Cosmological interpretation for the stochastic signal in pulsar timing arrays

The pulsar timing array (PTA) collaborations have recently reported compelling evidence for a stochastic signal consistent with a gravitational-wave background. In this paper, we combine the latest data sets from NANOGrav, PPTA, and EPTA to explore cosmological interpretations for the PTA signal from first-order phase transitions, domain walls, and cosmic strings, respectively. We find that the domain wall model is strongly disfavored with the Bayes factors compared with the first-order phase transitions and cosmic strings being 1/90 and 1/189, respectively, breaking the degeneracy among these models in individual data set. We also find that: (1) a strong phase transition at temperatures below the electroweak scale is favored, and the bubble collisions make the dominant contribution to the energy density spectrum; (2) a small reconnection probability p < 1.55 × 10⁻¹ allowed by strings in (super)string theory is favored at the 95% confidence level, and ground-based detectors can further constrain the parameter space.