Accretion Of Radiation Research Articles

Evolution of primordial black holes in an adiabatic FLRW universe with gravitational particle creation

We study the evolution of primordial black holes (PBHs) in an adiabatic FLRW universe with dissipation due to bulk viscosity which is considered to be in the form of gravitational particle creation. Assuming that the process of evaporation is quite suppressed during the radiation era, we obtain an analytic solution for the evolution of PBH mass by accretion during this era, subject to an initial condition. We also obtain an upper bound on the accretion efficiency $\epsilon$ for $a \sim a_r$, where $a_r$ is the point of transition from the early de Sitter era to the radiation era. Furthermore, we obtain numerical solutions for the mass of a hypothetical PBH with initial mass 100 g assumed to be formed at an epoch when the value of the Hubble parameter was, say, 1 km/s/Mpc. We consider three values of the accretion efficiency, $\epsilon=0.23,0.5$, and $0.89$ for our study. The analysis reveals that the mass of the PBH increases rapidly due to the accretion of radiation in the early stages of its evolution. The accretion continues but its rate decreases gradually with the evolution of the Universe. Finally, Hawking radiation comes into play and the rate of evaporation surpasses the accretion rate so that the PBH mass starts to decrease. As the Universe grows, evaporation becomes the dominant phenomenon, and the mass of the PBH decreases at a faster rate. As argued by Debnath and Paul, the evaporated mass of the PBHs might contribute towards the dark energy budget of the late Universe.

Observable characteristics of the charged black hole surrounded by thin disk accretion in Rastall gravity

The observable characteristics of the charged black hole (BH) surrounded by a thin disk accretion are investigated in the Rastall gravity. We found that the radii of the direct emission, lensing ring, and photon ring dramatically increased as the radiation field parameter increased, but they only weakly depend on the BH charge. Three positions of the radiation accretion disk relative to the BH are considered, i.e. the innermost accretion disk is closed to the radii of the innermost stable circular orbit, the photon ring of the BH, and the event horizon of the BH. The observed images in three cases respectively are obtained. It is found that the total observed flux is dominated by the direct emission, the lensing ring provides a small contribution, and the photon ring is negligible. The lensing and photon rings could not be observed in the blurred image with the EHT resolution. Our results suggest that the observable characteristics of the charged BH surrounded by the thin disk accretion in the Rastall gravity depend on both the BH space-time structure and the position of the radiating accretion disk with respect to the BH. The research of these BH images may serve as a probe for the BH-disk structure in M87* like nearby active galactic nuclei.

Supermassive gravitinos and giant primordial black holes

We argue that the stable (color singlet) supermassive gravitinos proposed in our previous work can serve as seeds for giant primordial black holes. These seeds are hypothesized to start out as tightly bound states of fractionally charged gravitinos in the radiation dominated era, whose formation is supported by the universally attractive combination of gravitational and electric forces between the gravitinos and anti-gravitinos (reflecting their `almost BPS-like' nature). When lumps of such bound states coalesce and undergo gravitational collapse, the resulting mini-black holes can escape Hawking evaporation if the radiation temperature exceeds the Hawking temperature. Subsequently the black holes evolve according to an exact solution of Einstein's equations, to emerge as macroscopic black holes in the transition to the matter dominated era, with masses on the order of the solar mass or larger. The presence of these seeds at such an early time provides ample time for further accretion of matter and radiation, and would imply the existence of black holes of almost any size in the universe, up to the observed maximum.

GUT-scale primordial black holes: mergers and gravitational waves

Tight constraints on the abundance of primordial black holes can be deduced across a vast range of masses, with the exception of those light enough to fully evaporate before nucleosynthesis. This hypothetical population is almost entirely unconstrained, to the point where the early Universe could pass through a matter-dominated phase with primordial black holes as the primary component. The only obvious relic of this phase would be Hawking radiated gravitons which would constitute a stochastic gravitational wave background in the present-day Universe, albeit at frequencies far beyond the scope of any planned detector technology. This paper explores the effects of classical mergers in such a matter dominated phase. For certain ranges of parameters, a significant fraction of the black holes merge, providing an additional, classical source of primordial gravitational waves. The resulting stochastic background typically has a lower amplitude than the Hawking background and lies at less extreme frequencies, but is unlikely to be easily detectable, with a maximal present day density of ΩGW ∼ 10−12 and frequencies between 1015–1019 Hz. We also assess the impact of radiation accretion on the lifetimes of such primordial black holes and find that it increases the black hole mass by ∼ 14% and the lifetimes by about 50%. However, this does not qualitatively change any of our conclusions.

