A Louder Gravitational Wave Bang from a Fast-Expanding Universe

TL;DR

This paper explores how a faster early-universe expansion rate can amplify gravitational wave signals from dark sector phase transitions, potentially explaining recent pulsar timing array observations.

Contribution

It introduces a simple model showing that modified expansion rates can significantly enhance gravitational wave signals from dark sector phase transitions.

Findings

Faster expansion rates boost gravitational wave signals from dark phase transitions.

Modified early-universe dynamics can reconcile dark sector models with NANOGrav data.

Enhanced signals expand viable dark sector scenarios for gravitational wave detection.

Abstract

A strong first-order phase transition in a dark sector may produce all or part of the low-frequency gravitational wave signal recently reported by the NANOGrav Collaboration and other pulsar timing arrays. Here we point out, with a simple toy model, that even if the amplitude of the gravitational wave background from the dark phase transition is insufficient to match the NANOGrav signal, a modified expansion rate at early times may considerably enhance the gravitational wave signal. In particular, a faster-than-standard expansion rate, triggered, for instance, by the presence of one or more additional sources of energy density redshifting with higher powers of temperatures than radiation, boosts upper limits on the gravitational wave signal from first-order cosmological phase transitions, enlarging the slate of possible dark sector scenarios matching the NANOGrav signal.