Institute for Advanced Study/Princeton University Early Universe/Cosmology Lunch Discussion

The direct detection of gravitational waves in 2015, thanks to the LIGO and Virgo interferometers, opened a new window on our Universe. The discoveries during the first three observing runs already had an extraordinary impact on astrophysics, cosmology, and fundamental physics, and the fourth run is ongoing.

The GW community is now looking at the next long-prepared step: `third-generation' detectors. Thanks to an increase of more than one order of magnitude in sensitivity and a larger bandwidth, Einstein Telescope and Cosmic Explorer will have an outstanding potential, capable of triggering fundamental discoveries.

I will focus on two techniques to exploit GW detectors to learn about the evolution of our Universe: `bright sirens' and the detection of stochastic GW backgrounds of cosmological origin. In the first part of the talk, I will review the basics of the bright siren method and discuss some of its subtleties. I will then present recent forecasts on the reconstruction of cosmological parameters and modified GW propagation at ET and CE through this method.

In the second part, I will instead focus on the problem of the characterization and subtraction of the astrophysical GW sources (resolved and as a background) from 3G data, to dig out an underlying cosmological background. I will discuss how the impact of astrophysical foregrounds can and should be evaluated with a filter function that optimizes the given cosmological search. I will then present prospects for a 3G network comprising ET and CE, a realistic CBC population, and example cosmological backgrounds.