Inhomogeneous initial conditions for inflation: A wibbly-wobbly timey-wimey path to salvation

TL;DR

This paper uses numerical relativity to show that inflationary models like Starobinsky and Higgs inflation are robust against various inhomogeneous initial conditions, including large fluctuations and inhomogeneities.

Contribution

It demonstrates through numerical simulations that inflation can emerge from highly inhomogeneous initial states, supporting its robustness across different inflation potentials.

Findings

Inflation occurs after oscillatory phases between gradient and kinetic energies.

Hubble-sized inhomogeneities can lead to primordial black hole formation.

Inflation remains robust regardless of initial inhomogeneity size or type.

Abstract

We use the 3+1 formalism of numerical relativity to investigate the robustness of Starobinsky and Higgs inflation to inhomogeneous initial conditions, in the form of either field gradient or kinetic energy density. Sub-Hubble and Hubble-sized fluctuations generically lead to inflation after an oscillatory phase between gradient and kinetic energies. Hubble-sized inhomogeneities also produce contracting regions that may end up forming primordial black holes, subsequently diluted by inflation. We analyse the dynamics of the preinflation era and the generation of vector and tensor fluctuations. Our analysis further supports the robustness of inflation to any size of inhomogeneity, in the field, velocity or equation of state. At large field values, the preinflation dynamics only marginally depends on the field potential and it is expected that such behaviour is universal and applies to any inflation potential of plateau-type, favoured by CMB observations after Planck.