Gravitational wave production from preheating with trilinear interactions

TL;DR

This paper studies how trilinear interactions during preheating after inflation can produce a significant stochastic gravitational wave background, which, despite being undetectable directly due to high frequencies, could be constrained indirectly via cosmological observations.

Contribution

It provides the first detailed analysis of gravitational wave production from preheating with trilinear couplings using lattice simulations, highlighting the amplitude and frequency characteristics of the resulting SGWB.

Findings

Large couplings produce high-amplitude SGWB with $h^2\Omega_{\rm GW}^{(0)} \simeq 5\times10^{-9}$.

The peak frequency of GWs is in the range $10^6-10^8$ Hz, beyond current detection capabilities.

Constraints on $\Delta N_{\rm eff}$ could indirectly probe these gravitational waves.

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 $\Delta N_{\rm eff}$.