Gravitational wave production from preheating with trilinear interactions

Abstract: We investigate the production of gravitational waves (GWs) during preheating with monomial/polynomial inflationary potentials, considering a trilinear coupling $\phi\chi^2$ between a daughter field $\chi$ and the inflaton $\phi$. For sufficiently large couplings, the trilinear interaction leads to an exponential production of $\chi$ particles, and as a result, a large stochastic GW background (SGWB) is generated throughout the process. We study the linear and non-linear dynamics of preheating with lattice simulations, following the production of GWs through all relevant stages. We find that large couplings lead to SGWBs with a large amplitude today, of the order of $h^2\Omega_{\rm GW}^{(0)} \simeq 5\cdot10^{-9}$. These backgrounds are however peaked at high frequencies $f_{\rm p} \sim 10^6-10^8$ Hz, which makes them undetectable by current/planned GW observatories. As the amount of GWs produced is in any case remarkable, we discuss the prospects for probing the SGWB indirectly by using constraints on the effective number of relativistic species in the universe $\Delta N_{\rm eff}$.