Speakers: Davide Racco (Ethz)
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.
https://indico.in2p3.fr/event/32452/
