Primordial gravity wave fossils and their use in testing inflation

TL;DR

This paper discusses a novel effect where primordial gravity waves imprint a lasting signature on the Universe's large-scale structure, offering a new way to test inflationary models through future observations.

Contribution

It introduces a second-order perturbation effect of primordial gravity waves on large-scale structure and forecasts its detectability with upcoming experiments, highlighting its potential to probe inflation.

Findings

Square Kilometre Array unlikely to surpass CMB constraints

Futuristic experiments could achieve high sensitivity to tensor modes

Potential to measure the tensor spectral index

Abstract

A new effect is described by which primordial gravity waves leave a permanent signature in the large scale structure of the Universe. The effect occurs at second order in perturbation theory and is sensitive to the order in which perturbations on different scales are generated. We derive general forecasts for the detectability of the effect with future experiments, and consider observations of the pre-reionization gas through the 21 cm line. It is found that the Square Kilometre Array will not be competitive with current cosmic microwave background constraints on primordial gravity waves from inflation. However, a more futuristic experiment could, through this effect, provide the highest ultimate sensitivity to tensor modes and possibly even measure the tensor spectral index. It is thus a potentially quantitative probe of the inflationary paradigm.