Cosmological Black Holes Research Articles

Models of cosmological black holes

Models of cosmological black holes

Scalar's quasibound states in cosmological black hole background

In this letter, we present in detail a novel exact solution of scalar quasibound states of a static spherically symmetric Schwarzschild de Sitter black hole. We investigate and work out the governing covariant scalar field wave equation, i.e., the Klein-Gordon equation and isolate the radial equation. In this letter, we show in detail, the derivation of the successfully obtained exact radial wave solutions which are expressed in terms of General Heun functions. Having the exact solutions in hand, the energy levels expression is obtained from special function's polynomial condition. In the last part, the Hawking radiation is investigated via the Damour-Ruffini method and the Hawking temperature is obtained from radiation distribution function.

Non-Minimally Coupled Electromagnetic Fields and Observable Implications for Primordial Black Holes

General relativity (GR) postulates have been verified with high precision, yet our understanding of how gravity interacts with matter fields remains incomplete. Various modifications to GR have been proposed in both classical and quantum realms to address these interactions within the strong gravity regime. One such approach is non-minimal coupling (NMC), where the space-time curvature (scalar and tensor) interacts with matter fields, resulting in matter fields not following the geodesics. To probe the astrophysical implications of NMC, in this work, we investigate non-minimally coupled electromagnetic (EM) fields in the presence of black holes. Specifically, we show that primordial black holes (PBHs) provide a possible tool to constrain the NMC parameter. PBHs represent an intriguing cosmological black hole class that does not conform to the no-hair theorem. We model the PBH as a Sultana–Dyer black hole and compare it with Schwarzschild. We examine observables such as the radius of the photon sphere, critical impact parameter, and total deflection angles for non-minimally coupled photons for Schwarzschild and Sultana–Dyer black holes. Both the black hole space-times lead to similar constraints on the NMC parameter. For a PBH of mass M=10−5 M⊙, the photon sphere will not be formed for one mode. Hence, the photons forming the photon sphere will be highly polarized, potentially leading to observable implications.

Scalar field dark energy: insights into cosmological evolution and black hole accretion

Scalar field dark energy: insights into cosmological evolution and black hole accretion

Cosmological LTB black hole in a quintom universe

We study cosmological Lemaitre–Tolman–Bondi (LTB) black hole thermodynamics immersed in a quintom universe. We investigate some thermodynamic aspects of such a black hole in detail. We apply two methods of treating particles’ tunneling from the apparent horizons and calculate the black hole’s temperature in each method; the results of which are the same. In addition, by considering specific time slices in the cosmic history, we study the thermodynamic features of this black hole in these specific cosmic epochs. Also, we discuss the information loss problem and the remnant content of the cosmological black hole in different cosmic epochs in this context. We show that approximately in all the cosmic history, the temperature of the black hole’s apparent horizon is more than the temperature of the cosmological apparent horizon.

Thermodynamics and phase transition structure of charged black holes in f(R) gravity background from Rényi statistics perception

In the current study, we investigate the Rényi statistics as an extension of the Boltzmann-Gibbs theory, in order to characterize the non-Boltzmannian phase structure of charged black holes in the f(R) gravity. Through this research, we discover that a Hawking-Page phase transition and a Van-der-Waals phase portrait, which are missing from the standard Boltzmann-Gibbs' description, emerge respectively within the grand canonical ensemble and canonical ensemble via the Rényi formalism.The thermodynamics associated with charged black holes without cosmological constant black holes using Rényi statistics processes the same properties as those of asymptotically AdS charged black holes via Gibbs-Boltzmann formalism, as further evidenced by such phase structures found here. This is another example of a potential and substantial relationship between the cosmological constant and the nonextensive Rényi parameter.

Tolman-Ehrenfest’s criterion of thermal equilibrium extended to conformally static spacetimes

With insight from examples and physical arguments, the Tolman-Ehrenfest criterion of thermal equilibrium for test fluids in static spacetimes is extended to local thermal equilibrium in conformally static geometries. The temperature of the conformally rescaled fluid scales with the inverse of the conformal factor, reproducing the evolution of the cosmic microwave background in Friedmann universes, the Hawking temperature of the Sultana-Dyer cosmological black hole, and a heuristic argument by Dicke.

Quantum Oppenheimer-Snyder and Swiss Cheese Models.

