Primordial magnetic fields in scale-invariant inflation

Chiara Cecchini, Massimiliano Rinaldi


Magnetic fields permeate the universe probed so far, from planetary scales to the Intergalactic Medium. An intriguing possibility is that they are remnants of primordial fields that originated during inflation. However, the amplification of vector perturbations in an exponentially expanding universe requires the breaking of EM conformal invariance. To this end, one can introduce a time-dependent coupling between the EM tensor and a scalar field with a non-trivial dynamics. In this work, we consider inflationary magnetogenesis in a fundamental scale-invariant model of quadratic gravity. We investigate the optimal functional form of the coupling to generate a nearly scale-invariant spectrum of magnetic energy density and sufficiently strong fields at the end of inflation. The aim is to avoid both the strong coupling and back reaction problems, which are known to plague the simplest models of inflationary magnetogenesis.