The low-frequency part of a primordial gravitational wave spectrum generated by local physics in a wide class of phenomena (such as a phase transition), is largely fixed by causality, offering a clean window into the early Universe. The physics of low-frequency modes allows to probe in a model-independent way the Hubble rate at the time of GW production, the equation-of-state of the Universe at subsequent times and the presence of free-streaming particles at primordial epochs.
I will highlight the importance of these considerations for the GW background recently measured at Pulsar Timing Arrays, where the Standard Model has an unavoidable and distinctive effect on a primordial GW signal.
