Gravitational wave astronomy is an emerging observational discipline that expands the astrophysical messengers astronomers can use to probe cosmic phenomena. The gravitational waveform from a source encodes the astrophysical properties and the dynamical motion of mass within the system; this is particularly evident in the case of binaries, where the overall amplitude of the system scales with physical parameters like mass and distance, but the phase structure of the waveform encodes the orbital evolution of the system. Strongly gravitating systems can show interesting and unusual orbital trajectories, as is the case for ``extreme mass ratio inspirals,'' observable in the millihertz gravitational wave band by space-based gravitational wave detectors. These sources can exhibit ``zoom-whirl'' orbits, which make complicated waveforms that are useful for mapping out the gravitational structure of the system. Zoom-whirl behavior can be intuitively understood in the context of effective potentials, which should be familiar to students from classical orbital theory in mechanics. Here we demonstrate and explain zoom-whirl orbits using effective potential theory around Schwarzschild black holes, and present an interactive tool that can be used in classroom and other pedagogical settings.
Black hole solutions can be used to shed light on general issues in General Relativity and Quantum Physics. Black--hole hair, entropy and naked singularities are considered here, along with some implications for the inflationary universe scenario.
We explore different (and complementary) views of spinors and their exotic counterparts, linking the very existence of the later to the presence of black holes. Moreover, we investigate the effects of the exotic term in the Hawking radiation emission rate, as well as its extremes, for asymptotically flat black holes solutions of general relativity. We show that, under certain circumstances, the emission rate extreme condition fixates an equation from which the exotic term could be inferred.
We study about an approximation method of the Hawking radiation. We analyze an massless scalar field in exotic black hole backgrounds models which have peculiar properties in black hole thermodynamics (monopole black hole in SO(3) Einstein-Yang-Mills-Higgs system and dilatoic black hole in Einstein-Maxwell-dilaton system). A scalar field is assumed not to be couple to matter fields consisting of a black hole background. Except for extreme black holes, we can well approximate the Hawking radiaition by `black body' one with Hawking temperature estimated at a radius of a critical impact parameter…
We examine the conjecture that black holes violating the reverse isoperimetric inequality have negative specific heat at constant volume $C_V$. We test this conjecture on the family of generalized exotic Bañados, Teitelbiom and Zanelli (BTZ) black holes and find that $C_V$ can be positive even when the reverse isoperimetric inequality is violated, providing a counter example to the conjecture. However in all cases where $C_V$ is positive, the specific heat at constant pressure $C_P$ is negative, indicating that generalized exotic black holes are thermodynamically unstable, suggesting that a br…
We explore the shadow of certain class generalized Kerr black holes, which are non vacuum solutions of the Einstein equations with exotic matter. The images depend on the angular momentum of the compact object, the characteristic parameter $l$ of the model, and the inclination angle of the observer. The results are compared with the case of the Kerr black hole and the Kerr naked singularity.
We report exotic orbital phenomena of spinning test particles orbiting around a Kerr black hole, i.e., some orbits of spinning particles are asymmetrical about the equatorial plane. When a nonspinning test particle orbits around a Kerr black hole in a strong field region, due to relativistic orbital precessions, the pattern of trajectories is symmetrical about the equatorial plane of the Kerr black hole. However, the patterns of the spinning particles' orbit are no longer symmetrical about the equatorial plane for some orbital configurations and large spins. We argue that these asymmetrical pa…
We construct the thermodynamic geometry of $2+1$ dimensional normal (exotic) BTZ black hole regarding to the fluctuation of cosmological constant. We argue that while the thermodynamic geometry of black hole without fluctuation of cosmological constant is a two dimensional flat spaces, the three dimensional thermodynamics parameters space including the cosmological constant as a fluctuating parameter will be curved. Some consequence of the fluctuation of cosmological constant will be investigated. We show that such fluctuation leads to a thermodynamics curvature which is correctly singular at …
We use a numerical code to consider the nonlinear processes arising when a Reissner-Nordstrom black hole is irradiated by an exotic scalar field (modelled as a free massless scalar field with an opposite sign for its energy-momentum tensor). These processes are quite different from the processes arising in the case of the same black hole being irradiated by a pulse of a normal scalar field. In our case, we did not observe the creation of a spacelike strong singularity in the T-region of the space-time. We investigate the antifocusing effects in the gravity field of the exotic scalar field with…
The purpose of this study is the derivation of the equation of motion for particles and light in the spacetime of Reissner-Nordström-(anti-)de Sitter black holes in the background of different kinds of regular and exotic matter fields. The complete analytical solutions of the geodesic equations are given in terms of the elliptic Weierstraß $\wp$-function and the hyperelliptic Kleinian $σ$-function. Finally after analyzing the geodesic motion of test particles and light using parametric diagrams and effective potentials, we present a list of all possible orbits.
