A viable scenario for Inflationary Magnetogenesis

Magnetic fields have been observed in galaxy, galaxy clusters and even in voids. A lower bound of strength 10^−15 G on Mpc scales has been suggested by gamma ray observation of Blazars. The precise mechanism for the origin of these large- scale magnetic fields still does not have a satisfactory explanation. The gener- ation of these fields is supposed to have happened by the amplification of seed fields. In order to generate the observed fields with known amplification mech- anisms, seed fields of sufficient strength need to be produced sufficiently early. Generation of these seed fields during inflation is one of the possible scenarios suggested in the literature. One of the model of inflationary magnetogenesis is f^2 F_μν F_μν type model (Ratra model) where f is a function of the inflaton field. This model, however, suffers from certain drawbacks such as strong coupling and back-reaction. A possible scenario where these problems are avoided has been suggested. In this scenario the coupling function (f) grows during inflation and transits to a decaying phase post-inflation. By choosing a suitable evolution of coupling function, it has been demonstrated that the above-mentioned difficul- ties (arising from strong coupling and back-reaction) are avoided. However, to avoid back-reaction post inflation, reheating has to be below ≈ 1.7 × 10^4 GeV. The generated magnetic energy spectrum is blue and it satisfies the gamma- ray bounds. If we consider reheating at QCD (150 MeV) epoch, the estimated present day magnetic field strength and the corresponding correlation length are 1.4 × 10^−12 G and 6.1 × 10^−4 Mpc, respectively.