By considering the quantum Oppenheimer-Snyder model in loop quantum cosmology, a new quantum black hole model whose metric tensor is a suitably deformed Schwarzschild one is derived. The quantum effects imply a lower bound on the mass of the black hole produced by the collapsing dust ball. For the case of larger masses where the event horizon does form, the maximal extension of the spacetime and its properties are investigated. By discussing the opposite scenario to the quantum Oppenheimer-Snyder, a quantum Swiss Cheese model is obtained with a bubble surrounded by the quantum universe. This model is analogous to black hole cosmology or fecund universes where the big bang is related to a white hole. Thus our models open a new window to cosmological phenomenology.

Hawking-like radiation as tunneling from a cosmological black hole in modified gravity: semiclassical approximation and beyond

Hawking radiation as a quantum phenomenon is generally attributed to the existence of the event horizon of a black hole. However, we demonstrate in this paper that there is indeed ingoing Hawking-like radiation associated with apparent horizons of the first cosmological black hole solution in the framework of Scalar-Tensor-Vector Gravity (STVG) theory living in the Friedmann-Lema\^{\i}tre-Robertson-Walker (FLRW) background. Such radiation can be attributed also to the cosmological apparent horizon of the FLRW universe and even to the cosmological event horizon of de Sitter spacetime. We see how STVG theory as a good theory for explaining black holes both on local and global scales affect the Hawking effect. Based on semiclassical approximation, we follow Hamilton-Jacobi and Parikh-Wilczek tunneling methods, respectively with and without back-reaction effects. We find out that back-reaction effects make a correlation between the emission modes in Parikh-Wilczek tunneling formalism, which can address the information paradox. We obtain the corresponding Hawking-like temperature as a function of inverse powers of apparent horizons radiuses of the cosmological black hole in STVG theory. We analyze the influence of the STVG parameter associated with a deviation of the STVG theory from General Theory of Relativity (GR) on both apparent horizons and the Hawking-like temperature of the cosmological black hole. We show that increasing the STVG parameter results in appearing the Hawking-like temperature in later cosmic times with some smaller values. Also, we follow the Hamilton-Jacobi approach beyond semiclassical approximation to involve all quantum correction terms in the deduced semiclassical outcomes for the cosmological black hole in the STVG theory.

Light deflection by rotating regular black holes with a cosmological constant

Using the Gauss-Bonnet theorem, we compute and examine the deflection angle of light rays by rotating regular black holes with a cosmological constant. By the help of optical geometries, we first deal with the Hayward black holes with cosmological contributions. Then, we reconsider the study of the Bardeen solutions. We inspect the cosmological constant effect on the deflection angle of light rays. Concretely, we find extra cosmological correction terms generalizing certain obtained findings. Using graphical analysis, we provide a comparative discussion with respect to the Kerr solutions. The results confirm that the non-linear electrodynamic charges affect the space-time geometry by decreasing the deflection angle of light rays by such cosmological black holes.

The Birth of a Relativistic Jet Following the Disruption of a Star by a Cosmological Black Hole

A black hole can launch a powerful relativistic jet after it tidally disrupts a star. If this jet fortuitously aligns with our line of sight, the overall brightness is Doppler boosted by several orders of magnitude. Consequently, such on-axis relativistic tidal disruption events (TDEs) have the potential to unveil cosmological (redshift $z>$1) quiescent black holes and are ideal test beds to understand the radiative mechanisms operating in super-Eddington jets. Here, we present multi-wavelength (X-ray, UV, optical, and radio) observations of the optically discovered transient \target at $z=1.193$. Its unusual X-ray properties, including a peak observed luminosity of $\gtrsim$10$^{48}$ erg s$^{-1}$, systematic variability on timescales as short as 1000 seconds, and overall duration lasting more than 30 days in the rest-frame are traits associated with relativistic TDEs. The X-ray to radio spectral energy distributions spanning 5-50 days after discovery can be explained as synchrotron emission from a relativistic jet (radio), synchrotron self-Compton (X-rays), and thermal emission similar to that seen in low-redshift TDEs (UV/optical). Our modeling implies a beamed, highly relativistic jet akin to blazars but requires extreme matter-domination, i.e, high ratio of electron-to-magnetic field energy densities in the jet, and challenges our theoretical understanding of jets.

How general is the strong cosmic censorship bound for quasinormal modes?

