizon. Can observations of purported BHs be fully explained by massive boson stars? See Ref. [183] for a review of such observations.
Neutron stars also lack horizons, but, in contrast to a boson star, have a hard surface. A hard surface is important because one would expect accretion onto such a surface to have observable consequences. Can a boson star avoid such consequences? Yuan, Narayan and Rees consider the the viability of 10 M _{⊙} boson stars as BH candidates in Xray binaries [223]. They find that accreting gas collects not at the surface (which the star lacks), but instead at the center, which ultimately should lead to Type I Xray bursts. Because these bursts are not observed, the case against boson stars as black hole mimickers is weakened (at least for BH candidates in Xray binaries).
Ref. [105] considers a simplified model of accretion and searches for bosonstar configurations that would mimic an accreting black hole. Although they find matches, they find that light deflection about a boson star will differ from the BH they mimic because of the lack of a photon sphere. Further work, studies the scalar field tails about boson stars and compares them to those of BHs [153]. If indeed a boson star collapses to a BH, then one could hope to observe the QNM of the massive scalar field, as described in [114].
Some of the strongest evidence for the existence of BHs is found at the center of most galaxies. The evidence suggests supermassive objects (of the order of millions of solar mass) occupying a small region (of order an astronomical unit) [32]. While definitive evidence for a BH horizon from conventional electromagnetic telescopes is perhaps just on the “horizon” [42], there are those who argue the viability of supermassive boson stars at galactic centers [214]. There could potentially be differences in the (electromagnetic) spectrum between a black hole and a boson star, but there is considerable freedom in adjusting the boson star potential to tweak the expected spectrum [103]. However, there are stringent constraints on BH alternatives to Sgr A* by the low luminosity in the near infrared [43]. In particular, the low luminosity implies a bound on the accretion rate assuming a hard surface radiating thermally and, therefore, the observational evidence favors a black hole because it lacks such a surface.
However, the observation of gravitational waves from such objects may be able to distinguish BHs from BSs [28]. Such a test would occur in the bandwidth for a spacebased observatory such as the beleaguered LISA mission. Because BSs allow for orbits within what would otherwise be a blackhole event horizon, geodesics will exhibit extreme pericenter precession resulting in potentially distinguishable gravitational radiation [128]. In any case, observations of supermassive objects at the centers of galaxies can be used to constrain the scalar field parameters of possible mimickers [17].
There are other possible BH mimickers, and a popular recent one is the gravastar [158]. Common among all these alternatives, and most significantly, is the lack of an event horizon. Both gravastars and BSs undergo an ergoregion instability for high spin (J/(GM^{2}) > 0.4) [44]. As mentioned above for BSs, gravitational waves may similarly be able to distinguish gravastars from BHs [175].
5.3 As origin of dark matter
Studies of stellar orbits within various galaxies produce rotation curves, which indicate galactic mass within the radius of the particular orbit. The discovery that these curves remain flat at large radius suggests the existence of a large halo of massive, yet dark, matter that holds the galaxy together despite its large rotation. However, what precise form of matter could fulfill the observational constraints is still very much unclear. Scalar fields are an often used tool in the cosmologist’s toolkit, but one cannot have a regular, static configuration of scalar field to serve as the halo [178] (see [69] as discussed in Section 6.3 for a discussion of rotating boson stars with embedded, rotating BH solutions). Instead, galactic scale boson stars are one possible candidate.
Boson stars can be matched onto the observational constraints for galactic dark matter halos [145, 199]. However, multistate boson stars that superpose various bosonstar solutions (e.g., an unexcited solution with an excited solution) can perhaps find better fits to the constraints [215]. Boson stars at the galactic scale may not exhibit general relativistic effects and can be effectively considered as BoseEinstein condensates (BEC) with angular momentum [185].
The solitonic nature of boson stars (see Figure 1) lends itself naturally to the wonderful observation of dark matter in the Bullet Cluster [146]. Ref. [144] attempts to determine a minimum galactic mass from such a match.
Interestingly, Ref. [19] foregoes boson stars and instead looks for quasistationary scalar solutions about a Schwarzschild black hole that could conceivably survive for cosmological times. Another approach is to use scalar fields for both the dark matter halo and the supermassive, central object. Ref. [8] looks for such a match, but finds no suitable solutions. Quite a number of more exotic models viably fit within current constraints, including those using Qballs [71].
6 Boson Stars in Mathematical Relativity
Although the experimental foundation for the existence of boson stars is completely lacking, on the theoretical and mathematical front, boson stars are well studied. Recent work includes a mathematical approach in terms of large and small data [87], followed up by studying singularity formation [151] and uniqueness [88, 150] for a certain boson star equation. In Ref. [48], they study radial solutions of the semirelativistic Hartree type equations in terms of global wellposedness. Ref. [30] demonstrates stationarity of time periodic scalar field solutions.
