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We examine the thermodynamic characteristics and phase structures of a black hole, where the black hole horizon could be a hypersurface with positive, zero, or negative constant curvature, within the framework of Einstein-Maxwell theory, incorporating a negative cosmological constant and a Gauss-Bonnet correction. Our research follows the topological approach to black hole thermodynamics where we treat anti-de Sitter (AdS) black holes as topological defects in thermodynamic space. We study the nature of the black hole's critical points and local stability by computing the winding numbers/topological charge associated with the zero point of the vector field, derived from the temperature of extremal points and generalized off-shell Gibbs free energy, respectively. Black holes are classified into different topological classes based on their topological number. In this study, we found unlike the charged AdS black hole, the charged GB AdS black hole exhibits a critical point. Our findings reveal the occurrence of a liquid/gas-like first-order phase transition between small-large black hole phases of the spherical charged GB AdS black hole. We conclude that the charged GB AdS and charged AdS black holes belong to different topological classes in the grand canonical ensemble. Furthermore, connecting with the previous studies, we conclude that the charged AdS and charged GB AdS black holes in canonical and charged GB in the grand canonical ensemble belong to the same topological classes., Comment: 29 pages, 21 figures, 3 Tables

We provide a transparent discussion of the high temperature asymptotic behaviour of Cosmology in a dilaton-Einstein-Gauss-Bonnet (dEGB) scenario of modified gravity with vanishing scalar potential. In particular, we show that it has a clear interpretation in terms of only three attractors (stable critical points) of a set of autonomous differential equations: $w=-\frac{1}{3}$, $w=1$ and $1

We present a family of charged rotating wormhole solutions to the Einstein-Maxwell equations, supported by anisotropic matter fields. We first revisit the charged static cases and analyze the conditions for the solution to represent a wormhole geometry. The rotating geometry is obtained by applying the Newman-Janis algorithm to the static geometry. We show the solutions to Maxwell equations in detail. We believe that our wormhole geometry offers a geometric realization corresponding to the concept of 'charge without charge'., Comment: 24 pages, 3 figures

We present a non-abelian cousin of the model presented in \cite{Lee:2022rtz} which induces cosmological anisotropies on top of standard FLRW geometry. This is in some sense doing a cosmological mean field approximation, where the mean field cosmological model under consideration would be the standard FLRW, and the induced anisotropies are small perturbative corrections on top of it. Here we mostly focus on the non-abelian $SU(2)$ gauge fields coupled to the gravity to generate the anisotropies, which can be a viable model for the axion-like particle (ALP) dark sector. The induced anisotropies are consequences of the non-trivial back-reaction of the gauge fields on the gravity sector, and by a clever choice of the parametrization, one can generate the Bianchi model we have studied in this note. We also show that the anisotropies influence the Sachs-Wolfe effect and we discuss the implications., Comment: 33 pages, 19 figures; updated references. Accepted for publication in Physics of the Dark Universe

Superradiance is the effect of field waves being amplified during reflection from a charged or rotating black hole. In this paper, we study the low-energy dynamics of super-radiant scattering of massive scalar and massless higher spin field perturbations in a generic axisymmetric stationary Kerr-Taub-NUT (Newman-Unti-Tamburino) spacetime, which represents sources with both gravitomagnetic monopole moment (magnetic mass) and gravitomagnetic dipole moment (angular momentum). We obtain a generalized Teukolsky master equation for all spin perturbation fields. The equations are separated into their angular and radial parts. The angular equations lead to spin-weighted spheroidal harmonic functions that generalize those in Kerr spacetime. We identify an effective spin as a coupling between frequency (or energy) and the NUT parameter. The behaviors of the radial wave function near the horizon and at the infinite boundary are studied. We provide analytical expressions for low-energy observables such as emission rates and cross sections of all massless fields with spin, including scalar, neutrino, electromagnetic, Rarita-Schwinger, and gravitational waves., Comment: 42 pages, 2 figures

