Evolution of primordial black holes in a radiation and phantom energy environment

In this work we extend previous work on the evolution of a primordial black hole (PBH) to address the presence of a dark energy component with a super-negative equation of state as a background, investigating the competition between the radiation accretion, the Hawking evaporation and the phantom accretion, the latter two causing a decrease on black hole mass. It is found that there is an instant during the matter-dominated era after which the radiation accretion becomes negligible compared to the phantom accretion. The Hawking evaporation may become important again depending on a mass threshold. The evaporation of PBHs is quite modified at late times by these effects, but only if the generalized second law of thermodynamics is violated.     

The Evolution of Primordial Black Hole Masses in the Radiation-Dominated Era

We discuss in this work the behaviour of primordial black holes (PBHs) in the radiation era. Taking into account the Hawking evaporation and the absorption of energy we revisit the complete differential equation for the evolution of the mass of a PBH. We show that the mass can grow in this cosmological phase in a very slow fashion (even when considering the very high temperature of the radiation) if at all, and give a strong upper limit to the maximum accretion of mass. We evaluate relativistic effects due to the peculiar motion relative to the CMBR and show that the existence of relativistic black holes with very high mass absorption is highly unlikely. Finally we demonstrate that thermodynamic equilibrium between black holes and the cosmic radiation can not exist for finite times, and therefore initially non-evaporating PBHs must jump to the evaporating regime. This analysis supports the several efforts performed to look for signatures of evaporating holes.     

Experimental observation and characterization of the magnetorotational instability

Differential rotation occurs in conducting flows in accretion disks and planetary cores. In such systems, the magnetorotational instability can arise from coupling Lorentz and centrifugal forces to cause large radial angular momentum fluxes. We present the first experimental observation of the magnetorotational instability. Our system consists of liquid sodium between differentially rotating spheres, with an imposed coaxial magnetic field. We characterize the observed patterns, dynamics, and torque increases, and establish that this instability can occur from a hydrodynamic turbulent background.     

Accretion onto a Kerr black hole in the presence of a dipole magnetic field

We analyze the accretion of charged matter onto a rotating black hole immersed in an aligned dipolar magnetic field. We specialize to motion in the equatorial plane and calculate the ‘Keplerian’ angular momentum distribution, the marginally stable and marginally bound orbits, and the efficiency of mass-to-energy conversion as functions of the angular momentum of the black hole and of the product of the dipole moment and the charge of the infalling matter. Although the detailed results are quite different from those previously obtained in the case of an uniform magnetic field, the astrophysically relevant results are very similar; when hydrodynamical accretion is considered, these effects of the magnetic field are always very small. But for test particles the efficiency can be significantly increased for limited ranges of the parameters.     

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 types of accretion—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 found 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 by Hawking evaporation. But for presently evaporating primordial black holes, radiation accretion and Hawking evaporation terms are dominant over the phantom accretion term and hence presently evaporating primordial black holes are not much affected by phantom accretion.     

Hawking Radiation of Charged Particles from a Rotating Black String

Using Damour-Ruffini method, Hawking radiation of rotating black strings is studied. Under the condition that the total energy, total angular momentum and total charge are conservative, the transition probability from initial state (energy M+ω, charge Q+e and angular momentum J+m) to final state (energy M, charge Q and angular momentum J) for black strings is derived considering the reaction of radiation particles to spacetime. That is, the probability that black strings radiate particles with energy ω, charge e and angular momentum m is obtained. The real spectrum is not a strictly pure thermal spectrum. Our result is consistent with Parikh and Wilczek’s result. It satisfies the unitary principle of quantum mechanics. However, in our result there are not only the term that denotes effect of energy and charge of radiation particles but also the term that denotes effect of radiation particles angular momentum on rotating black strings angular momentum. We provide a new way for investigating radiation of black strings.     

