27984646
Quantum general relativity and Hawking radiation
In a previous paper we set up the Wheeler-DeWitt equation which describes the quantum general relativistic collapse of a spherical dust cloud. In the present paper we specialize this equation to the case of matter perturbations around a black hole, and show that, in the WKB approximation, the wave functional describes an eternal black hole in equilibrium with a thermal bath at Hawking temperature.
119218867
Quantum Gravitational Collapse and Hawking Radiation in 2+1 Dimensions
We develop the canonical theory of gravitational collapse in $2+1$ dimensions with a negative cosmological constant and obtain exact solutions of the Wheeler-DeWitt equation regularized on a lattice. We employ these solutions to derive the Hawking radiation from black holes formed in all models of dust collapse. We obtain an (approximate) Planck spectrum near the horizon characterized by the Hawking temperature ${T}_{\mathrm{H}}=\ensuremath{\hbar}\sqrt{G\ensuremath{\Lambda}M}/2\ensuremath{\pi}$, where $M$ is the mass of a black hole that is presumed to form at the center of the collapsing matter cloud and $\ensuremath{-}\ensuremath{\Lambda}$ is the cosmological constant. Our solutions to the Wheeler-DeWitt equation are exact, so we are able to reliably compute the gray-body factors that result from going beyond the near-horizon region.
118717812
Thermodynamics and the naked singularity in the Gamma-metric
We investigate a possible way of establishing a parallel between the third law of black hole mechanics, and the strong version of the third law of thermodynamics. We calculate the surface gravity and area for a naked singular null surface in the Gamma-metric and explain in what sense this behaviour violates thermodynamics.
118799017
MODIFIED GRAVITY AS A COMMON CAUSE FOR COSMIC ACCELERATION AND FLAT GALAXY ROTATION CURVES
Flat galaxy rotation curves and the accelerating Universe both imply the existence of a critical acceleration, which is of the same order of magnitude in both the cases, in spite of the galactic and cosmic length scales being vastly different. Yet, it is customary to explain galactic acceleration by invoking gravitationally bound dark matter, and cosmic acceleration by invoking a "repulsive" dark energy. Instead, might it not be the case that the flatness of rotation curves and the acceleration of the Universe have a common cause? In this essay we propose a modified theory of gravity. By applying the theory on galactic scales we demonstrate flat rotation curves without dark matter, and by applying it on cosmological scales we demonstrate cosmic acceleration without dark energy.
36641344
On the relation between causality and topology in the semiclassical universe.
119522704
Toward a midisuperspace quantization of LeMaitre-Tolman-Bondi collapse models
LeMaitre-Tolman-Bondi models of spherical dust collapse have been used and continue to be used extensively to study various stellar collapse scenarios. It is by now well known that these models lead to the formation of black holes and naked singularities from regular initial data. The final outcome of the collapse, particularly in the event of naked singularity formation, depends very heavily on quantum effects during the final stages. These quantum effects cannot generally be treated semiclassically as quantum fluctuations of the gravitational field are expected to dominate before the final state is reached. We present a canonical reduction of LeMaitre-Tolman-Bondi space-times describing the marginally bound collapse of inhomogeneous dust, in which the physical radius R, the proper time of the collapsing dust {tau}, and the mass function F are the canonical coordinates R(r), {tau}(r) and F(r) on the phase space. Dirac's constraint quantization leads to a simple functional (Wheeler-DeWitt) equation. The equation is solved and the solution can be employed to study some of the effects of quantum gravity during gravitational collapse with different initial conditions.
73593699
String Cosmological Models with Magnetic Field
A new class of string cosmological models withand without magnetic field in the context of aspace-time with G3 symmetry has beenpresented. In order to study the effects of magneticfield, the standard energy-momentum tensor is modified byincorporating additional term due to magnetic field. Thephysical and kinematical behaviours of the stringcosmological models have been discussed.
117011561
On exact static spherically symmetric solutions in general relativity, Einstein-Cartan theory and higher-dimensional theories
The equations describing the hydrostatic equilibrium of a static general relativistic fluid sphere are obtained for magnetofluids, Einstein-Cartan theory and anisotropic fluids in four dimensions and perfect fluids in D (D ≥ 4) dimensions. The method due to Hojman and Santamarina and Berger et al. is used to obtain a new and larger class of solutions. Illustrative examples have been given which lead to generalization of the Schwarschild exterior solution and the relativistic models for spiral galaxies and dense stars (Hojman et al.).
121368055
GENERAL CLASS OF SCALAR-TENSOR THEORIES: A REVIEW
The general class of scalar-tensor theories is discussed in which, in contrast to Brans-Dicke theory, the coupling parameter ω is not a constant but a function of the scalar field ϕ. Some other generalizations of the Brans-Dicke theory have also been discussed in brief. The physical aspects of these theories have been dealt. The cosmological models in scalar-tensor theories have been discussed in detail. Finally some open problems in these theories have been pointed out.
120858933
Generalized static electromagnetic fields in Brans--Dicke theory
A general class of static, axially symmetric solutions of the Brans–Dicke–Maxwell equations is obtained under the assumption −r2⋅grr=gtt⋅gφφ⋅gzz These solutions have been subjected to conformal transformation and they are found to be new solutions of the static, axially symmetric Einstein–Maxwell–scalar fields. We have also developed a more general set of solutions under the unit transformations given by Morganstern.
122726731
Bianchi type-I, V and VIo models in modified generalized scalar-tensor theory
AbstractIn modified generalized scalar-tensor (GST) theory, the cosmological term Λ is a function of the scalar field ϕ and its derivatives $$
\dot \phi ^2 
$$. We obtain exact solutions of the field equations in Bianchi Type-I, V and VIo space-times. The evolution of the scale factor, the scalar field and the cosmological term has been discussed. The Bianchi Type-I model has been discussed in detail. Further, Bianchi Type-V and VIo models can be studied on the lines similar to Bianchi Type-I model.