The cosmological gravitational wave background from primordial density perturbations

TL;DR

This paper investigates how primordial density perturbations during the radiation era generate a gravitational wave background, which could be used by future detectors to probe the early universe's small-scale fluctuations.

Contribution

It provides calculations of the gravitational wave power spectra from primordial density perturbations, highlighting potential for future observational constraints on the primordial power spectrum.

Findings

Gravitational waves are produced at second-order from density fluctuations during radiation era.

Future detectors could constrain primordial power spectrum on small scales.

Computed GW background for power-law and delta-function spectra.

Abstract

We discuss the gravitational wave background generated by primordial density perturbations evolving during the radiation era. At second-order in a perturbative expansion, density fluctuations produce gravitational waves. We calculate the power spectra of gravitational waves from this mechanism, and show that, in principle, future gravitational wave detectors could be used to constrain the primordial power spectrum on scales vastly different from those currently being probed by large-scale structure. As examples we compute the gravitational wave background generated by both a power-law spectrum on all scales, and a delta-function power spectrum on a single scale.