Natural Chain Inflation

TL;DR

This paper develops a formalism for Chain Inflation, embedding it in high-energy physics, and demonstrates a viable model with an axion in supergravity that aligns with cosmological observations.

Contribution

It introduces a new effective formalism for calculating CMB observables in Chain Inflation and provides a concrete axion-based model compatible with cosmological constraints.

Findings

Upper limit on inflation scale: $V_*^{1/4}< 10^{12}$ GeV

Density perturbations originate from tunneling probability, not quantum fluctuations

Model consistent with $ ext{Lambda CDM}$ and cosmological data

Abstract

In Chain Inflation the universe tunnels along a series of false vacua of ever-decreasing energy. The main goal of this paper is to embed Chain Inflation in high energy fundamental physics. We begin by illustrating a simple effective formalism for calculating Cosmic Microwave Background (CMB) observables in Chain Inflation. Density perturbations seeding the anisotropies emerge from the probabilistic nature of tunneling (rather than from quantum fluctuations of the inflation). To obtain the correct normalization of the scalar power spectrum and the scalar spectral index, we find an upper limit on the scale of inflation at horizon crossing of CMB scales, $V_*^{1/4}< 10^{12}$~GeV. We then provide an explicit realization of chain inflation, in which the inflaton is identified with an axion in supergravity. The axion enjoys a perturbative shift symmetry which is broken to a discrete remnant by instantons. The model, which we dub `natural chain inflation' satisfies all cosmological constraints and can be embedded into a standard $\Lambda$CDM cosmology. Our work provides a major step towards the ultraviolet completion of chain inflation in string theory.