Detectable Gravitational Waves from (P)-reheating probes non-thermal
  Dark Matter
  We show that the stochastic Gravitational Waves (GW) signal generated during
the (p)reheating era can act as a novel probe of the non-thermal dark matter
production in the early Universe. Such scenarios are of utmost interest when no
other interaction between the visible and dark sectors is present, therefore
having no other detectability prospects. We consider the remnant energy in the
coherently oscillating inflaton zeroth mode to contribute as the observed relic
dark matter density in the Universe. To fully capture the nonlinear dynamics
and the effects of back-reactions during the oscillation, we resort to fully
nonlinear lattice simulation with pseudo-spectral methods to eliminate the
differencing noises. We investigate for models whose behavior during the
reheating era is of quadratic ($m_{\Phi}^2\Phi^2$ type) and find the typical
primordial stochastic GW background (SGWB) spectrum from scatterings among
highly populated inflaton modes behaving like matter, as expected, during this
nonlinear phase. We predict the detectable dark matter mass ranges within the
future GW detectors such as BBO, DECIGO, PTA, AION-MAGIS, and CE to be (MeV -
TeV) ranges. We conclude that with the current and future GW detectors
operating at high frequencies one will be able to probe non-thermal dark matter
which is otherwise impossible to test at laboratories and astrophysical
searches due to feeble or no interaction among the visible and dark sectors.
