Chain Inflation Reconsidered

TL;DR

This paper reevaluates density perturbations in chain inflation, showing that bubble collisions induce subsequent transitions, and finds that achieving the correct power spectrum requires many phase transitions per Hubble time, posing model-building challenges.

Contribution

It provides an analytical calculation of density fluctuations in chain inflation and discusses the implications of strong coupling and limited effective field theory validity.

Findings

Density perturbations are small, with non-Gaussianities of order 1.

Approximately 10^8 phase transitions per Hubble time are needed for the correct power spectrum.

Models of chain inflation are strongly coupled with limited effective field theory validity.

Abstract

We investigate density perturbations in the chain inflation scenario, where the inflaton undergoes successive tunneling transitions along one field direction. First we show that when the bubble walls associated with such a phase transition meet, they induce the next phase transition and continue to propagate as phase interfaces. Then we present an analytical calculation of the density fluctuations and an estimate of non-Gaussianities such as f_{NL}, which we find to be small (of order 1). To get the right amplitude for the power spectrum, there have to be 10^8 phase transitions per Hubble time, a significant model building challenge. We find that working models of chain inflation must be rather strongly coupled, and thus have a very limited range of validity as effective field theories. We discuss generalizations to the multiple field case, the curvaton scenario, and noncanonical kinetic terms.