Observational prospects for gravitational waves from hidden or dark chiral phase transitions

We study the gravitational wave (GW) signature of first-order chiral phase transitions ($\chi$PT) in strongly interacting hidden or dark sectors. We do so using several effective models in order to reliably capture the relevant non-perturbative dynamics. This approach allows us to explicitly calculate key quantities characterizing the $\chi$PT without having to resort to rough estimates. Most importantly, we find that the transition's inverse duration $\beta$ normalized to the Hubble parameter $H$ is at least two orders of magnitude larger than typically assumed in comparable scenarios, namely $\beta/H\gtrsim\mathcal{O}(10^4)$. The obtained GW spectra then suggest that signals from hidden $\chi$PTs occurring at around 100 MeV can be in reach of LISA, while DECIGO and BBO may detect a stochastic GW background associated with transitions between roughly 1 GeV and 10 TeV. Signatures of transitions at higher temperatures are found to be outside the range of any currently proposed experiment. Even though predictions from different effective models are qualitatively similar, we find that they may vary considerably from a quantitative point of view, which highlights the need for true first-principle calculations such as lattice simulations.