On the Anisotropy of the Gravitational Wave Background from Massless Preheating

TL;DR

This paper derives a formula for the anisotropy in the gravitational wave background caused by superhorizon modulations during preheating, and uses simulations to estimate the strength of these anisotropies, which could be detectable.

Contribution

It provides a general master formula for GW anisotropies from preheating and quantifies the expected anisotropy level using lattice simulations.

Findings

Angular power spectrum of GW anisotropies is approximately 3x10^{-4}.

Anisotropies are of order 1% on large scales.

Long wavelength modes are amplified during parametric resonance.

Abstract

When a light scalar field is present during inflation, its value will vary on superhorizon scales, modulating the preheating process at the end of inflation. Consequently, the amplitude of the gravitational wave (GW) background produced during preheating will also be modulated. The observed energy density of this background will therefore appear anisotropic at different angles in the sky. We provide a master formula for the angular power spectrum C_l of the anisotropies in the GW background from preheating, valid for any scenario where the anisotropies are due to the superhorizon modulation of a light degree of freedom. Using lattice field theory simulations of massless preheating with g^2/\lambda = 2, we find a flat angular spectrum l(l+1)C_l \approx 3x10^{-4}, which represents a strong anisotropy of order 1% variations on large angular scales. For our choice of couplings, long wavelengths are amplified most strongly during parametric resonance, which is crucial for the development of the anisotropies. If future direct detection GW observatories are capable of detecting backgrounds of cosmological origin, they should be able to detect this effect. This could eventually become a powerful tool to discriminate among inflationary and preheating scenarios.