Reheating constraints in inflationary magnetogenesis

Among primordial magnetogenesis models, inflation is a prime candidate to explain the current existence of cosmological magnetic fields. Assuming conformal invariance to be restored after inflation, their energy density decreases as radiation during the decelerating eras of the universe, and in particular during reheating. Without making any assumptions on inflation, on the magnetogenesis mechanism and on how the reheating proceeded, we show that requiring large scale magnetic fields to remain subdominant after inflation gives non-trivial constraints on both the reheating equation of state parameter and the reheating energy scale. In terms of the so-called reheating parameter, we find that ln(Rrad) > -10.1 for large scale magnetic fields of the order 5 x 10^(-15) Gauss today. This bound is then compared to those already derived from Cosmic Microwave Background (CMB) data by assuming a specific inflationary model. Avoiding magnetic field backreaction is always complementary to CMB and can give more stringent limits on reheating for all high energy models of inflation. For instance, a large field matter dominated reheating cannot take place at an energy scale lower than typically 500 GeV if the magnetic field strength today is Bo = 5 x 10^(-15) G, this scale going up to 10^(10) GeV if Bo = 10^(-9) G.