Gravitational Waves from Global Second Order Phase Transitions
  Global second-order phase transitions are expected to produce scale-invariant gravitational wave spectra. In this manuscript we explore the dynamics of a symmetry-breaking phase transition using lattice simulations. We explicitly calculate the stochastic gravitational wave background produced during the transition and subsequent self-ordering phase. We comment on this signal as it compares to the scale-invariant spectrum produced during inflation. 
John T. Giblin Jr and Larry R. Price and Xavier Siemens and Brian   Vlcek
astro-ph.CO gr-qc