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

Dark gauge fields have been discussed as candidates for dark matter recently. If they existed, primordial dark magnetic fields during inflation would have existed. It is believed that primordial gravitational waves (PGWs) arise out of quantum fluctuations during inflation. We study the graviton-dark photon conversion process in the presence of background primordial dark 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 103 Hertz for GUT scale inflation. Hence, the observation of PGWs could provide a new window for probing primordial dark magnetic fields.

Automated summary

Dark gauge fields have been proposed as potential candidates for dark matter, and if they exist, primordial dark magnetic fields would have existed during inflation. It is believed that primordial gravitational waves arise from quantum fluctuations during inflation. This study investigates the conversion of gravitons into dark photons in the presence of background primordial dark magnetic fields and finds that this process induces a tachyonic instability in the primordial gravitational waves, leading to a peak in their power spectrum. The height of the peak depends on the observation direction, and the peak frequency can range from 10-5 to 103 Hertz for GUT scale inflation, suggesting that the observation of primordial gravitational waves could be a new way to probe primordial dark magnetic fields.