Related papers: Anisotropic Four-Dimensional NS-NS String Cosmolog…
We consider the dynamics of a Bianchi type I spacetime in the presence of dilaton and magnetic fields. The general solution of the Einstein-Maxwell dilaton field equations can be obtained in an exact parametric form. Depending on the…
Motivated from recent string theoretic results, a tachyonic potential is constructed for a spatially homogeneous and anisotropic background cosmology.
We generalize a five dimensional black hole solution of low energy effective string theory to arbitrary constant spatial curvature. After interchanging the signature of time and radius we reduce the 5d solution to four dimensions and obtain…
In this manuscript, we investigate the oscillatory behaviour of the anisotropy in the diagonal Bianchi-I spacetimes. Our starting point is a simplification of Einstein's equations using only observable or physical variables. As a…
A homogeneous anisotropic four dimensional spacetime with Lorentzian signature is constructed from an ungauged WZW model based on a non-semisimple Lie group. The associated non-linear $\sigma $-model describes string propagation in an…
Accelerating universe or the existence of a small and positive cosmological constant is probably the most pressing obstacle as well as opportunity to significantly improving the models of four-dimensional cosmology from fundamental theories…
We show that massless solutions to the Einstein-Vlasov system in a Bianchi I space-time with small anisotropy, i.e. small shear and small trace-free part of the spatial energy momentum tensor, tend to a radiation fluid in an Einstein-de…
We present an anisotropic cosmological model based on a new exact solution of Einstein equations. The matter content consists of an anisotropic scalar field minimally coupled to gravity and of two isotropic perfect fluids that represent…
A qualitative analysis is presented for spatially flat, isotropic and homogeneous cosmologies derived from the string effective action when the combined effects of a dilaton, modulus, two-form potential and central charge deficit are…
We consider new cosmological solutions with a collapsing, an intermediate and an expanding phase. The boundary between the expanding (collapsing) phase and the intermediate phase is seen by comoving observers as a cosmological past (future)…
We study the Bianchi-I cosmological model motivated by signals of statistical isotropy violation seen in cosmic microwave background (CMB) observations and others. To that end, we consider various kinds of anisotropic matter that source…
We present an effective Eulerian description, in the non-relativistic regime, of the growth of cosmological perturbations around a homogeneous but anisotropic Bianchi I spacetime background. We assume a small deviation from isotropy,…
We study the evolution of Bianchi-I space-times filled with a global unidirectional electromagnetic field $F_{mn}$ interacting with a massless scalar dilatonic field according to the law \Psi(\phi) F^{mn} F_{mn} where \Psi(\phi) > 0 is an…
We investigate the cosmological solutions in closed bosonic string theory in the presence of non zero tachyon condensate. We specifically obtain time dependent solutions which describe an anisotropic universe. We also discuss the nature of…
Study the behaviour and the evolution of the cosmological field equations in an homogeneous and anisotropic spacetime with two scalar fields coupled in the kinetic term. Specifically, the kinetic energy for the scalar field Lagrangian is…
We synthesize and extend the previous ideas about appearance of both noncommutative and Finsler geometry in string theory with nonvanishing B--field and/or anholonomic (super) frame structures \cite{vstring,vstr2,vnonc,vncf}. There are…
We consider an Einstein-Dirac-Maxwell system with two charged massless spinors coupled with an electromagnetic field, and construct a family of exact solutions to the system. The solution spacetime is an anisotropic generalization of the…
A Bianchi type-VI cosmological model in the presence of a magnetic flux together with a cloud of cosmic strings is considered. In general, the presence of a magnetic field imposes severe restrictions regarding the consistency of the field…
We study the locally rotational symmetry Bianchi type-I dark energy model in the framework of $f(T)$ theory of gravity, where $T$ denotes the torsion scalar. A viable cosmological model is undertaken and the isotropization of this latter is…
In this paper we examine the classical evolution of a cosmological model derived from the low-energy tree-level limit of a generic string theory. The action contains the metric, dilaton, central charge and an antisymmetric tensor field. We…