Gravitational wave stochastic background from cosmological particle
  decay
  We assume that the cosmological dark matter is composed of massive neutral
scalar particles that decay into two massless particles. The decay produces a
stochastic background of gravitational waves (GWs) via a 'memory effect'
mechanism. We calculate the spectral amplitude and slope of the resulting
background, which is frequency independent (flat). We discuss its potential
observability and show that the resulting background might dominate the
cosmological GW background at frequencies above about 10 GHz. Penrose has
proposed a cosmological model in which dark matter particles have the Planck
mass and decay into two gravitons. For these, the spectrum has an additional
'direct' contribution from the decay products, which we also calculate. At low
frequencies, this direct contribution also has a flat spectrum but with a much
smaller amplitude than the memory part.
