Have pulsar timing array methods detected a cosmological phase transition

We show that the recent detection of a gravitational wave (GW) background reported by various pulsar timing array (PTA) collaborations including NANOGrav-15 yr, PPTA, EPTA/InPTA, and CPTA can be explained in terms of first order phase transitions (FOPTs) from dark sector models (DSM). Specifically, we explore a model for first order phase transitions that involves the Majoron, a Nambu-Goldstone boson that is emerging from the spontaneous symmetry breaking of a U(1)L or U(1)B-L symmetry. We show how the predicted GW power spectrum, with a realistic choice of the FOPT parameters, is consistent with 1-σ deviations from the estimated parameters of the background detected by the PTA collaborations.