118586619
Cosmology with non-minimally coupled k-field
We consider a non-minimally coupled (with gravity) scalar field with non-canonical kinetic energy. The form of the kinetic term is of Dirac–Born–Infeld form. We study the early evolution of the universe when it is sourced only by the k-field, as well as late time evolution when both the matter and k-field are present. For the k-field, we have considered constant potential as well as potential inspired from boundary string field theory. We show that it is possible to have an inflationary solution in early time as well as late time accelerating phases. The solutions also exhibit attractor properties in a sense that they do not depend on the initial conditions for certain values of the parameters.
119357931
Generalized Chaplygin gas, accelerated expansion and dark energy matter unification
We consider the scenario emerging from the dynamics of a generalized d-brane in a (d +1 ; 1) spacetime. The equation of state describing this system is given in terms of the energy density, , and pressure, p, by the relationship p = A= ,w hereA is a positive constant and 0 < 1. We discuss the conditions under which homogeneity arises and show that this equation of state describes the evolution of a universe evolving from a phase dominated by non-relativistic matter to a phase dominated by a cosmological constant via an intermediate period where the eective equation of state is given by p = .
119288549
Hořava-Lifshitz cosmology with generalized Chaplygin gas
We investigate cosmological scenarios of generalized Chaplygin gas in a universe governed by Hořava-Lifshitz gravity. We consider both the detailed and non-detailed balance versions of the gravitational background, and we include the baryonic matter and radiation sectors. We use observational data from Type Ia Supernovae, Baryon Acoustic Oscillations, and Cosmic Microwave Background, along with requirements of Big Bang Nucleosynthesis, to constrain the parameters of the model, and we provide the corresponding likelihood contours. We deduce that the present scenario is compatible with observations. Additionally, examining the evolution of the total equation-of-state parameter, we find in a unified way the succession of the radiation, matter, and dark energy epochs, consistently with the thermal history of the universe.
119358901
WMAP constraints on Cardassian model
We have investigated the constraints on the Cardassian model using the recent results from the Wilkinson Microwave Anisotropy Probe (WMAP) for the locations of the peaks of the Cosmic Microwave Background (CMB) anistropy spectrum. We find that the model is consistent with the recent observational data for certain range of the model parameter $n$ and the cosmological parameters. We find that the Cardassian model is favoured compared to $\Lambda$CDM model, for a higher spectral index ($n_s \approx 1$) together with lower value of Hubble parameter $h$ ($h\leq 0.71$). But for smaller values of $n_s$, both $\Lambda$CDM and Cardassian are equally favoured. Also, irrespective of Supernova constraint, CMB data alone predicts current acceleration of the universe in this model. We have also studied the constraint on the $\sigma_{8}$, the rms density fluctuations at $8h^{-1}$ Mpc scale.
15686768
Structure formation and CMBR anisotropy spectrum in the inflessence model
The inflessence model has recently been proposed in an attempt to explain both early inflation and present day accelerated expansion within a single mechanism. The model has been successfully tested against the Hubble diagram of Type Ia Supernovae, the shift parameter, and the acoustic peak parameter. As a further mandatory test, we investigate here structure formation in the inflessence model determining the evolution of matter density contrast $\delta \equiv \delta \rho_{\rm M}/\rho_{\rm M}$ in the linear regime. We compare the growth factor $D(a) \equiv \delta/a$ and the growth index $f(z) \equiv {\rm dln}{\delta}/{\rm dln}{a}$ to these same quantities for the successful concordance Λ CDM model with a particular emphasis on the role of the inflessence parameters $(\gamma, z_Q)$. We also evaluate the anisotropy spectrum of the cosmic microwave background radiation (CMBR) to check whether the inflessence model may be in agreement with the observations. We find that, for large values of $(\gamma, z_Q)$, structure formation proceeds in a similar way to that in the Λ CDM scenario, and it is also possible to nicely fit the CMBR spectrum.
120897327
VACUUMLESS COSMIC STRINGS IN BRANS–DICKE THEORY
The gravitational fields of vacuumless global and gauge strings have been studied in Brans–Dicke theory under the weak field assumption of the field equations. It has been shown that both global and gauge string can have repulsive as well as attractive gravitational effect in Brans–Dicke theory which is not so in General Relativity.
118381804
An Introduction to Duality Symmetries in String Theory
In this review I discuss some basic aspects of non-perturbative string theory. The topics include test of duality symmetries based on the analysis of the low energy effective action and the spectrum of BPS states, relationship between different duality symmetries, and an introduction to M-theory.