Beyond just existence, however, boson stars are often employed mathematically to study dynamics. Here, we concentrate on a few of these topics that have attracted recent interest.
6.1 Black hole critical behavior
If one considers some initial distribution of energy and watches it evolve, generally one arrives at one of three states. If the energy is sufficiently weak in terms of its gravity, the energy might end up dispersing to larger and large distances. However, if the energy is instead quite large, then perhaps it will concentrate until a black hole is formed. Or, if the form of the energy supports it, some of the energy will condense into a stationary state.
In his seminal work [52], Choptuik considers a real, massless scalar field and numerically evolves various initial configurations, finding either dispersion or blackhole formation. By parameterizing these initial configurations, say by the amplitude of an initial pulse p, and by tuning this parameter, he was able to study the threshold for blackhole formation at which he found fascinating blackhole critical behavior. In particular, his numerical work suggested that continued tuning could produce as small a black hole as one wished. This behavior is analogous to a phase transition in which the blackhole mass serves as an order parameter. Similar to phase transitions, one can categorize two types of transition that distinguish between whether the blackhole mass varies continuously (Type II) or discontinuously (Type I). For Choptuik’s work with a massless field, the transition is therefore of Type II because the blackhole mass varies from zero continuously to infinitesimal values.
Subsequent work has since established that this critical behavior can be considered as occurring in the neighborhood of a separatrix between the basins of attraction of the two end states. For p = p*, the system is precisely critical and remains on the (unstable) separatrix. Similarly other models find such threshold behavior occurring between a stationary state and blackhole formation. Critical behavior about stationary solutions necessarily involve blackhole formation “turningon” at finite mass, and is therefore categorized as Type I critical behavior.
The critical surface, therefore, appears as a codimension 1 surface, which evolutions increasingly approach as one tunes the parameter p. The distance from criticality p − p* serves as a measure of the extent to which a particular initial configuration has excited the unstable mode that drives solutions away from this surface. For Type II critical behavior, the mass of the resulting blackhole mass scales as a power law in this distance, whereas for Type I critical behavior, it is the survival time of the critical solution that scales as a power law. See [100] for a recent review.
We have seen that boson stars represent stationary solutions of Einstein’s equations and, thus, one would correctly guess that they may occur within Type I blackhole critical behavior. To look for such behavior, Hawley and Choptuik [109] begin their evolutions with bosonstar solutions and then perturb them both dynamically and gravitationally. They, therefore, included in their evolutionary system a distinct, free, massless, real scalar field which couples to the boson star purely through its gravity.
One can also consider these BSs in axisymmetry in which nonspherically symmetric modes could potentially become important. A first step in this direction studied spherically symmetric BSs within conformally flat gravity (which does not allow for gravitational waves) in axisymmetry [187]. Later, better resolution using adaptive mesh refinement within full general relativity was achieved by [141, 142], which upheld the results found within spherical symmetry. This work thus suggests that there are either no additional, unstable, axisymmetric modes or that such unstable modes are extremely slowly growing.
A very different type of critical behavior was also investigated by Lai [141]. By boosting identical boson stars toward each other and adjusting their initial momenta, he was able to tune to the threshold for blackhole formation. At the threshold, he found that the time till blackhole formation scaled consistent with Type I critical behavior and conjectured that the critical solution was itself an unstable boson star. This is one of the few fully nonlinear critical searches in less symmetry than spherical symmetry, and the first of Type I behavior in less symmetry. A related study colliding neutron stars instead of boson stars similarly finds Type I critical behavior [124] and subsequently confirmed by [127].
The gauged stars discussed in Section 3.9 also serve as critical solutions in spherical symmetry [53, 54, 166].
6.2 Hoop conjecture
An interesting use of boson stars was made by Choptuik and Pretorius [55]. They sought to answer classically whether the ultrarelativistic collision of two particles results in blackhole formation. Such a question clearly has relevance to hopes of producing black holes at the LHC (see, for example, [143, 177]). Guidance on this question is provided by Thorne’s Hoop Conjecture [213], which suggests that a black hole is formed if one squeezes energy into some spherical space of dimension less than the Schwarzschild radius for that energy.
Choptuik and Pretorius [55] find that indeed blackhole formation occurs at energies below that estimated by the Hoop Conjecture. This result is only a classical result consistent with the conjecture, but if it had not held, then there would have been no reason to expect a quantum theory to be consistent with it.
6.3 Other dimensions
Much work has been invested recently in considering physics in other dimensions. Motivation comes from various ideas including string theory (more dimensions) such as the AdS/CFT correspondence and holography (one fewer dimensions) [156, 159, 181]. Another source of motivation comes from the fact that higher dimensional black holes can have very different properties than those in three spatial dimensions [75]. Perhaps BSs will similarly display novel properties in other dimensions.