Hod's proposal claims that the least damped quasinormal mode of a black hole must have the imaginary part smaller than half of the surface gravity at the event horizon.The Strong Cosmic Censorship in General Relativity implies that this bound must be even weaker: half of the surface gravity at the Cauchy horizon. The appealing question is whether these bounds are limited by the Einstein theory only?Here we will present numerical evidence that once the black hole size is much smaller than then the radius of the cosmological horizon, both the Hod's proposal and the strong cosmic censorship bound for quasinormal modes are satisfied for general spherically symmetric black holes in an arbitrary metric theory of gravity. The low-lying quasinormal frequencies have the universal behavior in this regime and do not depend on the near-horizon geometry, but only on the asymptotic parameters: the value of the cosmological constant and black hole mass.

Conformally Schwarzschild cosmological black holes

We thoroughly investigate conformally Schwarzschild spacetimes in different coordinate systems to seek for physically reasonable models of a cosmological black hole. We assume that a conformal factor depends only on the time coordinate and that the spacetime is asymptotically flat Friedmann–Lemaître–Robertson–Walker Universe filled by a perfect fluid obeying a linear equation state p = wρ with w > −1/3. In this class of spacetimes, the McClure–Dyer spacetime, constructed in terms of the isotropic coordinates, and the Thakurta spacetime, constructed in terms of the standard Schwarzschild coordinates, are identical and do not describe a cosmological black hole. In contrast, the Sultana–Dyer and Culetu classes of spacetimes, constructed in terms of the Kerr–Schild and Painlevé–Gullstrand coordinates, respectively, describe a cosmological black hole. In the Sultana–Dyer case, the corresponding matter field in general relativity can be interpreted as a combination of a homogeneous perfect fluid and an inhomogeneous null fluid, which is valid everywhere in the spacetime unlike Sultana and Dyer’s interpretation. In the Culetu case, the matter field can be interpreted as a combination of a homogeneous perfect fluid and an inhomogeneous anisotropic fluid. However, in both cases, the total energy–momentum tensor violates all the standard energy conditions at a finite value of the radial coordinate in late times. As a consequence, the Sultana–Dyer and Culetu black holes for −1/3 < w ⩽ 1 cannot describe the evolution of a primordial black hole after its horizon entry.

Probing Our Universe’s Past Using Earth’s Geological and Climatological History and Shadows of Galactic Black Holes

In this short review, we discuss how Earth’s climatological and geological history and also how the shadows of galactic black holes might reveal our Universe’s past evolution. Specifically we point out that a pressure singularity that occurred in our Universe’s past might have left its imprint on Earth’s geological and climatological history and on the shadows of cosmological black holes. Our approach is based on the fact that the H0 tension problem may be resolved if some sort of abrupt physics change occurred in our Universe 70–150 Myrs ago, an abrupt change that deeply affected the Cepheid parameters. We review how such an abrupt physics change might have been caused in our Universe by a smooth passage of it through a pressure finite-time singularity. Such finite-time singularities might occur in modified gravity and specifically in F(R) gravity, so we show how modified gravity might drive this type of evolution, without resorting to peculiar cosmic fluids or scalar fields. The presence of such a pressure singularity can distort the elliptic trajectories of bound objects in the Universe, causing possible geological and climatological changes on Earth, if its elliptic trajectory around the Sun might have changed. Also, such a pressure singularity affects directly the circular photon orbits around supermassive galactic black holes existing at cosmological redshift distances, thus the shadows of some cosmological black holes at redshifts z≤0.01, might look different in shape, compared with the SgrA* and M87* supermassive black holes. This feature however can be checked experimentally in the very far future.

Dissimilar donuts in the sky? Effects of a pressure singularity on the circular photon orbits and shadow of a cosmological black hole

