Exact Scale-Invariant Background of Gravitational Waves from Cosmic
  Defects
  We demonstrate that any scaling source in the radiation era produces a
background of gravitational waves with an exact scale-invariant power spectrum.
Cosmic defects, created after a phase transition in the early Universe, are
such a scaling source. We emphasise that the result is independent of the
topology of the cosmic defects, the order of phase transition, and the nature
of the symmetry broken, global or gauged. As an example, using large-scale
numerical simulations, we calculate the scale invariant gravitational wave
power spectrum generated by the dynamics of a global O(N) scalar theory. The
result approaches the large N theoretical prediction as N^(-2), albeit with a
large coefficient. The signal from global cosmic strings is O(100) times larger
than the large N prediction.
