Gravitational Wave Probe of Gravitational Dark Matter from Preheating

Following our previous studies on gravitational dark matter (GDM) production in the early Universe of preheating,we forecast high-frequency gravitational wave (GW) spectra as the indirect probe of such GDM. We use proper lattice simulations to handle the resonance,and to solve the GW equation of motion with resonance induced scalar field excitations as the source term.Our numerical results show that the Higgs scalar excitations in the Higgs preheating model give rise to magnitudes of GW energy density spectra of order 10⁻¹⁰ at frequencies 10-10³ MHz, whereas the inflaton fluctuation excitations in the inflaton self-resonant preheating model yield magnitudes of GW energy density spectrum up to 10⁻⁹~(10⁻¹¹) at frequencies near $30~(2)$ MHz for the index $n=4~(6)$.