Shocks in the Early Universe
  We point out a surprising consequence of the usually assumed initial
conditions for cosmological perturbations. Namely, a spectrum of Gaussian,
linear, adiabatic, scalar, growing mode perturbations not only creates acoustic
oscillations of the kind observed on very large scales today, it also leads to
the production of shocks in the radiation fluid of the very early universe.
Shocks cause departures from local thermal equilibrium as well as creating
vorticity and gravitational waves. For a scale-invariant spectrum and standard
model physics, shocks form for temperatures $1$ GeV$<T<10^{7}$ GeV. For more
general power spectra, such as have been invoked to form primordial black
holes, shock formation and the consequent gravitational wave emission provides
a signal detectable by current and planned gravitational wave experiments,
allowing them to strongly constrain conditions present in the primordial
universe as early as $10^{-30}$ seconds after the big bang.
