Footprints of the QCD Crossover on Cosmological Gravitational Waves at Pulsar Timing Arrays

Journal Article · 20 February 2024 · Physical Review Letters

DOI: https://doi.org/10.1103/PhysRevLett.132.081001 · OSTI ID:2309824

Franciolini, Gabriele ; Racco, Davide ; Rompineve, Fabrizio

Pulsar timing arrays (PTAs) have reported evidence for a stochastic gravitational wave (GW) background at nanohertz frequencies, possibly originating in the early Universe. We show that the spectral shape of the low-frequency (causality) tail of GW signals sourced at temperatures around $T\gtrsim 1\mathrm{GeV}$ is distinctively affected by confinement of strong interactions (QCD), due to the corresponding sharp decrease in the number of relativistic species, and significantly deviates from $\sim f^3$ commonly adopted in the literature. Bayesian analyses in the NANOGrav 15 years and the previous international PTA datasets reveal a significant improvement in the fit with respect to cubic power-law spectra, previously employed for the causality tail. While no conclusion on the nature of the signal can be drawn at the moment, our results show that the inclusion of standard model effects on cosmological GWs can have a decisive impact on model selection. Published by the American Physical Society 2024

• Sponsoring Organization: USDOE
• OSTI ID: 2309824
• Journal Information: Physical Review Letters, Journal Name: Physical Review Letters Journal Issue: 8 Vol. 132; ISSN 0031-9007; ISSN PRLTAO
• Publisher: American Physical SocietyCopyright Statement
• Country of Publication: United States
• Language: English