We use the Weakly Interacting Massive Particle (WIMP) thermal decoupling scenario to probe Cosmologies in dilatonic Einstein Gauss-Bonnet (dEGB) gravity, where the Gauss-Bonnet term is non-minimally coupled to a scalar field with vanishing potential. We put constraints on the model parameters when the ensuing modified cosmological scenario drives the WIMP annihilation cross section beyond the present bounds from DM indirect detection searches. In our analysis we assumed WIMPs that annihilate to Standard Model particles through an s-wave process. For the class of solutions that comply with WIMP indirect detection bounds, we find that dEGB typically plays a mitigating role on the scalar field dynamics at high temperature, slowing down the speed of its evolution and reducing the enhancement of the Hubble constant compared to its standard value. For such solutions, we observe that the corresponding boundary conditions at high temperature correspond asymptotically to a vanishing deceleration parameter q, so that the effect of dEGB is to add an accelerating term that exactly cancels the deceleration predicted by General Relativity. The bounds from WIMP indirect detection are nicely complementary to late-time constraints from compact binary mergers. This suggest that it could be interesting to use other Early Cosmology processes to probe the dEGB scenario., Comment: 30 pages, 8 figures, 1 table. Updated to published version

We study the homoclinic orbit and the violation of chaos bound, which are obtained by particle motions around a black hole that coexist with anisotropic matter fields. The homoclinic one is associated with an unstable local maximum of the effective potential. By perturbing a particle located slightly away from the homoclinic one, we numerically compute Lyapunov exponents indicating the sensitivity of the initial value. Our results demonstrate that the violation of the chaos bound increases with higher angular momentum, and the anisotropic matter gives rise to violating the chaos bound further, even in the case of the nonextremal black hole. We utilize the Hamiltonian-Jacobi formalism to explicitly illustrate how the geodesic motion of a particle can be integrable in the procedure of obtaining our findings., Comment: 19 pages, 27 figures, references added, version to appear in PRD

We present a simple model which generates cosmological anisotropies on top of standard FLRW geometry. This is in some sense reminiscent of the mean field approximation, where the mean field cosmological model under consideration would be the standard FLRW, and the anisotropy is a small perturbative correction on top of it. Using a supergravity-inspired model, we confirm that the stable fixed point of our model corresponds to standard FLRW cosmology. We use a Bianchi VII$_0$-type model supplemented with a scalar and $U(1)$ gauge fields, and we show that the anisotropies of the geometry are generated by the non-trivial interaction between the gravity sector and the $U(1)$ gauge sector. Studying the attractor flow, we show that the anisotropies are present at early times (high redshift) and decay asymptotically to an FLRW attractor fixed point. With such a mechanism, observations of non-isotropy are not contradictory to FLRW geometry or indeed the $\Lambda$CDM model. Such models could in principle shed some insights on the present cosmological tensions., Comment: 46 pages, 15 figures; title of paper has been changed, added and clarified discussion of results, added figure, updated references. Accepted for publication in Physics of the Dark Universe

Local $H_0$ determinations currently fall in a window between $H_0 \sim 70$ km/s/Mpc (TRGB) and $H_0 \sim 76$ km/s/Mpc (Tully-Fisher). In contrast, BAO data calibrated in an early $\Lambda$CDM universe are largely consistent with Planck-$\Lambda$CDM, $H_0 \sim 67.5$ km/s/Mpc. Employing a generic two parameter family of evolving equations of state (EoS) for dark energy (DE) $w_{\textrm{DE}}(z)$ and mock BAO data, we demonstrate that if i) $w_{\textrm{DE}}(z=0) < -1$ and ii) integrated DE density less than $\Lambda$CDM, then $H_0$ increases. EoS that violate these conditions at best lead to modest $H_0$ increases within $1 \sigma$. Tellingly, Quintessence and K-essence satisfy neither condition, whereas coupled Quintessence can only satisfy ii). Beyond these seminal DE Effective Field Theories (EFTs), we turn to explicit examples. Working model agnostically in an expansion in powers of redshift $z$, we show that Brans-Dicke/$f(R)$ and Kinetic Gravity Braiding models within the Horndeski class can lead to marginal and modest increases in $H_0$, respectively. We confirm that as far as increasing $H_0$ is concerned, no DE EFT model can outperform the phenomenological two parameter family of the DE models. Evidently, the late universe may no longer be large enough to accommodate $H_0$, BAO and DE described by EFT., Comment: 24 pages, 15 figures, comments/correspondence welcome; v2 references and comments added; v3 references added, to appear in JCAP

