Nonsingular Cosmologies from Branes

Space - like branes , accelerating cosmologies and the near ‘ horizon ’ limit

It is known that there exist two different classes of time dependent solutions in the form of space-like (or S)-branes in the low energy M/string theory. Accelerating cosmologies are known to arise from S-branes in one class, but not in the other where the time-like holography in the dS/CFT type correspondence may be more transparent. We show how the accelerating cosmologies arise from S-branes...

Unstable growth of curvature perturbations in nonsingular bouncing cosmologies.

Bouncing cosmologies require an ekpyrotic contracting phase (w≫1) in order to achieve flatness, homogeneity, and isotropy. Models with a nonsingular bounce further require a bouncing phase that violates the null energy condition (w<-1). We show that the transition from the ekpyrotic phase to the bouncing phase creates problems for cosmological perturbations. A component of the adiabatic curvatu...

A class of homogeneous scalar-tensor cosmologies with a radiation fluid

We present a class of new exact homogeneous cosmological solutions with a radiation fluid for all scalar-tensor theories. The solutions belong to Bianchi type V Ih cosmologies. Explicit examples of nonsingular homogeneous scalartensor cosmologies are also given. 04.50.+h, 04.20.Jb, 98.80.Hw Typeset using REVTEX ∗E-mail: [email protected]

A Study of Accelerating Cosmologies from Superstring/M Theories

We study aspects of the accelerating cosmologies obtained from the compactification of vacuum solution and S2-branes of superstring/M theories. Parameter dependence of the resulting expansion of our universe and internal space is examined for all cases. We find that accelerated expansions are obtained also from spherical internal spaces, albeit the solution enters into contracting phase eventua...

Introduction to Palatini Theories of Gravity and Nonsingular Cosmologies