A black hole solution in a teleparallel theory of (2+1)-dimensional gravity, given in a previous paper, is examined. This solution is also a solution of the three-dimensional vacuum Einstein equation with a vanishing cosmological constant. Remarkable is the fact that this solution gives a black hole in a \lq \lq flat-land" in the Einstein theory and a Newtonian limit. Coordinate transformations to \lq \lq Minkowskian" coordinates, however, are singular not only at the origin, but also on the event horizon. {\em In the three-dimensional Einstein theory, vacuum regions of space-times can be loca…
In the context of three-dimensional gravity with torsion, the concepts of standard and "exotic" Banados-Teitelboim-Zanelli black holes are generalized by going over to black holes with torsion. This approach provides a unified insight into thermodynamics of black holes, with or without torsion.
Recently a new three-dimensional theory of gravity, dubbed Exotic Massive Gravity, was proposed as a unitary theory both in the bulk as well as in the dual CFT. This is the second simplest example, the first being Minimal Massive Gravity. Since the divergence of the field equations vanishes on-shell, Exotic Massive Gravity has third-way consistency. Here we show how to compute mass and angular momentum in this theory, and then implement our result in various solutions, both for generic values of the couplings as well as at chiral points of the theory. For the latter, the asymptotic AdS behavio…
An exotic Banados-Teitelboim-Zanelli (BTZ) black hole has an angular momentum larger than its mass in three dimension (3D), which suggests the possibility that cosmic censorship could be violated if angular momentum is extracted by the Penrose process. In this paper, we propose a mass formula for the exotic BTZ black hole and show no violation of weak cosmic censorship in the gedanken process above by understanding properly its mass formula. Unlike the other black holes, the total energy of the exotic BTZ black hole is represented by the angular momentum instead of the mass, which supports a b…
Exotic smooth manifolds, ${\bf R^2\times_ΘS^2}$, are constructed and discussed as possible space-time models supporting the usual Kruskal presentation of the vacuum Schwarzschild metric locally, but {\em not globally}. While having the same topology as the standard Kruskal model, none of these manifolds is diffeomorphic to standard Kruskal, although under certain conditions some global smooth Lorentz-signature metric can be continued from the local Kruskal form. Consequently, it can be conjectured that such manifolds represent an infinity of physically inequivalent (non-diffeomorphic) space-ti…
In this work, we develop a universal picture from topology in the thermodynamic parameters space to describe first order phase transitions of black holes. By employing an off-shell internal energy, we find two types of topological defects. The first is normal, describing black holes which move with a nonzero velocity and acceleration, in a central force field. The second type of defects are exotic: they are static in the space, not describing black holes but encoding information about first order transitions. For each defect, we assign a winding number and an inertial mass. By studying neutral…
A number of three-dimensional (3D) gravity models, such as 3D conformal gravity, admit "exotic" black hole solutions: the metric is the same as the BTZ metric of 3D Einstein gravity but with reversed roles for mass and angular momentum, and an entropy proportional to the length of the inner horizon instead of the event horizon. Here we show that the BTZ solutions of the "exotic" 3D Einstein gravity (with parity-odd action but Einstein field equations) are exotic black holes, and we investigate their thermodynamics. The first and second laws of black hole thermodynamics still apply, and the ent…
The Weak Gravity Conjecture arises from the requirement that it be possible for all (classically stable) extremal black holes to decay. The ``black hole version'' of the conjecture requires that it should be possible for this to occur through the emission of smaller black holes. We consider this version in the case of extremal AdS$_4$-Kerr-Newman black holes which are stable against a superradiant instability. One finds that the emitted black hole must be rather exotic, having an ``angular horizon'' analogous to the more familiar (radial) horizon.
