Simultaneously probing the sound speed and equation of state of the early Universe with pulsar timing arrays

• Published in: Journal of cosmology and astroparticle physics 2024-04, Vol.2024 (4), p.11
• Main Authors: Liu, Lang, Wu, You, Chen, Zu-Cheng
• Format: Article
• Language: English
• Subjects: Arrays; Equations of state; Gravitational waves; gravitational waves / experiments; gravitational waves / sources; Heating; physics of the early universe; primordial black holes; Pulsars; Universe
• ISSN: 1475-7516; 1475-7516
• DOI: 10.1088/1475-7516/2024/04/011

Recently, several major pulsar timing array (PTA) collaborations have assembled strong evidence for the existence of a gravitational-wave background at frequencies around the nanohertz regime. Assuming that the PTA signal is attributed to scalar-induced gravitational waves, we jointly employ the PTA data from the NANOGrav 15-year data set, PPTA DR3, and EPTA DR2 to probe the conditions of the early Universe. Specifically, we explore the equation of state parameter ( w ), the reheating temperature ( T rh ), and the sound speed ( c s ), finding w = 0.59 +0.36 -0.40 (median + 90% credible interval), and T rh ≲ 0.2 GeV at the 95% credible interval for a lognormal power spectrum of the curvature perturbation. Furthermore, we compute Bayes factors to compare different models against the power-law spectrum model, effectively excluding the pressure-less fluid domination model. Our study underscores the significance of scalar-induced gravitational waves as a powerful tool to explore the nature of the early Universe.