Magnetized hypermassive neutron-star collapse: a central engine for short gamma-ray bursts

A hypermassive neutron star (HMNS) is a possible transient formed after the merger of a neutron-star binary. In the latest axisymmetric magnetohydrodynamic simulations in full general relativity, we find that a magnetized HMNS undergoes "delayed" collapse to a rotating black hole (BH) as a result of angular momentum transport via magnetic braking and the magnetorotational instability. The outcome is a BH surrounded by a massive, hot torus with a collimated magnetic field. The torus accretes onto the BH at a quasisteady accretion rate [FORMULA: SEE TEXT]; the lifetime of the torus is approximately 10 ms. The torus has a temperature [FORMULA: SEE TEXT], leading to copious ([FORMULA: SEE TEXT]) thermal radiation that could trigger a fireball. Therefore, the collapse of a HMNS is a promising scenario for generating short-duration gamma-ray bursts and an accompanying burst of gravitational waves and neutrinos.     

黑洞自转角速度的演化特征

在同时考虑吸积和Blandford-Znajek过程的条件下,详细讨论了吸积盘（包括薄盘和厚盘）中心黑洞的自转角速度Ω_h的演化特征.结果表明,当黑洞的无量纲角动量a_*演化到某一临界时,Ω_h对时间的变化率由正变负,从而推断在顺行吸积过程事Ω_h先增后减,演化非单调,因而存在一个极大值.并指出黑洞的无量纲角动理a_*比黑洞的角动量J和自转角速度Ω_h更适合描写吸积盘中心黑洞自转状态的演 关键词：     

Energy dissipation and angular momentum transfer within a magnetically torqued accretion disc

We discuss transportation and redistribution of energy and angular momentum in the magnetic connection (MC) process and Blandford-Payne (BP) process. MC results in readjusting the interior viscous torque, and its effects are operative not only in but also beyond the MC region. The BP process is invoked to transfer the “excessive” angular momentum from an accretion disc. In addition, we derive a criterion for the interior viscous torque to resolve the puzzle of the overall equilibrium of angular momentum in disc accretion. It turns out that the efficiency of BP at extracting angular momentum and the intensity of the outflow are required to be greater than some critical values.

     

Two Mechanisms for Extracting Energy and Angular Momentum from a Rotating Black Hole

Two mechanisms of extracting energy and angular momentum from a rotating black hole, the Blandford-Znajek (BZ) process and magnetic coupling (MC) of a rotating black hole (BH) with the surrounding accretion disk, are discussed in detail by using a modified equivalent circuit. We obtain the same value for the BZ power as given by Lee. The strength of the power and torque produced by the rotating BH in the BZ process are compared with those in MC process. In addition, entropy production on the BH horizon due to the BZ process and that due to MC process are compared and discussed also by using the modified equivalent circuit.     

Structure,rearrangements and evaporation of rotating atomic clusters

Eigenvector-following techniques are used to explore the classical effective potential energy surface of rotating clusters composed of identical rare gas atoms. Precise determination of transition states reveals the mechanisms for atomic rearrangements and allows us to calculate rate constants for the evaporation of an atom from the cluster surface using RRK theory. We examine the variation in the effective potential energy surface with the magnitude of the angular momentum, with particular reference to centrifugal distortion and the resulting spectroscopic constants, the Hessian index of the stationary points, and the centrifugal barriers to evaporation. Most of the calculations are performed using a pairwise additive Lennard-Jones potential, but comparisons are made with the accurate Aziz potential augmented by a three-body Axilrod-Teller term.     

Hawking radiation of five-dimensional rotating black hole

Applying the Damour–Ruffini method, we have considered the Hawking radiation of the five-dimensional rotating black hole. When taking the energy conservation and angular momentum conservation into consideration and considering the reaction of the radiation of the particle to spacetime, we see that the exact radiation spectrum is not purely thermal and the angular momentum of the black hole is quantized.     

A new metric for rotating black holes in Gauss-Bonnet gravity

This paper presents a new metric and studies slowly rotating Gauss-Bonnet black holes with a nonvanishing angular momentum in five dimensional anti-de Sitter spaces.Taking the angular momentum parameter a up to second order,the slowly rotating black hole solutions are obtained by working directly in the action.In addition,it also finds that this method is applicable in higher order Lovelock gravity.     

