  We show that the recent detection of a gravitational wave (GW) background
reported by various pulsar timing array (PTA) collaborations including
NANOGrav-15yr, PPTA, EPTA, 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-$\sigma$ deviations from the estimated parameters of the background detected
by the PTA collaborations.

Cosmology and Nongalactic Astrophysics;High Energy Astrophysical Phenomena;High Energy Physics - Phenomenology;High Energy Physics - Theory