Regimes of mini black hole abandoned to accretion

Being inspired by the Eddington’s idea, along with other auxiliary arguments, it is unveiled that there exist regimes of a black hole that would prohibit accretion of ordinary energy. In explicit words, there exists a lower bound to black hole mass below which matter accretion process does not run for black holes. Not merely the baryonic matter, but, in regimes, also the massless photons could get prohibited from rushing into a black hole. However, unlike the baryon accretion abandoned black hole regime, the mass-regime of a black hole prohibiting accretion of radiation could vary along with its ambient temperature. For example, we discuss that earlier to [Formula: see text] s after the big-bang, as the cosmological temperature of the Universe grew above [Formula: see text] K, the mass range of black hole designating the radiation accretion abandoned regime, had to be in varying state being connected with the instantaneous age of the evolving Universe by an “one half” power law. It happens to be a fact that a black hole holding regimes prohibiting accretion of energy is gigantic by its size in comparison to the Planck length-scale. Hence the emergence of these regimes demands mini black holes for not being viable as profound suckers of energy. Consideration of accretion abandoned regimes could be crucial for constraining or judging the evolution of primordial black holes over the age of the Universe.

Primordial black hole and wormhole formation by domain walls

In theories with a broken discrete symmetry, Hubble sized spherical domain walls may spontaneously nucleate during inflation. These objects are subsequently stretched by the inflationary expansion, resulting in a broad distribution of sizes. The fate of the walls after inflation depends on their radius. Walls smaller than a critical radius fall within the cosmological horizon early on and collapse due to their own tension, forming ordinary black holes. But if a wall is large enough, its repulsive gravitational field becomes dominant much before the wall can fall within the cosmological horizon. In this ``supercritical'' case, a wormhole throat develops, connecting the ambient exterior FRW universe with an interior baby universe, where the exponential growth of the wall radius takes place. The wormhole pinches off in a time-scale comparable to its light-crossing time, and black holes are formed at its two mouths. As discussed in previous work, the resulting black hole population has a wide distribution of masses and can have significant astrophysical effects. The mechanism of black hole formation has been previously studied for a dust-dominated universe. Here we investigate the case of a radiation-dominated universe, which is more relevant cosmologically, by using numerical simulations in order to find the initial mass of a black hole as a function of the wall size at the end of inflation. For large supercritical domain walls, this mass nearly saturates the upper bound according to which the black hole cannot be larger than the cosmological horizon. We also find that the subsequent accretion of radiation satisfies a scaling relation, resulting in a mass increase by about a factor of 2.

Cosmological Black Holes and the Direction of Time

Macroscopic irreversible processes emerge from fundamental physical laws of reversible character. The source of the local irreversibility seems to be not in the laws themselves but in the initial and boundary conditions of the equations that represent the laws. In this work we propose that the screening of currents by black hole event horizons determines, locally, a preferred direction for the flux of electromagnetic energy. We study the growth of black hole event horizons due to the cosmological expansion and accretion of cosmic microwave background radiation, for different cosmological models. We propose generalized McVittie co-moving metrics and integrate the rate of accretion of cosmic microwave background radiation onto a supermassive black hole over cosmic time. We find that for flat, open, and closed Friedmann cosmological models, the ratio of the total area of the black hole event horizons with respect to the area of a radial co-moving space-like hypersurface always increases. Since accretion of cosmic radiation sets an absolute lower limit to the total matter accreted by black holes, this implies that the causal past and future are not mirror symmetric for any spacetime event. The asymmetry causes a net Poynting flux in the global future direction; the latter is in turn related to the ever increasing thermodynamic entropy. Thus, we expose a connection between four different "time arrows": cosmological, electromagnetic, gravitational, and thermodynamic.

