Gravitational waves from self-ordering scalar fields
  Gravitational waves were copiously produced in the early Universe whenever
the processes taking place were sufficiently violent. The spectra of several of
these gravitational wave backgrounds on subhorizon scales have been extensively
studied in the literature. In this paper we analyze the shape and amplitude of
the gravitational wave spectrum on scales which are superhorizon at the time of
production. Such gravitational waves are expected from the self ordering of
randomly oriented scalar fields which can be present during a thermal phase
transition or during preheating after hybrid inflation. We find that, if the
gravitational wave source acts only during a small fraction of the Hubble time,
the gravitational wave spectrum at frequencies lower than the expansion rate at
the time of production behaves as $\Omega_{\rm GW}(f) \propto f^3$ with an
amplitude much too small to be observable by gravitational wave observatories
like LIGO, LISA or BBO. On the other hand, if the source is active for a much
longer time, until a given mode which is initially superhorizon ($k\eta_* \ll
1$), enters the horizon, for $k\eta \gtrsim 1$, we find that the gravitational
wave energy density is frequency independent, i.e. scale invariant. Moreover,
its amplitude for a GUT scale scenario turns out to be within the range and
sensitivity of BBO and marginally detectable by LIGO and LISA. This new
gravitational wave background can compete with the one generated during
inflation, and distinguishing both may require extra information.
