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.
127634712
Axisymmetric stationary gravitational and Maxwell fields in the general scalar tensor theory
It is shown that the axisymmetric stationary gravitational and Maxwell fields in the general scalar tensor theory can be obtained from the axisymmetric stationary Einstein — Maxwell fields. The scalar-tensor analogue of the Kerr-Newman solution has been obtained by this method.
116059635
Viscous cosmological models with particle creation in Brans-Dicke theory
The effect of bulk viscosity on the evolution of the spatially flat Friedmann-Robertson-Walker models in the context of open thermodynamical systems, which allow particle creation, is analyzed within the framework of Brans-Dicke theory. Particle creation and bulk viscosity have been considered as separated irreversible processes. To accommodate the viscous pressure and creation pressure, which is associated with creation of matter out of gravitational field, the energy-momentum tensor is modified. Dynamical behaviour of the models have been discussed.
121342410
Bianchi type-V cosmological models with perfect fluid and dark energy
We consider a self-consistent system of Bianchi type-V cosmology and binary mixture of perfect fluid and dark energy. The perfect fluid is taken to be one obeying the usual equation of state p=γρ with γ∈[0,1]. The dark energy is considered to be either the quintessence or Chaplygin gas. Exact solutions to the corresponding Einstein equations are obtained as a quadrature. The cases of disordered radiation and models with power-law and exponential expansion have discussed in detail. For large t, the models tend to be isotropic.
121342913
Higher-dimensional cosmological model with variable gravitational constant and bulk viscosity in Lyra geometry
We have studied five-dimensional homogeneous cosmological models with variableG and bulk viscosity in Lyra geometry. Exact solutions for the field equations have been obtained and physical properties of the models are discussed. It has been observed that the results of new models are well within the observational limit.
122321601
Scalar-tensor theories of gravitation: Foundations and prospects
A generalization of Einstein's theory is discussed in which the gravitation is described by a tensor and a scalar field. The theory is more consistent with Mach's principle and less reliant on absolute properties of space. The modification involves a violation of the “strong principle of equivalence” on which Einstein's theory is based. In the original version of this new theory, the “constant” of gravitationG is varying and particle masses are fixed. Later on another version of the theory was given in whichG is truly a constant and the particle masses vary. The two versions are related by a conformal transformation. The physical and mathematical foundations of this theory have been discussed and the field equations have been derived. The astrophysical and cosmological consequences of the theory have been elaborately reviewed.
119551280
Explicit cosmological coarse graining via spatial averaging
The present matter density of the Universe, while highly inhomogeneous on small scales, displays approximate homogeneity on large scales. We propose that whereas it is justified to use the Friedmann–Lemaître–Robertson–Walker (FLRW) line element (which describes an exactly homogeneous and isotropic universe) as a template to construct luminosity distances in order to compare observations with theory, the evolution of the scale factor in such a construction must be governed not by the standard Einstein equations for the FLRW metric, but by the modified Friedmann equations derived by Buchert (Gen Relat Gravit 32:105, 2000; 33:1381, 2001) in the context of spatial averaging in Cosmology. Furthermore, we argue that this scale factor, defined in the spatially averaged cosmology, will correspond to the effective FLRW metric provided the size of the averaging domain coincides with the scale at which cosmological homogeneity arises. This allows us, in principle, to compare predictions of a spatially averaged cosmology with observations, in the standard manner, for instance by computing the luminosity distance versus red-shift relation. The predictions of the spatially averaged cosmology would in general differ from standard FLRW cosmology, because the scale-factor now obeys the modified FLRW equations. This could help determine, by comparing with observations, whether or not cosmological inhomogeneities are an alternative explanation for the observed cosmic acceleration.
15678593
A simple derivation of the naked singularity in spherical dust collapse
We describe a simple method of determining whether the singularity that forms in the spherically symmetric collapse of inhomogeneous dust is naked or covered. This derivation considerably simplifies the analysis given in the earlier literature, while giving the same results as have been obtained previously.
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.
15099305
Gravitational collapse of an infinite dust cylinder
We examine the gravitational collapse of an infinite cylindrical distribution of time like dust. In order to simplify the calculation we make an assumption that the axial and azimuthal metric functions are equal. It is shown that the resulting solution describes homogeneous collapse. We show that the interior metric can be matched to a time dependent exterior. We also discuss the nature of the singularity in the matter region and show that it is covered.
118799695
The three and a half layers of dynamics : analog, digital, semi-digital, analog
Quantum theory is extremely successful in explaining most physical phenomena, and is not contradicted by any experiment. Yet, the theory has many puzzling features : the occurrence of probabilities, the unclear distinction between the microscopic and the macroscopic, the unexplained absence of superpositions in positions of macroscopic objects, the dependence of the theory on an external classical time, and the experimentally verified but peculiar `influence' outside the light-cone in EPR experiments. These puzzles point towards a conflict between quantum theory and our present understanding of spacetime structure, and suggest the existence of a deeper theory. In this essay we make the case that in the underlying theory the matter and spacetime degrees of freedom are non-commuting matrices, and yet the dynamics is analog. A digital quantum-theory like dynamics for matter as well as spacetime emerges in the statistical thermodynamic approximation to this deeper theory. When most of the matter clumps into macroscopic structures, it is shown to behave classically, and it induces classical dynamics on spacetime; this is the eventual analog limit, our macroscopic world. In between the digital layer and the uppermost analog layer is the realm of standard quantum theory - microscopic objects and their interaction with measuring apparatuses on a classical spacetime background : the semi-digital approximation. Such a multi-layered description of dynamics can explain the puzzling features of quantum theory, and is testable by ongoing laboratory experiments.