In lower dimensional AdS (2+1) spacetimes, early work in 1998 studied exact solutions of boson stars [191, 67, 192]. Higher dimensional scenarios were apparently first considered qualitatively a few years later in the context of brane world models [205]. This discussion was followed with a detailed analysis of the 3, 4, and 5 dimensional AdS solutions [11].
More recently, Ref. [84] considers oscillatons in higher dimensions and measure the scalar mass loss rate for dimensions 3, 4, and 5. They extend this work considering inflationary spacetimes [83].
The axisymmetric rotating BSs discussed in Section 3.5 satisfy a coupled set of nonlinear, elliptic PDEs in two dimensions. One might, therefore, suspect that adding another dimensions will only make things more difficult. As it turns out, however, moving to four spatial dimensions provides for another angular momentum, independent of the one along the zdirection (for example). Each of these angular momenta are associated with their own orthogonal plane of rotation. And so if one chooses solutions with equal magnitudes for each of these momenta, the solutions depend on only a single radial coordinate. This choice results in the remarkable simplification that one need only solve ODEs to find rotating solutions [138].
The work of [69] makes ingenious use of this 5D ansatz to construct rotating black holes with only a single Killing vector. They set the potential of [108] to zero so that the scalar fields are massless and they add a (negative) cosmological constant to work in antide Sitter (AdS). They find solutions for rotating black holes in 5D AdS that correspond to a bar mode for rotating neutron stars in 3D (see also [202] for a numerical evolution of a black hole in higher dimensions, which demonstrates such bar formation). One might expect such a nonsymmetric black hole to settle into a more symmetric state via the emission of gravitational waves. However, AdS provides for an essentially reflecting boundary in which the black hole can be in equilibrium. The distortion of the higher dimensional black hole also has a correspondence with the discrete values of the angular momentum of the corresponding boson star. For higher values of the rotational quantum number, the black hole develops multiple “lobes” about its center.
These solutions are extended to arbitrary odddimensional AdS spacetimes in [206]. Finding the solutions perturbatively, they explicitly show that these solutions approach (i) the boson star and (ii) the MyersPerry blackhole solutions in AdS [170] in different limits. See [75] for a review of black holes in higher dimensions.
The same authors of [69] also report on the existence of geons in 3+1 AdS “which can be viewed as gravitational analogs of boson stars” [70] (recall that boson stars themselves arose from Wheeler’s desire to construct local electrovacuum solutions). These bundles of gravitational energy are stable to first order due to the confining boundary condition adopted with AdS. However, these geons and the blackhole solutions above [69] are unstable at higher order because of the turbulent instability reported in [31].
Ref. [21] also studies blackhole solutions in 5D AdS. They find solutions for black holes with scalar hair that resemble a boson star with a BH in its center. See Ref. [92] for a review of charged scalar solitons in AdS.
7 Final Remarks
Boson stars have a long history as candidates for all manner of phenomena, from fundamental particle, to galactic dark matter. A huge variety of solutions have been found and their dynamics studied. Mathematically, BS are fascinating solitonlike solutions. Astrophysically, they represent possible explanations of blackhole candidates and dark matter, with observations constraining BS properties.
Further constraints appear to be right around the corner in a few directions. The LHC is rapidly narrowing the possible mass range for a Higgs particle (the quantized version of a fundamental scalar field), and some hold hope that black holes might be produced that would indicate the existence of higher dimensions. COGENT, Pamela, and others are finding intriguing (and frustratingly contradictory) clues about the true nature of dark matter. And Planck and JWST, if it can overcome its funding and budgetary problems, promise to further refine our view of the cosmos and the role of scalar fields within. Advanced LIGO will be complete in a few years and the future of spacebased GW telescopes such as LISA is currently being determined. GW observations will be groundbreaking on so many fronts, but could potentially help distinguish BSs from neutron stars or black holes.
Perhaps future work on boson stars will be experimental, if fundamental scalar fields are observed, or if evidence arises indicating the boson stars uniquely fit galactic dark matter. But regardless of any experimental results found by these remarkable experiments, there will always be regimes unexplored by experiments where boson stars will find a natural home.
Notes
Acknowledgments
It is our pleasure to thank Juan Barranco, Francisco Guzman, and Luis Lehner for their helpful comments on the manuscript. We especially appreciate the careful and critical reading by Bruno Mundim. We also thank Gyula Fodor and Péter Forgacs for their kind assistance with the section on oscillatons and oscillons. This research was supported by Perimeter Institute for Theoretical Physics. Research at Perimeter Institute is supported by the Government of Canada through Industry Canada and by the Province of Ontario through the Ministry of Research and Innovation. This work was also supported by NSF grants PHY0969827 and PHY0803624 to Long Island University.
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