Starting inflation from inhomogeneous initial conditions with momentum

TL;DR

This study uses numerical relativity to explore how cosmic inflation can start from highly inhomogeneous initial conditions, including cases with non-zero momentum, and finds that inflation is generally robust even under extreme inhomogeneities.

Contribution

The paper introduces methods to construct and evolve inhomogeneous initial conditions with momentum in inflationary models, addressing previous technical challenges and demonstrating inflation's robustness.

Findings

Inflation can originate from highly inhomogeneous states with large gradients and kinetic energies.

Large scalar field velocities towards non-inflationary values can be mitigated by inhomogeneities, enabling inflation.

Inflation remains robust even when initial conditions include black hole formation or near-Planckian densities.

Abstract

We investigate the circumstances under which cosmic inflation can arise from very inhomogeneous initial conditions using numerical relativity simulations. Previous studies have not considered cases with non-zero momentum density due to technical challenges with solving the coupled Einstein constraint equations. Here we address these, introducing and comparing several different ways of constructing cosmological initial conditions with inhomogeneous scalar field and time derivative profiles. We evolve such initial conditions with large inhomogeneities in both single- and two-field inflationary models. We study cases where the initial gradient and kinetic energy are much larger than the inflationary energy scale, and black holes can form, as well as cases where the initial scalar potential energy is comparable, as in scenarios where inflation occurs at nearly Planckian densities, finding large-field inflation to be generally robust. We consider examples of initial conditions where a large scalar field velocity towards non-inflationary values would prevent inflation from occurring in the homogeneous case, finding that the addition of large gradients in the scalar field can actually dilute this effect, with the increased expansion and non-vanishing restoring force leading to inflation.