We find a class of charged black hole solutions in third order Lovelock Gravity. To obtain this class of solutions, we are not confined to the usual assumption of maximal symmetry on the horizon and will consider the solution whose boundary is Einstein space with supplementary conditions on its Weyl tensor. The Weyl tensor of such exotic horizons exposes two charge-like parameter to the solution. These parameters in addition with the electric charge, cause different features in compare with the charged solution with constant-curvature horizon. For this class of asymptotically (A)dS solutions, …
In this work, we obtain uncharged\charged Kiselev-like black holes as a new class of black hole solutions surrounded by perfect fluid in the context of Rastall theory. Then, we study the specific cases of the uncharged\charged black holes surrounded by regular matter like dust and radiation, or exotic matter like quintessence, cosmological constant and phantom fields. By comparing the Kiselev-like black hole solutions in Rastall theory with the Kiselev black hole solutions in GR, we find an effective perfect fluid behaviour for the black hole's surrounding field. It is shown that the correspon…
Gravitational wave astronomy provides an unprecedented opportunity to test the nature of black holes and search for exotic, compact alternatives. Recent studies have shown that exotic compact objects (ECOs) can ring down in a manner similar to black holes, but can also produce a sequence of distinct pulses resembling the initial ringdown. These "echoes" would provide definite evidence for the existence of ECOs. In this work we study the generation of these echoes in a generic, parametrized model for the ECO, using Green's functions. We show how to reprocess radiation in the near-horizon region…
Gravitational waves (GWs) from presumed binary black hole mergers are now being detected on a regular basis with the Advanced LIGO and Advanced Virgo interferometers. Exotic compact objects (ECOs) have been proposed that differ from Kerr black holes, and which could leave an imprint upon the GW signal in a variety of ways. Here we consider excitations of ECOs during inspiral, which may occur when the monotonically increasing GW frequency matches a resonant frequency of an exotic object. This causes orbital energy to be taken away, leading to a speed-up of the orbital phase evolution. We show t…
Classically, black holes are compact objects with perfect semi-permeable horizons: Anything may enter, nothing may leave. We consider an axiomatic approach that applies to any black hole type, including arbitrarily near-extremal black holes, that can unitarily evaporate away completely by any mechanism. We show that a quantum black hole must either have a leaky horizon (allowing quantum information out), or it must look very different from its classical counterpart having an external neighborhood consisting of exotic matter with super-ordinary entropic content (such as an `atmosphere' of micro…
We examine the thermodynamics of a new class of asymptotically AdS black holes with non-constant curvature event horizons in Gauss-Bonnet Lovelock gravity, with the cosmological constant acting as thermodynamic pressure. We find that non-trivial curvature on the horizon can significantly affect their thermodynamic behaviour. We observe novel triple points in 6 dimensions between large and small uncharged black holes and thermal AdS. For charged black holes we find a continuous set of triple points whose range depends on the parameters in the horizon geometry. We also find new generalizations o…
A unified framework for black holes and traversible wormholes is described, where both are locally defined by outer trapping horizons, two-way traversible for wormholes and one-way traversible for black or white holes. In a two-dimensional dilaton gravity model, examples are given of: construction of wormholes from black holes; operation of wormholes for transport, including back-reaction; maintenance of an operating wormhole; and collapse of wormholes to black holes. In spherically symmetric Einstein gravity, several exotic matter models supporting wormhole solutions are proposed: ghost scala…
We study the possibility of shielding a regular extremal black hole by means of a matter thin-shell. While the surface energy density $σ$ on the static shell is negative, the tangential pressures $p$ are positive, both of them being finite when the shell approaches the black hole horizon. The Darmois-Israel junction conditions are used to find $σ$ and $p$ in terms of the radius $a$ of the shell. The surface gravitational energy $E_{S}$ is computed, keeping track of the pressure contribution. The stability conditions are briefly investigated.
