A peak in the power spectrum of primordial gravitational waves induced by primordial magnetic fields

Abstract

It has been argued that there exist extragalactic magnetic fields in the range from 10 −17 to 10 −9 Gauss with a cosmological coherence length. One plausible explanation for the origin of the extragalactic magnetic fields would be quantum fluctuations of the electromagnetic fields during inflation. It is also believed that primordial gravitational waves (PGWs) arise out of quantum fluctuations during inflation. We study the graviton-photon conversion process in the presence of background magnetic fields and find that the process induces the tachyonic instability of the PGWs. As a consequence, a peak appears in the power spectrum of PGWs.It turns out that the peak height depends on the direction of observation. The peak frequency could be in the range from 10 −5 to 10 3 Hertz for GUT scale inflation. Hence, the observation of PGWs could provide a new window for probing primordial magnetic fields.