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Pulsar Timing Arrays (PTAs) have reported evidence for a stochasticgravitational wave (GW) background at nHz frequencies, possibly originating inthe early Universe. We show that the spectral shape of the low-frequency(causality) tail of GW signals sourced at temperatures around $T\gtrsim 1$ GeVis distinctively affected by confinement of strong interactions (QCD), due tothe corresponding sharp decrease in the number of relativistic species.Bayesian analyses in the NANOGrav 15 years and the previous International PTAdatasets reveal a significant improvement in the fit with respect to cubicpower-law spectra, previously employed for the causality tail. This suggeststhat the inclusion of Standard Model effects on GWs can have a potentiallydecisive impact on model selection.

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