Constraining cosmological ultra-large scale structure using numerical relativity

TL;DR

This paper uses numerical relativity to simulate nonlinear ultra-large scale inhomogeneities from cosmic inflation, providing new constraints on their amplitude based on CMB data and highlighting the importance of nonlinear effects in early universe cosmology.

Contribution

It introduces a numerical relativity approach to constrain ultra-large scale structure from inflation, accounting for nonlinear gravitational effects.

Findings

ULSS curvature perturbations of order unity are compatible with CMB data.

Nonlinear gravitational effects significantly impact constraints on early universe inhomogeneities.

Numerical relativity is a valuable tool for cosmological structure analysis.

Abstract

Cosmic inflation, a period of accelerated expansion in the early universe, can give rise to large amplitude ultra-large scale inhomogeneities on distance scales comparable to or larger than the observable universe. The cosmic microwave background (CMB) anisotropy on the largest angular scales is sensitive to such inhomogeneities and can be used to constrain the presence of ultra-large scale structure (ULSS). We numerically evolve nonlinear inhomogeneities present at the beginning of inflation in full General Relativity to assess the CMB quadrupole constraint on the amplitude of the initial fluctuations and the size of the observable universe relative to a length scale characterizing the ULSS. To obtain a statistically significant number of simulations, we adopt a toy model in which inhomogeneities are injected along a preferred direction. We compute the likelihood function for the CMB quadrupole including both ULSS and the standard quantum fluctuations produced during inflation. We compute the posterior given the observed CMB quadrupole, finding that when including gravitational nonlinearities, ULSS curvature perturbations of order unity are allowed by the data, even on length scales not too much larger than the size of the observable universe. Our results illustrate the utility and importance of numerical relativity for constraining early universe cosmology.