BLACK HOLES IN AN ACCELERATING UNIVERSE

Abstract

In this thesis, I have considered the scenario of the accretion of phantom energy onto various types of black holes. The phantom energy is a strange kind of energy existing in the universe that drives the cosmic accelerated expansion. Among various black holes, we have chosen the following for our study: Schwarzschild, Riessner-Nordstrom, primordial and BTZ black holes. I also discuss various possibilities of phantom energy as viscous Chaplygin gas and modified variable Chaplygin gas.

The first two chapters are devoted to introductions to cosmology and black holes. In the former, I discuss the standard cosmological model and focus on dark energy theory. In the second chapter, we briefly discuss the astrophysical implications of black holes along with the derivation of two well-known black hole solutions in general relativity. I also discuss a lower space dimensional black hole for our further use.

In the third chapter, we shall discuss the phantom energy accretion onto primordial black holes of various masses. These black holes were formed from the gravitational collapse of primordial soup in the early Universe. They hypothetically radiate energy via Hawking evaporation process. I have found some interesting consequences of the accretion of phantom energy onto a primordial black hole. One of which is that to have the primordial black hole decay now it would have to be more massive initially.

In the fourth chapter, I study the accretion of dark energy on a ReissnerNordstrom (RN) black hole. Since the RN black hole contains charge, the charge remains unaffected during the accretion of phantom energy. The phantom energy interacts only with the mass of the black hole to decrease it. Due to mass reduction, a stage is naturally reached when the magnitude of charge exceeds the mass of the black hole. At this point, there appears a naked singularity. The appearance of a naked singularity is forbidden according to the Cosmic Censorship Hypothesis (CCH). Hence charged black holes serves the purpose of making naked singularities through my suggested mechanism. In chapter five, I study the accretion of phantom energy onto a Schwarzschild black hole but taking the former to be represented by extended forms of Chaplygin gas. Our analysis is an extension of the results of [1] who used a linear equation of state for phantom energy. I discuss two cases of polytropic equation of state for phantom energy.

In the sixth chapter, I study the same problem of the accretion of phantom energy on to a Schwarzschild black hole but assuming the former to be viscous. The presence of bulk viscosity in the dark energy can produce the affect of phantom energy (i.e. the equation of state becomes super-negative) and hence cause the accelerated expansion. The accretion mechanism of phantom energy is adapted from the fourth chapter. I will find that the mass of the black hole decreases faster in the viscous phantom case compared to the non-viscous one. Thus bulk viscosity plays a crucial role in the overall evolution of black holes. The origin of bulk viscosity is not clear and is purely ad hoc in the present context.

In seventh chapter, we study the accretion of phantom energy onto a lower space dimensional black hole. An interesting finding is that the rate of change mass of this black hole is independent of its mass and depends only on energy density and pressure of the phantom energy. I also discuss some thermodynamical aspects of the accretion process. In particular, we find that the first and second laws of thermodynamics are violated. I then employ the generalized second law of thermodynamics. Assuming that the later law holds, it yields a condition on the pressure and mass parameters of the model.

Finally in chapter eight, I conclude the thesis and present some open problems related to this work.