Brans–Dicke Theory and Primordial Black Holes in Early Matter-Dominated Era

We show that primordial black holes can be formed in the matter-dominated era with gravity described by the Brans–Dicke theory. Considering an early matter-dominated era between inflation and reheating, we found that the primordial black holes formed during that era evaporate at a quicker rate than those of early radiation-dominated era. Thus, in comparison with latter case, less number of primordial black holes could exist today. Again the constraints on primordial black hole formation tend towards the larger value than their radiation-dominated era counterparts indicating a significant enhancement in the formation of primordial black holes during the matter-dominaed era.     

Primordial braneworld black holes: Significant enhancement of lifetimes through accretion

The Randall-Sundrum (RS-II) braneworld cosmological model with a fraction of the total energy density in primordial black holes is considered. Due to their 5d geometry, these black holes undergo modified Hawking evaporation. It is shown that during the high-energy regime, accretion from the surrounding radiation bath is dominant compared to evaporation. This effect increases the mass of the black holes till the onset of matter (or black hole) domination of the total energy density. Thus black holes with even very small initial masses could survive till several cosmologically interesting eras.     

Growth of black holes at the centers of galaxies: Absorption of stars and activity of galactic nuclei

We study the influence of supermassive black holes on the distribution of stars at the centers of galaxies. We analyze relaxation processes associated with encounters between stars and their absorption by black holes. For an isothermal distribution of stars, we obtain the growth law and estimate the current masses of black holes. The tidal disruption of stars near black holes is considered as a possible cause of the activity of galactic nuclei.     

On primordial black holes and secondary gravitational waves generated from inflation with solo/multi-bumpy potential

It is well known that a primordial black hole (PBH) can be generated in the inflation process of the early universe, especially when the inflation field has a number of non-trivial features that could break the slow-roll condition. In this study, we investigate a toy model of inflation with bumpy potential, which has one or several bumps. We determined that the potential with multi-bump can generate power spectra with multi-peaks in small-scale region, which can in turn predict the generation of primordial black holes in various mass ranges. We also consider the two possibilities of PBH formation by spherical and elliptical collapses. Finally, we discuss the scalar-induced gravitational waves (SIGWs) generated by linear scalar perturbations at second-order.     

A New Model for The Formation of Mini Black Holes

A new model different from the Hawking model is suggested to describe the formation of mini (primordial) black holes. It is based on an idea that the fluids, which fill in the universe at the beginning just after the big bang, are separated into mini clusters, moving with supersonic velocities and colliding with each other under the forces of gravitational attractions. We show that between the time era from 10^–43 to 10^–8 sec. about 36 various generations of mini black holes with masses ranging in the interval 10^–5—10^–30 gm are possible.     

Neutrino masses, dark matter and the mysterious early quasars

The lightest right-handed or sterile neutrino, that is embedded in a renormalizable seesaw-like extension of the standard model, with a mass and a tiny mixing θ∼10^−6.5 to one of the left-handed active neutrinos, is an attractive quasi-stable dark matter particle candidate. This sterile neutrino is produced in the early universe with the dark matter abundance required by WMAP. It is quasi-stable, decaying in about 10¹⁹ years into two neutrinos and an antineutrino, and it may be observed directly through its subdominant radiative decay into an active neutrino and a photon in about 10²¹ years. In contrast to the galaxies, that are known to form hierarchically, the supermassive black holes are formed anti-hierarchically, i.e. the most massive quasars first, and the least massive active galactic nuclei last. Here we argue that the anti-hierarchical formation of the supermassive black holes may be due to the possibility that both, the quasars and active galactic nuclei, may originate from supermassive degenerate neutrino balls that are swallowed up by stellar-mass black holes, produced by supernova explosions of massive stars at the centers of the neutrino balls.     

Primordial black holes and the observed Galactic 511 keV line

The observed 511 keV line from the Galactic Bulge is a real challenge for theoretical astrophysics: despite a lot of suggested mechanisms, there is still no convincing explanation and the origin of the annihilated positrons remains unknown. Here we discuss the possibility that a population of slowly evaporating primordial black holes with the mass around 10¹⁶–10¹⁷ g ejects (among other particles) low-energy positrons into the Galaxy. In addition to positrons, we have also calculated the spectrum and number density of photons and neutrinos produced by such black holes and found that the photons are potentially observable in the near future, while the neutrino flux is too weak and below the terrestrial and extra-terrestrial backgrounds. Depending on their mass distribution, such black holes could make a small fraction or the whole cosmological dark matter.     

