Searching for Inflation in Simple String Theory Models: An Astrophysical Perspective

TL;DR

This paper explores the potential for inflation within simple string theory models, demonstrating that many vacua do not support inflation and highlighting the complexity of identifying inflating solutions in the string landscape.

Contribution

It provides a pedagogical framework for calculating cosmological predictions from string theory and analyzes specific models, revealing limitations in their inflationary potential.

Findings

Numerical analysis fails to find inflation in studied models.

Many vacua differ only by a constant factor in the potential.

Simple models may not capture the complexity needed for inflation.

Abstract

Attempts to connect string theory with astrophysical observation are hampered by a jargon barrier, where an intimidating profusion of orientifolds, Kahler potentials, etc. dissuades cosmologists from attempting to work out the astrophysical observables of specific string theory solutions from the recent literature. We attempt to help bridge this gap by giving a pedagogical exposition with detailed examples, aimed at astrophysicists and high energy theorists alike, of how to compute predictions for familiar cosmological parameters when starting with a 10-dimensional string theory action. This is done by investigating inflation in string theory, since inflation is the dominant paradigm for how early universe physics determines cosmological parameters. We analyze three explicit string models from the recent literature, each containing an infinite number of "vacuum" solutions. Our numerical investigation of some natural candidate inflatons, the so-called "moduli fields", fails to find inflation. We also find in the simplest models that, after suitable field redefinitions, vast numbers of these vacua differ only in an overall constant multiplying the effective inflaton potential, a difference which affects neither the potential's shape nor its ability to support slow-roll inflation. This illustrates that even having an infinite number of vacua does not guarantee having inflating ones. This may be an artifact of the simplicity of the models that we study. Instead, more complicated string theory models appear to be required, suggesting that explicitly identifying the inflating subset of the string landscape will be challenging.