Dark Holograms and Gravitational Waves

TL;DR

This paper computes gravitational wave spectra from cosmological phase transitions in strongly coupled holographic models, showing potential detectability by current and future GW observatories, and explores unique spectral features like double peaks.

Contribution

It provides the first detailed holographic calculations of GW spectra from dark sector phase transitions, including corrections for redshift effects in multi-transition scenarios.

Findings

GW signals may be detectable by interferometers and Pulsar Timing Arrays.

Spectra can exhibit double peaks when two phase transitions occur at different temperatures.

Potential explanation for NANOGrav's recent GW observations.

Abstract

Spectra of stochastic gravitational waves (GW) generated in cosmological first-order phase transitions are computed within strongly correlated theories with a dual holographic description. The theories are mostly used as models of dark sectors. In particular, we consider the so-called Witten-Sakai-Sugimoto model, a $SU(N)$ gauge theory coupled to different matter fields in both the fundamental and the adjoint representations. The model has a well-known top-down holographic dual description which allows us to perform reliable calculations in the strongly coupled regime. We consider the GW spectra from bubble collisions and sound waves arising from two different kinds of first-order phase transitions: a confinement/deconfinement one and a chiral symmetry breaking/restoration one. Depending on the model parameters, we find that the GW spectra may fall within the sensibility region of ground-based and space-based interferometers, as well as of Pulsar Timing Arrays. In the latter case, the signal could be compatible with the recent potential observation by NANOGrav. When the two phase transitions happen at different critical temperatures, characteristic spectra with double frequency peaks show up. Moreover, in this case we explicitly show how to correct the redshift factors appearing in the formulae for the GW power spectra to account for the fact that adiabatic expansion from the first transition to the present times cannot be assumed anymore.