Topological inflation with graceful exit

TL;DR

This paper presents a model of topological inflation that naturally ends via a nonminimally coupled scalar field, producing cosmological perturbations consistent with CMB data and solving the graceful exit problem.

Contribution

It introduces a nonminimally coupled scalar field to topological inflation, enabling a natural end to inflation and matching observational data.

Findings

Inflation ends naturally with the scalar field dynamics.

Monopole size shrinks after inflation, becoming negligible.

Predicted scalar spectral index around 0.955, consistent with Planck data.

Abstract

We investigate a class of models of topological inflation in which a super-Hubble-sized global monopole seeds inflation. These models are attractive since inflation starts from rather generic initial conditions, but their not so attractive feature is that, unless symmetry is again restored, inflation never ends. In this work we show that, in presence of another nonminimally coupled scalar field, that is both quadratically and quartically coupled to the Ricci scalar, inflation naturally ends, representing an elegant solution to the graceful exit problem of topological inflation. While the monopole core grows during inflation, the growth stops after inflation, such that the monopole eventually enters the Hubble radius, and shrinks to its Minkowski space size, rendering it immaterial for the subsequent Universe's dynamics. Furthermore, we find that our model can produce cosmological perturbations that source CMB temperature fluctuations and seed large scale structure statistically consistent (within one standard deviation) with all available data. In particular, for small and (in our convention) negative nonminimal couplings, the scalar spectral index can be as large as $n_s\simeq 0.955$, which is about one standard deviation lower than the central value quoted by the most recent Planck Collaboration.