Simulations of helical inflationary magnetogenesis and gravitational waves

Abstract

Using numerical simulations of helical inflationary magnetogenesis in a low reheating temperature scenario, we show that the magnetic energy spectrum is strongly peaked at a particular wavenumber that depends on the reheating temperature. Gravitational waves (GWs) are produced at frequencies between 3 nHz and 50 mHz for reheating temperatures between 150 MeV and 3 × 10 5 GeV, respectively. At and below the peak frequency, the stress spectrum is always found to be that of white noise. This implies a linear increase of GW energy per logarithmic wavenumber interval, instead of a cubic one, as previously thought. Both in the helical and nonhelical cases, the GW spectrum is followed by a sharp drop for frequencies above the respective peak frequency. In this magnetogenesis scenario, the presence of a helical term extends the peak of the GW spectrum and therefore also the position of the aforementioned drop toward larger frequencies compared to the case without helicity. This might make a difference in it being detectable with space interferometers. The efficiency of GW production is found to be almost the same as in the nonhelical case, and independent of the reheating temperature, provided the electromagnetic energy at the end of reheating is fixed to be a certain fraction of the radiation energy density. Also, contrary to the case without helicity, the electric energy is now less than the magnetic energy during reheating. The fractional circular polarization is found to be nearly hundred per cent in a certain range below the peak frequency range.

Leading-order nonlinear gravitational waves from reheating magnetogeneses

“…where RD refers to the radiation-dominated era, which starts at the end of reheating, a is the scale factor, α is chosen to be 2, to avoid the backreaction problem [57], or 1, which enables reheating temperatures above the electroweak (EW) scale [56] (see Sec. IV), and β > 0 parameterizes the reheating temperature T r .…”

“…In Fourier space, the relevant equations for the fourpotential modes ñ during reheating take the form [56,58]…”

“…We are interested in studying the leading-order nonlinear term producing gravitational radiation, for which we require strong sources. For this reason, we consider in this work the production of GWs by reheating magnetogeneses, which yield electromagnetic (EM) strengths that depend only on the initial field and can grow several orders of magnitude [55,56]. For simplicity, we defer the study of the nonlinear effect produced by hydromagnetic turbulence from cosmological phase transitions to future work, since it requires a fully relativistic framework to reach plasma and/or Alfvén velocities near speed of light, as expected for very strong sources.…”

“…There have been recent analytical works on nonhelical and helical magnetogeneses during the reheating era [57,58], which circumvent known difficulties such as the backreaction and strong coupling problems [59]. Numerical simulations of GWs from these magnetogeneses have also been performed [55,56] using the Pencil Code [60].…”

Gauge Field Production and Schwinger Reheating in Runaway Axion Inflation

“…These phenomena during inflation have been studied to constrain the strength of the coupling through the cosmological observations [40][41][42][43][44][45][46]. Moreover, if the U(1) gauge field is the one in the Standard Model of particle physics (SM), it can also explain the baryon asymmetry of the Universe [47][48][49][50] and the origin of the intergalactic magnetic fields [51][52][53][54][55] (See also the studies on the gauge field amplification during reheating [56][57][58][59][60]). It is natural that we expect that it would also lead to a successful reheating after kination, if the tachyonic instability of the U(1) gauge fields is sufficiently effective during kination.…”