Accretion of radiation and rotating primordial black holes

We consider rotating primordial black holes (PBHs) and study the effect of accretion of radiation in the radiation-dominated era. The central part of our analysis deals with the role of the angular momentum parameter on the evolution of PBHs. We find that both the accretion and evaporation rates decrease with an increase in the angular momentum parameter, but the rate of evaporation decreases more rapidly than the rate of accretion. This shows that the evaporation time of PBHs is prolonged with an increase in the angular momentum parameter. We also note that the lifetime of rotating PBHs increases with an increase in the accretion efficiency of radiation as in the case of nonrotating PBHs.

Vacuum Energy and Primordial Black Holes in Brans-Dicke Theory

In our work, we study the evolution of primordial black holes within the context of Brans-Dicke theory by assuming present universe as vacuum dominated. We also consider the accretion of radiation, matter and vacuum energy only during respective dominant periods. From our study, we found that the accretion rate is slower in both radiation and vacuum energy dominated eras in Brans-Dicke theory in comparision with General Theory of Relativity. Thus the PBHs evaporate at a faster rate in Brans-Dicke theory than Standard Cosmology, if we consider the presence of vacuum energy in both cases. We also find that vacuum energy accretion efficiency should be less than 0.61.

The effect of cosmic rays on the stability of hot dilute plasmas

Many factors are considered in instability analysis, such as self-gravity, differential rotation, and magnetic field. Cosmic rays (CRs) are another factor that have been recently considered and shown to play a dynamic role. However, the effect of CRs on the stability of hot dilute plasma systems has not yet been considered. This type of plasma is believed to be found in the intracluster medium of galaxy clusters and inefficient radiation accretion flows around black holes. Here, we study the dynamic effects of CRs on instability modes in such systems. We discuss analytically some special cases and work out general cases numerically. We show that the growth rate of some unstable modes such as magnetothermal instability (MTI), gyroviscous magnetorotational instability, and convective instability which depend on the ratio of CR pressure to gas pressure (βcr), the ratio of the radial gradient of CR pressure to the radial gradient of gas pressure (Φ), and CR diffusion coefficient. CRs can alter MTI criterion. In this paper, we show that a plasma which is stable in ideal MTI criterion, i.e. dT/dR > 0, such as cooling flow clusters, can be unstable in the presence of CRs if |Φ| > 1.

Evolution of Primordial Black Holes in Loop Quantum Cosmology

In this work, we study the evolution of Primordial Black Holes within the context of Loop Quantum Gravity. First we calculate the scale factor and energy density of the universe for different cosmic era and then taking these as inputs we study evolution of primordial black holes. From our estimation it is found that accretion of radiation does not affect evolution of primordial black holes in loop quantum gravity even though a larger number of primordial black holes may form in early universe in comparison with Einstein's or scalar-tensor theories.

PBH mass growth through radial accretion during the radiation dominated era

We model the radial accretion of radiation on Primordial Black Holes (PBH) by numerically solving Einstein's equations coupled to an ultrarelativistic ideal gas with equation of state p = ρ/3. We calculate the final mass of a black hole by the integration of the accreted radiation energy density during the leptonic era between t ∼ 10−4s to t ∼ 102s after the Big Bang. Our results indicate that small PBHs with initial masses between 10−4 to 1M⊙ may grow up to hundreds of solar masses, and thus can be SMBH seeds. On the other hand, PBHs formed at t ∼ 1s with initial mass between 900 and ∼ 980M⊙, by the time t ∼ 100s show masses of 104 to 106M⊙ which are masses of seeds or already formed SMBHs. The fact that we consider only radial flow implies that our results work well as limiting cases, and it is expected that under more general scenarios the accretion rates may change significantly. Nevertheless we show that it is possible that SMBHs can be PBHs that grew due to the accretion of radiation.

Primordial Black Holes Evolution in f(T) Gravity

We consider the effect of accretion of radiation, matter and dark energy in the early universe on primordial black holes (PBH) in f(T) gravity. Due to the Hawking radiation, mass of the primordial black hole decreases. We show that for the phantom accretion inclusion with the Hawking evaporation, the mass of the PBH decreases faster whereas for the accretion of radiation, matter and quintessence together with Hawking evaporation, the mass increases in f(T) gravity.