We study a rotating black hole with anisotropic matter and electromagnetic fields. We show that the electromagnetic field has the same form as the corresponding one in the Kerr-Newman geometry. The reason is thanks to the specific form of the metric function. We also mention energy extraction mechanisms from the black hole., Comment: 14 pages, Typo fixed. This article is prepared for the proceedings of the 17th Italian-Korean Symposium for Relativistic Astrophysics

We study black hole solutions in dilatonic Einstein-Gauss-Bonnet theory with a coupling constant $\alpha$ between the dilaton field and the Gauss-Bonnet term. In a previous study, we considered the black hole with the vanishing cosmological constant in this theory and constructed the hairy black hole solution with the negative $\alpha$. In this study, we present black hole solutions numerically with various physical properties in anti-de Sitter spacetime. We describe the procedure for constructing the black hole solutions in detail., Comment: 14 pages, 10 figures. This article is prepared for the Proceedings of the 17th Italian-Korean Symposium for Relativistic Astrophysics

We investigate the gravitational wave spectrum resulted from the cosmological first-order phase transition. We compare two models; one is a scalar field model without gravitation, while the other is a scalar field model with gravitation. Based on the sensitivity curves of the LISA space-based interferometer on the stochastic gravitational-wave background, we compare the difference between the gravitational wave spectra of the former and the latter cases resulted from the bubble collision process. Especially, we calculated the speed of the bubble wall before collision for the two models numerically. We show that the difference between the amplitudes of those spectra can clearly distinguish between the two models. We expect that the LISA with Signal to Noise Ratio =10 could observe the spectrum as the fast first-order phase transition.

We obtain the shadow cast induced by the rotating black hole with an anisotropic matter. A Killing tensor representing the hidden symmetry is derived explicitly. The existence of separability structure implies a complete integrability of the geodesic motion. We analyze an effective potential around the unstable circular photon orbits to show that one side of the black hole is brighter than the other side. Further, it is shown that the inclusion of the anisotropic matter ($Kr^{2(1-w)}$) has an effect on the observables of the shadow cast. The shadow observables include approximate shadow radius $R_s$, distortion parameter $\delta_s$, area of the shadow $A_s$, and oblateness $D_{os}$., Comment: 23 pages, 7 figures, references added, typos corrected

We present a family of new rotating black hole solutions to Einstein's equations that generalizes the Kerr-Newman spacetime to include an anisotropic matter. The geometry is obtained by employing the Newman-Janis algorithm. In addition to the mass, the charge and the angular momentum, an additional hair exists thanks to the negative radial pressure of the anisotropic matter. The properties of the black hole are analyzed in detail including thermodynamics. This black hole can be used as a better engine than the Kerr-Newman one in extracting energy., Comment: 20 pages, 4 figures, reference added

We study a hairy black hole solution in the dilatonic Einstein-Gauss-Bonnet theory of gravitation, in which the Gauss-Bonnet term is non-minimally coupled to the dilaton field. Hairy black holes with spherical symmetry seem to be easily constructed with a positive Gauss-Bonnet coefficient $\alpha$ within the coupling function, $f(\phi) = \alpha e^{\gamma \phi}$, in an asymptotically flat spacetime, i.e., no-hair theorem seems to be easily evaded in this theory. Therefore, it is natural to ask whether this construction can be expanded into the case with the negative coefficient $\alpha$. In this paper, we present numerically the dilaton black hole solutions with a negative $\alpha$ and analyze the properties of GB term through the aspects of the black hole mass. We construct the new integral constraint allowing the existence of the hairy solutions with the negative $\alpha$. Through this procedure, we expand the evasion of the no-hair theorem for hairy black hole solutions., Comment: 9 pages, 7 figures, 1 table, discussion and reference added

