Insights on fundamental physics from Gravitational Wave backgrounds

Speaker: 

Dr. Davide Racco, Postdoctoral Researcher, Eidgenössische Technische Hochschule Zürich  - ETH

Abstract:

The era of Gravitational Wave (GW) astronomy is an exciting time to explore what GW backgrounds can uncover about fundamental physics and cosmology. I will discuss some of these directions, spanning from ground- and space-based interferometers to Pulsar Timing Arrays (PTAs).
The incoming experiment LISA will ultimately confirm or disprove a Dark Matter candidate like Primordial Black Holes, whose only unconstrained mass range selects a frequency band for the induced GWs where LISA is most sensitive. 
Additional targets of LISA and ground-based experiments like ET are primordial GW backgrounds produced in a finite time, as e.g. phase transitions. I will discuss how the low-frequency part of their spectrum 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 comment on the discrimination of this signal from astrophysical backgrounds in LISA. I will finally highlight the importance of these considerations for the GW background recently measured at PTAs, where the QCD crossover of the Standard Model has an unavoidable and distinctive effect on a primordial GW signal.