Time Delay in Black Hole Gravitational Lensing as a Distance Estimator

We calculate the time delay between different relativistic images formed by black hole gravitational lensing in the strong field limit. For spherically symmetric black holes, it turns out that the time delay between the first two images is proportional to the minimum impact angle. Their ratio gives a very interesting and precise measure of the distance of the black hole. Moreover, using also the separation between the images and their luminosity ratio, it is possible to extract the mass of the black hole. The time delay for the black hole at the center of our Galaxy is just few minutes, but for supermassive black holes with M=10⁸ ÷10⁹ in the neighbourhood of the Local Group the time delay amounts to few days, thus being measurable with a good accuracy.     

The formation of primary galactic nuclei during phase transitions in the early universe

A new mechanism describing the formation of protogalaxies is proposed, based on the second-order phase transition in the inflation stage and the domain wall formation upon the end of inflation. The presence of closed domain walls with the size markedly exceeding the cosmological horizon at the instant of their formation and the wall collapse in the postinflation epoch (when the wall size becomes comparable with the cosmological horizon) lead to the formation of massive black hole clusters that can serve as nuclei for the future galaxies. The black hole mass distributions obtained do not contradict the available experimental data. The number of black holes with M ～ 100 solar masses (M _⊙) and above is comparable with the number of galaxies in the visible Universe. Development of the proposed approach gives grounds for a principally new scenario of galaxy formation in the model of a hot Universe.     

Weighing black holes in the universe

The determination of the mass of black holes in our universe is crucial to understand their physics nature but is a great challenge to scientists. In this paper I briefly review some methods that are currently used to estimate the mass of black holes, especially those in X-ray binary systems and in galactic nuclei. Our recent progress in improving the mass estimates of supermassive black holes in active galactic nuclei by involving some empirical relations is presented. Finally I point out the similarities and common physics in Galactic black hole X-ray binaries and active galactic nuclei, and demonstrate that the black hole mass estimation is very much helpful to understand the accretion physics around black holes.     

黑洞照亮宇宙——银河系中心黑洞及其物理意义

2020年度诺贝尔物理学奖颁发给为黑洞和超大质量致密天体做出突出贡献的三位科学家,他们分别从理论和观测上提供了令人信服的证明和证据.他们的工作打开了理解宇宙中大质量天体命运的窗口.人们普遍相信超大质量黑洞存在于每一个星系的中心,是这些黑洞照亮了再电离时期的宇宙,也是它们为揭开宇宙膨胀历史、暗能量宇宙演化性质、纳赫兹低频...     

Extreme astrophysical sources

Extreme astrophysical sources, those where the largest energy transfers occur, give all way to gravity's fatal attraction to sustain their unrivalled power. With a special emphasis on space observations, we present an inventory of extreme astrophysical sources which follows the filiation supernovae, neutron stars and black holes, before ending with supermassive black holes. This ill-assorted population of cosmic sites has some traits in common, the black-hole relativistic-jet connection being commonplace, from solar-mass specimens up to supermassive ones.     

A common origin of neutralino stars and supermassive black holes

To account for the microlensing events observed in the Galactic halo, Gurevich, Zybin, and Sirota have proposed a model of gravitationally bound, noncompact objects with masses of ～(0.01–1)M _⊙. These objects are formed in the expanding Universe from adiabatic density perturbations and consist of weakly interacting particles of dark matter, for example, neutralinos. They assumed the perturbation spectrum on some small scale to have a distinct peak. We show that the existence of this peak would inevitably give rise to a large number of primordial black holes (PBHs) with masses of ～10⁵ M _⊙ at the radiation-dominated evolutionary stage of the Universe. Constraints on the coefficient of nonlinear contraction and on the compactness parameter of noncompact objects were derived from constraints on the PBH number density. We show that noncompact objects can serve as gravitational lenses only at a large PBH formation threshold, δ_c > 0.5, or if noncompact objects are formed from entropic density perturbations.     

Tracing activity in distant supermassive black holes with X-rays

Supermassive black holes at the centres of galaxies have long been thought to be the engines of quasars, which emit more energy than any other sources in the Universe. In the local Universe, dormant supermassive black holes have been detected through the motions of stars and gas near the galactic centres. In the distant Universe, high energy X-ray observations are now revealing the accretion of matter onto supermassive black holes, even when the black holes are highly obscured by gas and dust. Great advances are being made in obtaining a cosmic census of supermassive black holes. The duration, times, and mass inflow rates to these black holes are being traced via multiwavelength follow-up observations with ground-based telescopes and a time history of the accretion is thereby being reconstructed.     

Gauss-bonnet black holes and possibilities for their experimental search

Corollaries of gravity models with second-order curvature corrections in the form of a Gauss-Bonnet term and possibilities (or impossibilities) for their experimental search or observations are discussed. The full version of the four-dimensional Schwarzschild-Gauss-Bonnet black hole solution and the constraint on the possible minimal black hole mass following from this model are considered. Using our solution as a model for the final stages of Hawking evaporation of black holes with a low initial mass (up to 10¹⁵ g) whose lifetime is comparable to that of our Universe, we have revealed differences in the patterns of evaporation: we have obtained high values of the emitted energy and showed the impossibility of an experimental search for primordial black holes by their evaporation products. Scenarios for the evaporation of Gauss-Bonnet black holes in multidimensional gravity models and possibilities for their experimental search are also discussed.     

