Dark Holograms and Gravitational Waves
  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.