In this work, we present a study to probe the nature of interactions between black hole microstructures for the case of the BTZ black holes. Even though BTZ black holes without any angular momentum or electric charge thermodynamically behave as an ideal gas, i.e. with non-interacting microstructures; in the presence of electric charge or angular momentum, BTZ black holes are associated with repulsive interactions among the microstructures. We extend the study to the case of exotic BTZ black holes with mass $M = αm + γ\frac{j}{l}$ and angular momentum $J=αj + γl m$, for arbitrary values of $ (α…
In this work we implement the Minimal Geometric Deformation method to obtain the isotropic sector and the decoupler matter content of any anisotropic solution of the Einstein field equations with cosmological constant in $2+1$ dimensional space--times. We obtain that the solutions of both sectors can be expressed analytically in terms of the metric functions of the original anisotropic solutions instead of formal integral as in its $3+1$ counterpart. As a particular example we study a regular black hole solution and we show that, depending on the sign of the cosmological constant, the solution…
During the early phase of in-spiral of a binary system, the tidal heating of a compact object due to its companion plays a significant role in the determination of the orbital evolution of the binary. The phenomenon depends crucially on the `hairs', as well as on the nature of the compact object. It turns out that the presence of extra dimension affects both these properties, by incorporating an extra tidal charge for braneworld black holes and also by introducing quantum effects, leading to possible existence of exotic compact objects. It turns out that the phasing information from tidal heat…
We present new traversable wormhole and non-singular black hole solutions in pure, scale-free $R^2$ gravity. These exotic solutions require no null energy condition violating or "exotic" matter and are supported only by the vacuum of the theory. It is well known that $f(R)$ theories of gravity may be recast as dual theories in the Einstein frame. The solutions we present are found when the conformal transformation required to move to the dual frame is singular. For quadratic $R^2$ gravity, the required conformal factor is identically zero for spacetimes with $R=0$. Solutions in this case are a…
We study the response of a static Unruh-DeWitt detector outside an exotic compact object (ECO) with a general reflective boundary condition in 3+1 dimensions. The horizonless ECO, whose boundary is extremely close to the would-be event horizon, acts as a black hole mimicker. We find that the response rate is notably distinct from the black hole case, even when the ECO boundary is perfectly absorbing. For a (partially) reflective ECO boundary, we find resonance structures in the response rate that depend on the different locations of the ECO boundary and those of the detector. We provide a deta…
Exotic Spinor Deformation of a Schwarzschild Black Hole: Perturbations and Stability
Faculty of Engineering and Science of Guaratinguetá, UNESP（圣保罗州立大学瓜拉廷格塔工程与科学学院）；São Paulo State Department of Education (SEDUC-SP)（圣保罗州教育部）；Engineering School of Lorena - University of Sao Paulo (EEL-USP)（圣保罗大学洛雷达纳工程学院）
We find the general solution of the 6-dimensional Einstein-Gauss-Bonnet equations in a large class of space and time-dependent warped geometries. Several distinct families of solutions are found, some of which include black string metrics, space and time-dependent solutions and black holes with exotic horizons. Among these, some are shown to verify a Birkhoff type staticity theorem, although here, the usual assumption of maximal symmetry on the horizon is relaxed, allowing exotic horizon geometries. We provide explicit examples of such static exotic black holes, including ones whose horizon ge…
The spacetime solution for a black hole, surrounded by an exotic matter field, in Rastall gravity, is calculated in an arbitrary d-dimensional spacetime. After this, we calculate the scalar quasinormal modes of such solution, and study the shift on the modes caused by the modification of the theory of gravity, i.e., by the introduction of a new term due to Rastall. We conclude that the shift strongly depends on the kind of exotic field one is studying, but for a low density matter that supposedly pervades the universe, it is unlikely that Rastall gravity will cause any instability on the probe…
We present numerical waveforms of gravitational-wave echoes from spinning exotic compact objects (ECOs) that result from binary black hole coalescence. We obtain these echoes by solving the Teukolsky equation for the $ψ_4$ associated with gravitational waves that propagate toward the horizon of a Kerr spacetime, and process the subsequent reflections of the horizon-going wave by the surface of the ECO, which lies right above the Kerr horizon. The trajectories of the infalling objects are modified from Kerr geodesics, such that the gravitational waves propagating toward future null infinity mat…
Recently it has been argued that near-horizon modifications of the standard (classical) black hole spacetime could lead to observable alterations of the gravitational waveform generated by a binary black hole coalescence. Such modifications can be inspired by quantum gravity considerations, resulting in speculative horizonless exotic compact objects (ECOs) with no singularities, which may be an alternative to the classical black hole solution. A largely model-independent description of these objects proposed in the literature relies on the introduction of a partially reflective wall at some sm…
Exotic compact objects can be difficult to distinguish from black holes in the inspiral phase of the binaries observed by gravitational-wave detectors, but significant differences may be present in the merger and post-merger signal. We introduce a toy model capturing the salient features of binaries of exotic compact objects with compactness below $0.2$, which do not collapse promptly following the merger. We use it to assess their detectability with current and future detectors, and whether they can be distinguished from black hole binaries. We find that the Einstein Telescope (LISA) could ob…
A Black Hole (BH) is a spacetime region with a horizon and where geodesics converge to a singularity. At such a point, the gravitational field equations fail. As an alternative to the problem of the singularity arises the existence of Exotic Compact Objects (ECOs) that prevent the problem of the singularity through a transition phase of matter once it has crossed the horizon. ECOs are characterized by a closeness parameter or cutoff, $ε$, which measures the degree of compactness of the object. This parameter is established as the difference between the radius of the ECO's surface and the gravi…
Classical phenomenological aspects of acoustic perturbations on a draining bathtub geometry where a surface with reflectivity $\mathcal{R}$ is set at a small distance from the would-be acoustic horizon, which is excised, are addressed. Like most exotic compact objects featuring an ergoregion but not a horizon, this model is prone to instabilities when $|\mathcal{R}|^2\approx 1$. However, stability can be attained for sufficiently slow drains when $|\mathcal{R}|^2\lesssim70\%$. It is shown that the superradiant scattering of acoustic waves is more effective when their frequency approaches one o…
Recently, exotic black holes whose masses and angular momenta are interchanged have been found, and it is known that their entropies depend only on the $inner$ horizon areas. But a basic problem of these entropies is that the second law of thermodynamics is not guaranteed, in contrast to the Bekenstein-Hawking entropy. Here, I find that there is another entropy formula which recovers the usual Bekenstein-Hawking form, but the characteristic angular velocity and temperature are identified with those of the inner horizon, in order to satisfy the first law of black hole thermodynamics. The temper…
We extend the treatment of the problem of the gravitational waves produced by a particle of negligible mass orbiting a Kerr black hole using black hole perturbation theory in the time domain, to elliptic and inclined orbits. We model the particle by smearing the singularities in the source term using narrow Gaussian distributions. We compare results (energy and angular momentum fluxes) for such orbits with those computed using the frequency domain formalism.