We study the effect of vortices on the tunneling decay of a symmetry-breaking false vacuum in three spacetime dimensions with gravity. The scenario considered is one in which the initial state, rather than being the homogeneous false vacuum, contains false vortices. The question addressed is whether, and, if so, under which circumstances, the presence of vortices has a significant catalyzing effect on vacuum decay. After studying the existence and properties of vortices, we study their decay rate through quantum tunneling using a variety of techniques. In particular, for so-called thin-wall vortices we devise a one-parameter family of configurations allowing a quantum-mechanical calculation of tunneling. Also for thin-wall vortices, we employ the Israel junction conditions between the interior and exterior spacetimes. Matching these two spacetimes reveals a decay channel which results in an unstable, expanding vortex. We find that the tunneling exponent for vortices, which is the dominant factor in the decay rate, is half that for Coleman-de Luccia bubbles. This implies that vortices are short-lived, making them cosmologically significant even for low vortex densities. In the limit of the vanishing gravitational constant we smoothly recover our earlier results for the decay of the false vortex in a model without gravity., Comment: 27 pages, 8 figures, Typos corrected

We investigate various types of Fubini instantons in Dilatonic Einstein-Gauss-Bonnet theory of gravitation, which describe the decay of the vacuum state at a hilltop potential through the tunneling without a barrier. It is shown that the vacuum states are modified by the non-minimally coupled higher-curvature term. Accordingly, we present the new type of solutions describing the tunneling from new vacuum states in anti-de Sitter and de Sitter backgrounds. The decay probabilities of vacuum states are also influenced. Consequently, we show that the semiclassical exponents can be decreased for specific parameter ranges, which make the tunneling probabilities enhanced., Comment: 9 pages, 9 figures, Typo corrections, Reference added, Published version

We study the thermodynamic properties of a black hole and the Hawking-Page phase transition in the asymptotically anti--de Sitter spacetime in the dilatonic Einstein-Gauss-Bonnet theory of gravitation. We show how the higher-order curvature terms can influence both the thermodynamic properties and the phase transition. We evaluate both heat capacity and free energy difference to determine the local and global thermodynamic stabilities, respectively. We find that the phase transition occurs from the thermal anti--de Sitter to a small spherical black hole geometry and occurs to a hyperbolic black hole geometry in the (dilatonic) Einstein-Gauss-Bonnet theory of gravitation unlike those in Einstein's theory of gravitation., Comment: 24 pages, 8 figures, errors corrected, references added, version published in PRD

We investigate dynamics of magnetic thin-shells in three dimensional anti de Sitter background. Because of the magnetic field, an oscillatory solution is possible. This oscillating shell can tunnel to a collapsing shell or a bouncing shell, where both of tunnelings induce an event horizon and a singularity. In the entire path integral, via the oscillating solution, there is a non-zero probability to maintain a trivial causal structure without a singularity. Therefore, due to the path integral, the entire wave function can conserve information. Since an oscillating shell can tunnel after a number of oscillations, in the end, it will allow an infinite number of different branchings to classical histories. This system can be a good model of the effective loss of information, where information is conserved by a solution that is originated from gauge fields., Comment: 16 pages, 9 figures

We investigate the fragmentation instability of hairy black holes in the theory with a Gauss$-$Bonnet(GB) term in asymptotically flat spacetime. Our approach is through the non-perturbative fragmentation instability. By this approach, we investigate whether the initial black hole can be broken into two black holes by comparing the entropy of the initial black hole with the sum of those of two fragmented black holes. The relation between the black hole instability and the GB coupling with dilaton hair are presented. We describe the phase diagrams with respect to the mass of the black hole solutions and coupling constants. We find that a perturbatively stable black hole can be unstable under fragmentation., Comment: 20 pages, 10 figures, published version in EPJC