Implications of a viscosity bound on black hole accretion

Motivated by the viscosity bound in gauge/gravity duality, we consider the ratio of shear viscosity (η) to entropy density (s  ) in black hole accretion flows. We use both an ideal gas equation of state and the QCD equation of state obtained from lattice for the fluid accreting onto a Kerr black hole. The QCD equation of state is considered since the temperature of accreting matter is expected to approach ¹⁰¹² K${10}^{12}\text{K}$ in certain hot flows. We find that in both the cases η/s$\eta /s$ is small only for primordial black holes and several orders of magnitude larger than any known fluid for stellar and supermassive black holes. We show that a lower bound on the mass of primordial black holes leads to a lower bound on η/s$\eta /s$ and vice versa. Finally we speculate that the Shakura–Sunyaev viscosity parameter should decrease with increasing density and/or temperatures.     

Can Superconducting Cosmic Strings Piercing Seed Black Holes Generate Supermassive Black Holes in the Early Universe?

The discovery of a large number of supermassive black holes (SMBH) at redshifts , when the Universe was only 900 million years old, raises the question of how such massive compact objects could form in a cosmologically short time interval. Each of the standard scenarios proposed, involving rapid accretion of seed black holes or black hole mergers, faces severe theoretical difficulties in explaining the short‐time formation of supermassive objects. In this work we propose an alternative scenario for the formation of SMBH in the early Universe, in which energy transfer from superconducting cosmic strings piercing small seed black holes is the main physical process leading to rapid mass increase. As a toy model, the accretion rate of a seed black hole pierced by two antipodal strings carrying constant current is considered. Using an effective action approach, which phenomenologically incorporates a large class of superconducting string models, we estimate the minimum current required to form SMBH with masses of order by . This corresponds to the mass of the central black hole powering the quasar ULAS J112001.48+064124.3 and is taken as a test case scenario for early‐epoch SMBH formation. For GUT scale strings, the required fractional increase in the string energy density, due to the presence of the current, is of order 10⁻⁷, so that their existence remains consistent with current observational bounds on the string tension. In addition, we consider an “exotic” scenario, in which an SMBH is generated when a small seed black hole is pierced by a higher‐dimensional string, predicted by string theory. We find that both topological defect strings and fundamental strings are able to carry currents large enough to generate early‐epoch SMBH via our proposed mechanism.     

Possible effects of a cosmological constant on black hole evolution

We explore possible effects of vacuum energy on the evolution of black holes. If the universe contains a cosmological constant, and if black holes can absorb energy from the vacuum, then black hole evaporation could be greatly suppressed. For the magnitude of the cosmological constant suggested by current observations, black holes larger than 4×10²⁴ g would accrete energy rather than evaporate. In this scenario, all stellar and supermassive black holes would grow with time until they reach a maximum mass scale of 6×10⁵⁵ g, comparable to the mass contained within the present day cosmological horizon.     

Gravitational wave formation from the collapse of dark energy field configurations

Dark energy is the dominant component of the energy density in the Universe. In a previous paper, we have shown that the collapse of dark energy fields leads to the formation of supermassive black holes with masses comparable to the masses of black holes at the centers of galaxies. Thus, it becomes a pressing issue to investigate the other physical consequences of the collapse of dark energy fields. Given that the primary interactions of dark energy fields with the rest of the Universe are gravitational, it is particularly interesting to investigate the gravitational wave signals emitted during the collapse of dark energy fields. This is the focus of the current work described in this paper. We describe and use the 3+1 BSSN formalism to follow the evolution of the dark energy fields coupled with gravity and to extract the gravitational wave signals. Finally, we describe the results of our numerical computations and the gravitational wave signals produced by the collapse of dark energy fields.     

Effect of vacuum energy on evolution of primordial black holes in Einstein gravity

We study the evolution of primordial black holes by considering present universe is no more matter dominated rather vacuum energy dominated. We also consider the accretion of radiation, matter and vacuum energy during respective dominance period. In this scenario, we found that radiation accretion efficiency should be less than 0.366 and accretion rate is much larger than previous analysis by Nayak et al. (2009) [1]. Thus here primordial black holes live longer than previous works Nayak and Singh (2011) [1]. Again matter accretion slightly increases the mass and lifetime of primordial black holes. However, the vacuum energy accretion is slightly complicated one, where accretion is possible only up to a critical time. If a primordial black hole lives beyond critical time, then its? lifespan increases due to vacuum energy accretion. But for presently evaporating primordial black holes, critical time comes much later than their evaporating time and thus vacuum energy could not affect those primordial black holes.