Phase transition during inflation and the gravitational wave signal at pulsar timing arrays

The gravitational wave (GW) signal offers a promising window into the dynamics of the early Universe. The recent results from the pulsar timing arrays (PTAs) could be the first glimpse of such new physics. In particular, they could point to new details during inflation, which cannot be probed by other means. We explore the possibility that the new results could come from the secondary GWs sourced by curvature perturbations, generated by a first-order phase transition during inflation. Based on the results of a field-theoretic lattice simulation of the phase transition process, we show that the GW signal generated through this mechanism can account for the new results from the PTAs. We analyze the spectral shape of the signal in detail. Future observations can use such information to distinguish the GW signal considered here from other possible sources.