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## The dimensional reduction of eleven dimensional supergravity into a product of Robertson-Walker spacetime and the seven sphere

### Pramana (1991-03-01) 36: 245-252 , March 01, 1991

The dimensional reduction of eleven dimensional supergravity is discussed. It is shown that there is no dimensional reduction onto Robertson-Walker space with the asymmetric tensor*F* giving a realistic fluid. Furthermore it is shown that the ansatz’s for the scale factor*R:R=at*^{n}, R=a exp (*bt*^{n}), and*R=aZ*^{n}, there is no dimensional reduction except the known example of the Freund-Rubin-Englert solution.

## Role of a scalar field in the radiation dominated epoch of the development of the universe

### Astrophysics (2008-01-01) 51: 121-129 , January 01, 2008

The recently discovered accelerated expansion of the universe is of current interest in theoretical research on the evolution of the universe. The cause of this behavior is presumably the presence of dark energy, which has been estimated to form up to 70% of the universe and generates a “repulsive force.” In this paper a cosmological model is constructed which takes the dark energy into account in a Jordan-Brans-Dicke tensor-scalar model with a dominant, nonminimally coupled scalar field in the presence of a cosmological scalar. The radiation dominant epoch is discussed.

## Black hole physics in globally hyperbolic space-times

### Pramana (1982-05-01) 18: 385-396 , May 01, 1982

The usual definition of a black hole is modified to make it applicable in a globally hyperbolic space-time. It is shown that in a closed globally hyperbolic universe the surface area of a black hole must eventually decrease. The implications of this breakdown of the black hole area theorem are discussed in the context of thermodynamics and cosmology. A modified definition of surface gravity is also given for non-stationary universes. The limitations of these concepts are illustrated by the explicit example of the Kerr-Vaidya metric.

## Non-singular cosmologies in the conformally invariant gravitation theory

### Pramana (1976-11-01) 7: 344-354 , November 01, 1976

It is shown that in the framework of a conformally invariant gravitation theory, the singularity which is present in some anisotropic universes in general relativity is due to a wrong choice of conformal frame. Frames exist in which these models can be made singularity free.

## Angular fluctuations in the cosmic microwave background owing to initial perturbations in the radiation temperature

### Astrophysics (2007-10-01) 50: 515-524 , October 01, 2007

A new method is proposed to account for multiple scattering by electrons in calculations of the correlation functions describing the angular fluctuations in the cosmic microwave background radiation (CMBR). The apparatus of the theory of radiative transport with Rayleigh scattering is used. The problem is reduced to solving an integral equation for the vector source function (dependent only on time), along with differential equations for the other quantities (scalar potentials, baryon velocities, etc.) which show up in the problem. The quantities which describe the angular fluctuations in the CMBR (in the temperature and in the polarization) are then calculated by integrating the vector source function along the line of sight. As an illustration, the correlation functions and power spectra are calculated for the case where the fluctuations are produced by some initial gaussian perturbations of the CMBR.

## Gravitational fields with space-times of Bianchi type IX

### Pramana (1986-07-01) 27: 63-72 , July 01, 1986

Spatially homogeneous space-times of Bianchi type IX are considered. A general scheme for the derivation of exact solutions of Einstein’s equations corresponding to perfect fluid plus pure radiation fields is outlined. Some simple rotating Bianchi type IX cosmological models are presented. The details of these solutions are also discussed.

## Causal dissipation in Robertson-Walker cosmological models

### Pramana (1998-02-01) 50: 105-108 , February 01, 1998

Some of the Robertson-Walker cosmological models filled with a fluid with bulk viscosity have been derived which are consistent with causal thermodynamics. The models are discussed briefly.

## Statistical isotropy of the cosmic microwave background

### Pramana (2004-03-01) 62: 793-796 , March 01, 2004

The breakdown of statistical homogeneity and isotropy of cosmic perturbations is a generic feature of ultra-large scale structure of the cosmos, in particular, of non-trivial cosmic topology. The statistical isotropy (SI) of the cosmic microwave background temperature fluctuations (CMB anisotropy) is sensitive to this breakdown on the largest scales comparable to, and even beyond the cosmic horizon. We propose a set of measures,*K*_{l} (*l* = 1, 2,3,...) which for non-zero values indicate and quantify statistical isotropy violations in a CMB map. We numerically compute the predicted*K*_{l} spectra for CMB anisotropy in flat torus universe models. Characteristic signatures of different models in the*K*_{l} spectrum are noted.

## String cosmology

### Pramana (1999-12-01) 53: 1105-1114 , December 01, 1999

In this review, we discuss various cosmological issues related to our Universe from a string theoretic perspective. We analyse the pre-big bang cosmological scenario which appears naturally in this context due to the existence of scale factor duality symmetry in string theory. We then discuss some of the attractive and problematic features of this scenario. Finally, we introduce a method which is powerful enough to search for cosmological solutions in various low energy limits of string theories.

## String cosmology in Bianchi I space-time

### Pramana (1990-01-01) 34: 1-11 , January 01, 1990

Some cosmological solutions of massive strings are obtained in Bianchi I space-time following the techniques used by Letelier and Stachel. A class of solutions corresponds to string cosmology associated with/without a magnetic field and the other class consists of pure massive strings, obeying the Takabayashi equation of state*ρ*=(1+*W*)*λ*.