The Hawking temperature in the context of dark energy for Kerr–Newman and Kerr–Newman–AdS backgrounds

We show that the Hawking temperature is modified in the presence of dark energy in an emergent gravity scenario for Kerr–Newman(KN) and Kerr–Newman–AdS(KNAdS) background metrics. The emergent gravity metric is not conformally equivalent to the gravitational metric. We calculate the Hawking temperatures for these emergent gravity metrics along θ=0\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\theta =0$$\end{document}. Also we show that the emergent black hole metrics are satisfying Einstein’s equations for large r and θ=0\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\theta =0$$\end{document}. Our analysis is done in the context of dark energy in an emergent gravity scenario having k-\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$k-$$\end{document}essence scalar fields ϕ\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\phi $$\end{document} with a Dirac–Born–Infeld type Lagrangian. In KN and KNAdS background, the scalar field ϕ(r,t)=ϕ1(r)+ϕ2(t)\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\phi (r,t)=\phi _{1}(r)+\phi _{2}(t)$$\end{document} satisfies the emergent gravity equations of motion at r→∞\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$r\rightarrow \infty $$\end{document} for θ=0\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\theta =0$$\end{document}.

The motivation of this work is to calculate the Hawking temperature in the presence of dark energy for an emergent gravity metric which is also a blackhole metric. We consider two cases, (a) when the gravitational metric is a Kerr-Newman and (b) when the gravitational metric Kerr-Newman-AdS.
In Sect. 2, we have described k-essence and emergent gravity where the metricG μν contains the dark energy field φ and this field should satisfy the emergent gravity equations of motion. Again, forG μν is to be a blackhole metric, it has to satisfy the Einstein field equations. The formalism for k-essence and emergent gravity used is as described in [18][19][20][21][22].
In Sects. 3 and 5, we have shown that for Kerr-Newman and Kerr-Newman-AdS both cases, the emergent gravity metrics are mapped on to the Kerr-Newman and Kerr-Newman-AdS type metrics in the presence of dark energy. The emergent metric satisfies Einstein equations for large r and the dark energy field φ satisfies the emergent gravity equations of motion for along θ = 0 at r → ∞.
We have calculated the Hawking temperature for emergent gravity metrics for Kerr-Newman and Kerr-Newman-AdS backgrounds in Sects. 4 and 6, respectively. We have clarified that the Hawking temperature is spherically symmetric from very general conditions and taking θ = 0 does not therefore affect this property of the Hawking temperature. It has been shown elaborately in [52], how the Hawking temperature is independent of θ , although the metric functions depend on θ . Also Hawking temperature is purely horizon phenomenon of the spacetime where the temperature is not depending on θ . So we can say that the Hawking temperature is spherically symmetric.

k−essence and emergent gravity
The k-essence scalar field φ minimally coupled to the gravitational field g μν has action [18][19][20][21][22] where X = 1 2 g μν ∇ μ φ∇ ν φ. The energy momentum tensor is and ∇ μ is the covariant derivative defined with respect to the gravitational metric g μν . The equation of motion is and 1 + 2X L X X L X > 0. Carrying out the conformal transfor- Then the inverse metric of G μν is A further conformal transformation [13,14]Ḡ μν ≡ c s L X G μν gives Here one must always have L X = 0 for the sound speed c 2 s to be positive definite and only then equations (1) − (4) will be physically meaningful, since L X = 0 implies L is independent of X , then from Eq. (1), L(X, φ) ≡ L(φ) i.e., L becomes a function of pure potential and the very definition of k-essence fields becomes meaningless because such fields correspond to lagrangians where the kinetic energy dominates over the potential energy. Also the very concept of minimal coupling of φ to g μν becomes redundant, so the Eq. (1) meaningless and Eqs. (4)(5)(6) ambiguous.
For the non-trivial configurations of the k− essence field φ, ∂ μ φ = 0 (for a scalar field, ∇ μ φ ≡ ∂ μ φ) andḠ μν is not conformally equivalent to g μν . So this k− essence field φ field has the properties different from canonical scalar fields defined with g μν and the local causal structure is also different from those defined with g μν . Further, if L is not an explicit function of φ then the equation of motion (3) is reduces to; We shall take the Lagrangian as This is a particular case of the DBI Lagrangian [13][14][15][16][17] This is typical for the k-essence field where the kinetic energy dominates over the potential energy. Then Note the rationale of using two conformal transformations: the first is used to identify the inverse metric G μν , while the second realises the mapping onto the metric given in (9) for the Lagrangian

