Simulating relic gravitational waves from inflationary magnetogenesis

Abstract

We present three-dimensional direct numerical simulations of the production of magnetic fields and gravitational waves (GWs) in the early Universe during a low energy scale matter-dominated postinflationary reheating era, and during the early subsequent radiative era, which is strongly turbulent. The parameters of the model are determined such that it avoids a number of known physical problems and produces magnetic energy densities between 0.2% and 2% of the critical energy density at the end of reheating. During the subsequent development of a turbulent magnetohydrodynamic cascade, magnetic fields and GWs develop a spectrum that extends to higher frequencies in the millihertz (nanohertz) range for models with reheating temperatures of around 100 GeV (150 MeV) at the beginning of the radiation-dominated era. However, even though the turbulent cascade is fully developed, the GW spectrum shows a sharp drop for frequencies above the peak value. This suggests that the turbulence is less efficient in driving GWs than previously thought. The peaks of the resulting GW spectra may well be in the range accessible to space interferometers, pulsar timing arrays, and other facilities.

Tensor spectrum of turbulence-sourced gravitational waves as a constraint on graviton mass

“…Given that the drop extends over several orders of magnitude, the differences between the spatial and temporal spectra occur where the spatial spectrum is already very weak and below the noise level of the temporal spectrum. Note that this feature of a sharp drop is also present in a physically more realistic turbulence source driven by inflationary magnetogenesis [41]. The weak temporal signal beyond this drop is therefore most likely no longer reliable.…”