121240575
Stationary Universe in General Relativity and Lyra's Geometry
The stationary universe solutions have been studied using the metrics of VAN Stockum (1937) and Godel (1949) in a New ST theory (Saez and Ballester, 1985), general relativity with variable cosmological term (Narlikar et al., 1991), Lyra's geometry (Sen, 1957) and Cosmic strings. It has been shown that the stationary universe has rigid rotation in ST theory. In the presence of variable cosmological term, there is non-rigid rotation in the presence of Zeldovich fluid for van Stockum metric. In Lyra's geometry the only solution is possible when the gauge function β is variable and there is Zeldovich fluid. However, in the presence of cosmic strings, there is no solution with van Stockum or Godel metrics.
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.
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.
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.
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.
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.
119183596
Fully Characterizing Axially Symmetric Szekeres Models with Three Data Sets
Inhomogeneous exact solutions of General Relativity with zero cosmological constant have been used in the literature to challenge the \Lambda CDM model. From one patch Lema\^itre-Tolman-Bondi (LTB) models to axially symmetric quasi-spherical Szekeres (QSS) Swiss-cheese models, some of them are able to reproduce to a good accuracy the cosmological data. It has been shown in the literature that a zero-\Lambda -LTB model with a central observer can be fully determined by two data sets. We demonstrate that an axially symmetric zero-\Lambda -QSS model with an observer located at the origin can be fully reconstructed from three data sets, number counts, luminosity distance and redshift drift. This is a first step towards a future demonstration involving five data sets and the most general Szekeres model.
120882934
Redshift drift as a test for discriminating between different cosmological models
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.
17484415
Einstein gravity as the thermodynamic limit of an underlying quantum statistics
The black hole area theorem suggests that classical general relativity is the thermodynamic limit of a quantum statistics. The degrees of freedom of the statistical theory cannot be the spacetime metric. We argue that the statistical theory should be constructed from a noncommutative gravity, whose classical, and thermodynamic, approximation is Einstein gravity. The noncommutative gravity theory exhibits a duality between quantum fields and macroscopic black holes, which is used to show that the black hole possesses an entropy of the order of its area. The principle on which this work is based also provides a possible explanation for the smallness of the cosmological constant, and for the quantum measurement problem, indicating that this is a promising avenue towards the merger of quantum mechanics and gravity.
30095666
Hawking radiation from the quantum Lemaitre-Tolman-Bondi model
In an earlier paper, we obtained exact solutions of the Wheeler-DeWitt equation for the Lemaitre-Tolman-Bondi model of gravitational collapse, employing a lattice regularization. In this paper, we derive Hawking radiation in nonmarginally bound models from our exact solutions. We show that a nonvanishing energy function does not spoil the (approximate) Planck spectrum near the horizon. We can also reliably compute corrections to the Bogoliubov coefficient because our solutions are exact. The corrections are obtained by going beyond the near-horizon region and are shown to introduce additional greybody factors, which modify the blackbody spectrum of radiation from the black hole.