An Analytical Study of the Primordial Gravitational-Wave-Induced Contribution to the Large-Scale Structure of the Universe

TL;DR

This paper analytically investigates how primordial gravitational waves induce corrections to the matter density contrast in large-scale structures, providing new insights into their potential observational signatures.

Contribution

It presents the first full analytical solution for the density contrast correction caused by primordial GWs during radiation dominance, considering different phases of the early universe.

Findings

Cold dark matter density contrast surpasses radiation contrast during radiation era.

Analytical solutions for GW-induced density perturbations during different cosmic epochs.

Discussion of the evolution of these perturbations into the matter and dark energy eras.

Abstract

The imprint of gravitational waves (GWs) on large-scale structures (LSS) is a useful and promising way to detect or to constrain them. Tensor fossils have been largely studied in the literature as an indirect way to detect primordial GWs. In this paper we analyze a new effect induced by primordial GWs: a correction to the density contrast of the underlying matter distribution of LSS, as well as its radiation counterpart, induced by the energy density fluctuation of the gravitational radiation. We perform our derivation of the full analytical solution of the density contrast for waves entering the horizon during radiation dominance. We account for two phases in the radiation era, depending on the main contributor to the perturbed energy density of the Universe. By comparing the density contrast of cold dark matter and radiation -- sourced by linear gravitational waves only -- we conclude that the former overcomes the latter at some time in the radiation era, a behaviour analogous to their linear counterpart. Then we conclude by discussing the case of density perturbations produced by GWs entering the Hubble radius during the matter era as well as their evolution in the late dark-energy dominated phase.