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We demonstrate that any scaling source in the radiation era produces abackground of gravitational waves with an exact scale-invariant power spectrum.Cosmic defects, created after a phase transition in the early Universe, aresuch a scaling source. We emphasise that the result is independent of thetopology of the cosmic defects, the order of phase transition, and the natureof the symmetry broken, global or gauged. As an example, using large-scalenumerical simulations, we calculate the scale invariant gravitational wavepower spectrum generated by the dynamics of a global O(N) scalar theory. Theresult approaches the large N theoretical prediction as N^(-2), albeit with alarge coefficient. The signal from global cosmic strings is O(100) times largerthan the large N prediction.

Exact Scale-Invariant Background of Gravitational Waves from Cosmic Defects