Binary-black-hole encounters, gravitational bursts, and maximum final spin

The spin of the final black hole in the coalescence of nonspinning black holes is determined by the "residual" orbital angular momentum of the binary. This residual momentum consists of the orbital angular momentum that the binary is not able to shed in the process of merging. We study the angular momentum radiated, the spin of the final black hole, and the gravitational bursts in a sequence of equal mass encounters. The initial orbital configurations range from those producing an almost direct infall to others leading to numerous orbits before infall, with multiple bursts of radiation. Our sequence consists of orbits with fixed impact parameter. What varies is the initial linear momentum of the black holes. For this sequence, the final black hole of mass M_{h} gets a maximum spin parameter a/M_{h} approximately 0.823, with this maximum occurring for initial orbital angular momentum L/M_{h};{2} approximately 1.176.     

Radiation spectrum of a high-dimensional rotating black hole

This study extends the classical Damour-Ruffini method and discusses Hawking radiation in a (n + 4)-dimensional rotating black hole. Under the condition that the total energy and angular momentum of spacetime are conservative, but angular momentum a = J/M of unit mass of the black hole is variable, taking into consideration the reaction of the radiation of the particle to the spacetime, a new Tortoise coordinate transformation and discuss the black hole radiation spectrum is discussed. The radiation spectru...     

Fate of an accretion disc around a black hole when both the viscosity and dark energy is in effect

This paper deals with the viscous accretion flow of a modified Chaplygin gas towards a black hole as the central gravitating object. A modified Chaplygin gas is a particular type of dark energy model which mimics of radiation era to phantom era depending on the different values of its parameters. We compare the dark energy accretion with the flow of adiabatic gas. An accretion disc flowing around a black hole is an example of a transonic flow. To construct the model, we consider three components of the Navier–Stokes equation, the equation of continuity and the modified Chaplygin gas equation of state. As a transonic flow passes through the sonic point, the velocity gradient being apparently singular there, it gives rise to two flow branches: one in-falling, the accretion and the other outgoing, the wind. We show that the wind curve is stronger and the wind speed reaches that of light at a finite distance from the black hole when dark energy is considered. Besides, if we increase the viscosity, the accretion disc is shortened in radius. These two processes acting together make the system deviate much from the adiabatic accretion case. It shows a weakening process for the accretion procedure by the work of the viscous system influencing both the angular momentum transport and the repulsive force of the modified Chaplygin gas.     

Standing Rankine-Hugoniot Shocks in Black Hole Accretion Discs

We study the problem of standing shocks in viscous disc-like accretion flows around black holes. For the first time we parametrize such a flow with two physical constants, namely the specific angular momentum accreted by the black hole j and the energy quantity K. By providing the global dependence of shock formation in the j- K parameter space, we show that a significant parameter region can ensure solutions with Rankine-Hugoniot shocks; and that the possibilities of shock formation are the largest for inviscid flows, decreasing with increasing viscosity, and ceasing to exist for a strong enough viscosity. Our results support the view that the standing shock is an essential ingredient in black hole accretion discs and is a general phenomenon in astrophysics, and that there should be a continuous change from the properties of inviscid flows to those of viscous ones.     

Black hole solutions in heterotic string theory on a torus

We construct the general electrically charged, rotating black hole solution in the heterotic string theory compactified on a six-dimensional torus and study its classical properties. This black hole is characterized by its mass, angular momentum, and a 28-dimensional electric charge vector. We recover the axion-dilaton black holes and Kaluza-Klein black holes for special values of the charge vector. For a generic black hole of this kind, the 28-dimensional magnetic dipole moment vector is not proportional to the electric charge vector, and we need two different gyromagnetic ratios for specifying the relation between these two vectors. We also give an algorithm for constructing a 58 parameter rotating dyonic black hole solution in this theory, characterized by its mass, angular momentum, a 28-dimensional electric charge vector and a 28-dimensional magnetic charge vector. This is the most general asymptotically flat black hole solution in this theory consistent with the no-hair theorem.     

Accretion,primordial black holes and standard cosmology

Primordial black holes evaporate due to Hawking radiation. We find that the evaporation times of primordial black holes increase when accretion of radiation is included. Thus, depending on accretion efficiency, more primordial black holes are existing today, which strengthens the conjecture that the primordial black holes are the proper candidates for dark matter.