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.
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.
122267670
Some static solutions in the general scalar‐tensor theory
Exact solutions are found for the static gravitational fields for a matter‐free space in the general scalar‐tensor theory of Nordtvedt–Barker–Schwinger. The singular behavior of some of the invariants has also been discussed for the solutions in Barker and Schwinger theories.
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.
124426353
Lyra's geometry and cosmology: A Review
The Lyra's geometry, which is a generalization of Weyl's geometry (removing the defect of non-integrability of length transfer) and the gravitational theory and cosmology based on this geometry have been reviewed. Further, cosmological models with both constant and time dependent displacement field have been discussed. Some of these models solve the singularity, entropy and horizon problems. Finally some possibilities of further problems and their investigations have been pointed out.
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.
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.
2151015
Quantum stress tensor in self-similar spherical dust collapse
We calculate the quantum stress tensor for a massless scalar field in the 2D self-similar spherical dust collapse model which admits a naked singularity. We find that the outgoing radiation flux diverges on the Cauchy horizon. This may have two consequences. The resultant back reaction may prevent the naked singularity from forming, thus preserving cosmic censorship through quantum effects. The divergent flux may lead to an observable signature differentiating naked singularities from black holes in astrophysical observations.
115706838
The effect of cosmic inhomogeneities on the average cosmological dynamics
It is generally assumed that on sufficiently large scales the Universe is well-described as a homogeneous, isotropic FRW cosmology with a dark energy. Does the formation of nonlinear cosmic inhomogeneities produce a significant effect on the average large-scale FLRW dynamics? As an answer, we suggest that if the length scale at which homogeneity sets in is much smaller than the Hubble length scale, the back-reaction due to averaging over inhomogeneities is negligible. This result is supported by more than one approach to study of averaging in cosmology. Even if no single approach is sufficiently rigorous and compelling, they are all in agreement that the effect of averaging in the real Universe is small. On the other hand, it is perhaps fair to say that there is no definitive observational evidence yet that there indeed is a homogeneity scale which is much smaller than the Hubble scale, or for that matter, if today's Universe is indeed homogeneous on large scales. If the Copernican principle can be observationally established to hold, or is theoretically assumed to be valid, this provides strong evidence for homogeneity on large scales. However, even this by itself does not say what the scale of homogeneity is. If that scale is today comparable to the Hubble radius, only a fully non-perturbative analysis can establish or rule out the importance of cosmological back-reaction. This brief elementary report summarizes some recent theoretical developments on which the above inferences are based.
119261020
Models of Wave-function Collapse, Underlying Theories, and Experimental Tests
We describe the state of the art in preparing, manipulating and detecting coherent molecular matter. We focus on experimental methods for handling the quantum motion of compound systems from diatomic molecules to clusters or biomolecules. Molecular quantum optics offers many challenges and innovative prospects: already the combination of two atoms into one molecule takes several well-established methods from atomic physics, such as for instance laser cooling, to their limits. The enormous internal complexity that arises when hundreds or thousands of atoms are bound in a single organic molecule, cluster or nanocrystal provides a richness that can only be tackled by combining methods from atomic physics, chemistry, cluster physics, nanotechnology and the life sciences. We review various molecular beam sources and their suitability for matter-wave experiments. We discuss numerous molecular detection schemes and give an overview over diffraction and interference experiments that have already been performed with molecules or clusters. Applications of de Broglie studies with composite systems range from fundamental tests of physics up to quantum-enhanced metrology in physical chemistry, biophysics and the surface sciences. Nanoparticle quantum optics is a growing field, which will intrigue researchers still for many years to come. This review can, therefore, only be a snapshot of a very dynamical process.