String Cosmology

TL;DR

This paper introduces string cosmology, focusing on effective actions, the role of the dilaton in inflation, and challenges of modeling eternal acceleration within string theory.

Contribution

It provides an overview of low-energy string effective actions, analyzes exact solutions with a linear dilaton, and discusses scenarios for cosmic acceleration and exit strategies.

Findings

Exact solutions with linear dilaton induce expanding universes.

Dilaton-driven inflationary scenarios are explored.

Challenges in modeling eternal acceleration in string theory are discussed.

Abstract

`Old' String Theory is a theory of one-dimensional extended objects, whose vibrations correspond to excitations of various target-space field modes including gravity. In these lectures I will give an introduction to low-energy Effective Target-Space Actions derived from conformal invariance conditions of the underlying sigma models in string theory, discuss cosmology, emphasizing the role of the dilaton field in inducing inflationary scenaria and in general expanding string universes. Specifically, I shall analyse some exact solutions of string theory with a linear dilaton, and discuss their role in inducing expanding Robertson-Walker Universes. I will mention briefly pre-Big-Bang scenaria of String Cosmology, in which the dilaton plays a crucial role. In view of recent claims on experimental evidence on the existence of cosmic acceleration in the universe today, with a positive non-zero cosmological constant (de Sitter type), I shall also discuss difficulties of incorporating such Universes with eternal acceleration in the context of critical string theory, and present scenaria for a graceful exit from such a phase.