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 super horizon at the time of production. Such gravitational waves are expected from the self ordering of randomly oriented scalar fields which can be present during at her mal phase transition or during preheating after hybrid inflation. We find that, if the gravitational wave source acts only during as mall fraction of the Hubble time, the gravitational wave spectrum at frequencies lower than the expansion rate at the time of production behaves as Omega(GW)(f) proportional to f(3) with an amplitude much too small to be observable by gravitational wave observatories like LIGO, LISA or BBO. On the otherhand, if the source is active for a much longer time, until a given mode which is initially superhorizon (k(eta*) << 1), enters the horizon, for k(eta) greater than or similar to 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 wavebackground can compete with the one generated during inflation, and distinguishing both may require extra information.