The possible detection of echoes in late gravitational-wave signals is the most promising way to test horizonless alternatives to general relativistic black holes, and probe the physics of these hypothetical ultra-compact objects. While there is currently no evidence for the presence of such signatures, better accuracy is expected with the growing wealth of data from gravitational waves observatories. So far, several searches for these specific signals have been performed considering equidistant intervals between consecutive echoes, i.e. quasi-periodic wave-forms, and ignoring possible backrea…
This paper discusses the theoretical construction of thin-shell wormholes from Kiselev black holes. We assume a barotropic equation of state for the exotic matter on the shell. While most of these wormholes are unstable to linearized radial perturbations, a limit argument is used to show that under certain conditions, stable solutions can be found.
In the context of higher dimensional braneworld scenario, we have argued that the occurrence of horizonless exotic compact objects, as an alternative to classical black holes, are more natural. These exotic compact objects carry a distinctive signature of the higher dimension, namely a tidal charge parameter, inherited from the projection of the higher dimensional Weyl tensor onto the four dimensional spacetime we live in. Due to the absence of any horizon, rotating exotic compact objects are often unstable because of superradiance. Interestingly, these higher dimensional exotic compact object…
We summarize recent results on the properties of near-horizon metrics in different spherically symmetric space-times, including Kantowski-Sachs cosmological models whose evolution begins with a horizon (the so-called Null Big Bang) and static metrics related to black holes. We describe the types of matter compatible with cosmological and black-hole horizons. It turns out, in particular, that a black hole horizon can be in equilibrium with a fluid of disordered cosmic strings ("black holes can have curly hair"). We also discuss different kinds of horizons from the viewpoint of the behavior of t…
Quantum gravitational effects on the near horizon may alter the black hole's horizon drastically to be partially reflective, portrayed by a quantum membrane. With this modification, the object can be considered as an exotic compact object (ECO). Quantum effects on the strong gravitational regime may also lead to a non-conserved matter tensor that can be described phenomenologically using Rastall gravity. In this work, we study the properties of black holes and ECOs within Rastall gravity using Kerr/CFT correspondence. We systematically investigate the properties of the most general rotating bl…
Astrophysical and electromagnetic emissivity properties of black holes surrounded by a quintessence type exotic fluid in the Scalar-Vector-Tensor Modified Gravity
The radiation emitted by horizonless exotic compact objects (ECOs), such as wormholes, 2-2-holes, fuzzballs, gravastars, boson stars, collapsed polymers, superspinars etc., is expected to be strongly suppressed when compared to the radiation of black holes. If large primordial curvature fluctuations collapse into such objects instead of black holes, they do not evaporate or evaporate much slower than black holes and could thus constitute all of the dark matter with masses below $M < 10^{-16}M_\odot.$ We reevaluate the relevant experimental constraints for light ECOs in this mass range and show…
The event horizon of a black hole in general relativity absorbs all infalling radiation. Any observation of the contrary would immediately challenge the expectation that astrophysical black holes are described by the vacuum Kerr geometry. If a putative black hole does reflect part of the ingoing radiation, its quasinormal mode structure is drastically altered. Low frequency modes can be introduced that are resonantly excited during the inspiral of a binary system. We study the resulting phase shift of the gravitational wave signal. Building on neutron star results, we obtain a model-independen…