The black hole observations obtained so far indicate one thing: similar “donuts” exist in the sky. But what if some of the observed black hole shadows that will be obtained in the future are different from the others? In this work the aim is to show that a difference in the shadow of some observed black holes in the future might explain the H 0-tension problem. In this letter we investigate the possible effects of a pressure cosmological singularity on the circular photon orbits and the shadow of galactic supermassive black holes at cosmological redshifts. Since the pressure singularity is a global event in the Universe, the effects of the pressure singularity will be imposed on supermassive black holes at a specific redshift. As we show, the pressure singularity affects the circular photon orbits around cosmological black holes described by the McVittie metric, and specifically, for some time before the time instance that the singularity occurs, the photon orbits do not exist. We discuss the possible effects of the absence of circular photon orbits on the shadow of these black holes. Our idea indicates that if a pressure singularity occurred in the near past, then this could have a direct imprint on the shadow of supermassive galactic black holes at the redshift corresponding to the time instance that the singularity occurred in the past. Thus, if a sample of shadows is observed in the future for redshifts , and for a specific redshift differences are found in the shadows, this could be an indication that a pressure singularity occurred, and this global event might resolve the H 0-tension as discussed in previous work. However, the observation of several shadows at redshifts is a rather far future task.

Thakurta metric does not describe a cosmological black hole

Recently, the Thakurta metric has been adopted as a model of primordial black holes. We show that the spacetime described by this metric has neither black-hole event horizon nor black-hole trapping horizon and involves the violation of all the standard energy conditions as a solution of the Einstein equation. Therefore, this metric does not describe a cosmological black hole in the early universe. It is pointed out that a contradictory claim by the other group stems from an incorrect choice of sign.

Static and spherically symmetric general relativity solutions in minimal theory of bigravity

We investigate static and spherically symmetric solutions in the Minimal Theory of Bigravity (MTBG). First, we show that a pair of Schwarzschild-de Sitter spacetimes with different cosmological constants and black hole masses written in the spatially-flat Gullstrand-Painlev\'e (GP) coordinates is a solution in the self-accelerating branch of MTBG, while it cannot be a solution in the normal branch. We then illustrate how Schwarzschild-de Sitter solutions can become compatible with the normal branch when using different coordinates. We also confirm that the self-accelerating branch of MTBG admits static and spherically symmetric general relativity solutions with matter written in the spatially-flat coordinates, including neutron stars with arbitrary matter equations of state. Finally, we show that in the self-accelerating branch nontrivial solutions are given by the Schwarzschild-de Sitter metrics written in nonstandard coordinates.

New evidence and analysis of cosmological-scale asymmetry in galaxy spin directions

In the past several decades, multiple cosmological theories that are based on the contention that the Universe has a major axis have been proposed. Such theories can be based on the geometry of the Universe, or multiverse theories such as black hole cosmology. The contention of a cosmological-scale axis is supported by certain evidence such as the dipole axis formed by the CMB distribution. Here I study another form of the cosmological-scale axis, based on the distribution of the spin direction of spiral galaxies. Data from four different telescopes are analyzed, showing nearly identical axis profiles when the distribution of the redshifts of the galaxies is similar.

Apparent horizons of the Thakurta spacetime and the description of cosmological black holes

We discuss the validity of the Thakurta metric to describe cosmological black holes by analysing the nature of its horizon. By adopting the preferred foliation of the Thakurta spacetime associated with the Kodama time, we demonstrate that the Thakurta horizon is indeed a future outer trapping horizon. Therefore, the respective observers see it as a cosmological black hole, contrary to some claims in the literature.

Critical phenomena of charged dilatonic black holes through Rényi statistics approach

In this paper, in order to describe the non-Boltzmannian phase structure of charged dilatonic black holes in the absence of the cosmological constant, we consider the Rényi statistics as an extension of the Boltzmann–Gibbs theory which also satisfies Abe’s formula. This description is achieved by considering such a black hole either as a closed system (canonical ensemble) or as an open system (grand canonical ensemble). Through this study, we find that a Hawking–Page phase transition and a Van der Waals phase portrait appear within the grand canonical ensemble and canonical ensemble, respectively, these transitions are absent in the usual Boltzmann–Gibbs’s description. Furthermore, such phase structures provide another example wherein the thermodynamics associated with a vanishing cosmological constant black holes via Rényi statistics processes the same proprieties as those of asymptotically AdS charged black holes via Gibbs–Boltzmann formalism. This provides more insight into a possible and deep connection between the nonextensive Rényi parameter [Formula: see text] and the cosmological constant [Formula: see text]. Among others, we have addressed a generalization of [C. Promsiri, E. Hirunsirisawat and W. Liewrian, Phys. Rev. D 102 (2020) 064014] to dilaton gravity background and [R. Zhao, H.-H. Zhao, M.-S. Ma and L.-C. Zhang, Eur. Phys. J. C 73 (2013) 2645] to Rényi statistics approach.