EVOLUTION OF PRIMORDIAL BLACK HOLE MASS SPECTRUM IN BRANS–DICKE THEORY

We investigate the evolution of primordial black hole mass spectrum by including both accretion of radiation and Hawking evaporation within Brans–Dicke (BD) cosmology in radiation-, matter- and vacuum-dominated eras. We also consider the effect of evaporation of primordial black holes on the expansion dynamics of the universe. The analytic solutions describing the energy density of the black holes in equilibrium with radiation are presented. We demonstrate that these solutions act as attractors for the system ensuring stability for both linear and nonlinear situations. We show, however, that inclusion of accretion of radiation delays the onset of this equilibrium in all radiation-, matter- and vacuum-dominated eras.

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 itsʼ 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.

Dark matter and dark energy accretion on to intermediate-mass black holes

In this work we investigate the accretion of cosmological fluids onto an intermediate-mass black hole at the centre of a globular cluster, focusing on the influence of the parent stellar system on the accretion flow. We show that the accretion of cosmic background radiation and the so-called dark energy onto an intermediate-mass black hole is negligible. On the other hand, if cold dark matter has a nonvanishing pressure, the accretion of dark matter is large enough to increase the black hole mass well beyond the present observed upper limits. We conclude that either intermediate-mass black holes do not exist, or dark matter does not exist, or it is not strictly collisionless. In the latter case, we set a lower limit for the parameter of the cold dark matter equation of state.

Phantom energy accretion and primordial black holes evolution in Brans–Dicke theory

In this work, we study the evolution of primordial black holes within the context of Brans-Dicke theory by considering the presence of a dark energy component with a super-negative equation of state called phantom energy as a background. Besides Hawking evaporation, here we consider two type of accretions - radiation accretion and phantom energy accretion. We found that radiation accretion increases the lifetime of primordial black holes whereas phantom accretion decreases the lifespan of primordial black holes. Investigating the competition between the radiation accretion and phantom accretion, we got that there is an instant during the matter-dominated era beyond which phantom accretion dominates radiation accretion. So the primordial black holes which are formed in the later part of radiation dominated era and in matter dominated era are evaporated at a quicker rate than the Hawking evaporation. But for presently evaporating primordial black holes, radiation accretion and Hawking evaporation terms are dominant over phantom accretion term and hence presently evaporating primordial black holes are not much affected by phantom accretion.

Accretion, primordial black holes and standard cosmology

Primordial Black Holes evaporate due to Hawking radiation. We find that the evaporation time of primordial black holes increase when accretion of radiation is included.Thus depending on accretion efficiency more and more number of primordial black holes are existing today, which strengthens the idea that the primordial black holes are the proper candidate for dark matter.

Note on nonstationarity and accretion by primordial black holes in Brans-Dicke theory

We consider the evolution of primordial black holes by including non-stationarity in their formation process and accretion of radiation in Brans-Dicke theory. Specifically, we focus on how $\eta$, the fraction of the horizon mass the black hole comprises capturing nonstationarity, affects the lifetimes of these primordial black holes. Our calculation reveals that the primordial black hole dynamics is controlled by the product $f\eta$ where $f$ is the accretion efficiency. We also estimate the impact of $\eta$ through $f\eta$ on the primordial black holes' initial mass fraction constraint obtained from the $\gamma$-ray background limit.

Astrophysical constraints on primordial black holes in Brans-Dicke theory

We consider cosmological evolution in Brans-Dicke theory with a populationof primordial black holes. Hawking radiation from the primordial black holesimpacts various astrophysical processes during the evolution of the Universe.The accretion of radiation by the black holes in the radiation dominated eramay be effective in imparting them a longer lifetime. We present a detailed study of how this affects various standard astrophysical constraints coming from the evaporation of primordial black holes. We analyze constraints arisingfrom the present density of the Universe, the present photonspectrum, the distortion of the cosmic microwave background spectrum and also from processes affecting light element abundances after nucleosynthesis. We find that the constraints on the initial primordialblack hole mass fractions are tightened with increased accretion efficiency.