Kerr-Newman metric and emergent gravity
We consider the gravitational metric g μν is Kerr-Newman (KN) and denote ∂ 0 φ ≡φ, ∂ r φ ≡ φ . We consider that the k-essence scalar field φ ≡ φ(r, t). The line element of Kerr-Newman metric is [44][45][46][47][48] where, ; It is to be noted that the above metric (10) also rediscovered in [50,51]. In [52], elaborately shown how the Hawking temperature is not depending on θ although the metric functions depend on θ . In our case the emergent gravity metric (9)Ḡ μν contains extra terms (first derivative of k-essence scalar fields) but these extra terms are still not depended on θ . Therefore, the modified Hawking temperature will still be independent of θ . For this reason, we will choose our evaluation for some fixed θ ,i.e., θ = 0 only. Assuming the Kerr-Newman metric along θ = 0. Then the above line element (10) becomes with F(r ) = Δ(r ) Σ and Σ = r 2 + α 2 . Also in [53] have shown that the four dimensional spherically non-symmetric Kerr-Newman metric (10) transformed into a two dimensional spherically symmetric metric (11) in the region near the horizon by the method of dimensional reduction.
The emergent gravity metric (9) components arē Then the emergent gravity line element (12) along θ = 0 becomes Now transform the coordinates [13,14] from (t, r ) to (ω, r ) such that and considerinġ we get the line element (13): We consider the solution of Eq. (15) Then the Eq. (15) reduces tȯ where K is a constant and K = 0 since k-essence scalar field will have non-zero kinetic energy. Now from (17) we and φ 2 (t) = √ kt and choosing integration constant to be zero. Therefore the line element (16) becomes where This new metric (19) is also Kerr-Newman (KN) type along θ = 0 in the presence of dark energy. Note that K = 1 since β cannot be zero, as then the metric (19) becomes singular. Also we have the total energy density is unity (Ω matter + Ω radiation + Ω darkenergy = 1) [14,49]. So we can say that the dark energy density i.e., kinetic energy (φ 2 2 = K ) of k-essence scalar field (in unit of critical density) cannot be greater than unity. Again K cannot be greater than 1 because the metric (19) will lead to wrong signature. The possibility of non-zero K appears because that would imply the absence of dark energy. Therefore, the only allowed values of K are 0 < K < 1. So there is no question of K approaching towards unity and confusions regarding this limit is avoided. It can be shown that, for r → ∞, this metric (19) is an approximate solution of Einstein's equation.
Also mention that the mass and charge of this type black hole are modified as M = M 1−K , Q = Q √ 1−K respectively in the presence of dark energy density term K =φ 2 2 . Now we can show that the k−essence scalar field φ(r, t) given by equation (18) to satisfy the emergent equation of motion (7) along the symmetry axis θ = 0 at r → ∞. For θ = 0, the emergent equation of motion (7) takes the form The first term vanishes since φ 2 (t) is linear in t and the last two terms vanish becauseḠ 01 =Ḡ 10 = 0. Using the expression for the second and third terms for r → ∞ goes as ( From the Planck collaboration results [54,55], we have the value of dark energy density (in unit of critical density) K is about 0.696. Therefore, the second and third terms of (20) is negligible as the denominator goes to infinity. Therefore, in this limit the emergent equation of motion is satisfied.
A massless particle in a black hole background is described by the Klein-Gordon equation We can expands Ψ as to obtain the leading order inh the Hamilton-Jacobi equation is We consider S is independent of θ and φ. Then the above Eq. (30) The action S is assumed to be of the form Then Then The two values of W (r ) correspond to the outer and inner horizons, respectively. Therefore the Eq. (32) becomes So the tunneling rates are and where K B is Boltzman constant. From these above two expressions (37) and (38) the corresponding Hawking temperatures of the two horizons are and The usual Hawking temperature for Kerr-Newman black hole is [52] The above temperatures (39,40) are modified in the presence of dark energy. These temperatures are different from usual Hawking temperature (41) as the presence of terms β = 1 − K , M = M 1−K and Q = Q √ 1−K where K is the dark energy density (in unit of critical density).

Kerr-Newman-AdS background
We consider the gravitational metric g μν is Kerr-Newman-AdS (KNAdS). The line element of KNAdS metric [62][63][64][65][66] is The parameters M and α are related to the mass and angular momentum of the black hole, G is the gravitational constant and l is the curvature radius determined by the negative cosmological constant (Λ < 0) Λ = − 3 l 2 . Again we choose symmetric axis along θ = 0 as before since in [52] elaborately shown that the Hawking temperature is independent of θ . Then the line element (42) reduces to with F(r ) = Δ r Σ and Σ = r 2 + α 2 . Using this (45) the emergent gravity metric (9) components arē Again we consider the k-essence scalar field φ(r, t) is spherically symmetric. So the emergent gravity line element for KNAdS background along θ = 0 is Transform the coordinates (t, r ) to (ω, r ) as and we choosė Then the line element (47) becomes We consider again the solution of Eq. (49) as φ(r, t) = φ 1 (r ) + φ 2 (t).
Then the Eq. (49) iṡ where K is a constant and K = 0. From (51) we geṫ where and Now we clarify the parameters of the above Eq. (52): C = −1 For this type of k-essence scalar field φ (52), the line element (50) reduces to Similar reasons as before here also the only allowed values of K are 0 < K < 1. Also it can be shown that this metric (55) is an approximate solution of Einstein's equations at r → ∞ along θ = 0. Note that the parameters M, Q, l are also modified in the presence of dark energy density (K ).

The Hawking temperature for KNAdS type metric in the presence of dark energy
We calculate the Hawking temperature using tunneling formalism [58,[64][65][66]. The horizons of the metric (55) in the presence of dark energy are determined by Δ r = (r 2 + α 2 ) 1 + r 2 l 2 − 2G M r + Q 2 = β l 2 [r 4 + r 2 (α 2 + l 2 ) − 2G M l 2 r + l 2 (α 2 + Q 2 )] = β The equation Δ r = 0 has four roots, two real positive roots and two complex roots. We denote r d ++ and r d −− are complex roots and r d + and r d − are positive real roots in the presence of dark energy (K ). Here we consider r d + > r d − so that r d + is the black hole event horizon and r d − is the Cauchy horizon of the KNAdS type black hole (55). Now we use the Eddington-Finkelstein coordinates (v, r ) or (u, r ) along θ = 0 i.e., advanced and retarded null coordinates [14] v = ω + r * ; u = ω − r * with dr * = (r 2 + α 2 )dr we get the emergent gravity line element (55) becomes