Inflation, space-borne interferometers and the expansion history of the Universe

TL;DR

This paper demonstrates that the spectral energy density of relic gravitons can be significantly higher at certain frequencies than previously thought if the universe's expansion history included phases faster or slower than radiation before nucleosynthesis, affecting gravitational wave detection and cosmological constraints.

Contribution

It reveals how non-standard post-inflationary expansion histories can enhance relic graviton signals in space-borne detectors, challenging conventional assumptions.

Findings

Spectral energy density can exceed standard predictions by up to eight orders of magnitude.

The shape of the gravitational wave spectrum encodes information about inflation and subsequent expansion phases.

Constraints on gravitational wave backgrounds inform both inflationary models and the universe's expansion history.

Abstract

According to the common wisdom, between a fraction of the mHz and few Hz the spectral energy density of the inflationary gravitons can be safely disregarded even assuming the most optimistic sensitivities of the space-borne detectors. In this analysis we show that this conclusion is evaded if, prior to nucleosynthesis, the post-inflationary evolution includes a sequence of stages expanding either faster or slower than radiation. As a consequence, contrary to the conventional lore, it is shown that below a fraction of the Hz the spectral energy density of the relic gravitons may exceed (even by eight orders of magnitude) the signal obtained under the hypothesis of radiation dominance throughout the whole expansion history prior to the formation of light nuclei. Since the slopes and the amplitudes of the spectra specifically reflect both the inflationary dynamics and the subsequent decelerated evolution, it is possible to disentangle the contribution of the relic gravitons from other (late-time) bursts of gravitational radiation associated, for instance, with a putative strongly first-order phase transition at the TeV scale. Hence, any limit on the spectral energy density of the relic gravitons in the mHz range simultaneously constrains the post-inflationary expansion history and the inflationary initial data.