  Drastic changes in the early universe such as first-order phase transition
can produce a stochastic gravitational wave (GW) background. We investigate the
testability of a scale invariant extension of the standard model (SM) using the
GW background produced by the chiral phase transition in a strongly interacting
QCD-like hidden sector, which, via a SM singlet real scalar mediator, triggers
the electroweak phase transition. Using the Nambu--Jona-Lasinio method in a
mean field approximation we estimate the GW signal and find that it can be
tested by future space based detectors.
