Most of the observable consequences of Dark Energy (DE) follow from its impact on the expansion history of the universe, which in turn affects the growth of density perturbations, and the age of the universe. The expansion history of the universe maps to a distance-redshift relation

Gravitational Wave (GW) signals from binary compact object mergers act as standard sirens, as they provide a self-calibrated luminosity distance to the source, while redshift information can be obtained from their electromagnetic (EM) counterparts and/or inferred through LSS observations.


The next generation of GW detectors, such as Einstein Telescope (ET) and Cosmic Explorer (CE) will increase the detection rate of binary neutron star (BNS) mergers and binary black hole (BBH) mergers by an enormous factor, reaching more than 100,000 events per year. This will enable probing the expansion rate of the universe without the distance calibration issues that currently affect analyses with type Ia supernovae, thanks to joint GW-EM detections from BNS mergers. Concerning GW signals from BBH mergers, in the absence of optical emission, redshift can be obtained, e.g., via the intensity mapping (IM) of neutral hydrogen (HI) 21 cm emission.


The goal of this “Focus Week” is to discuss advanced model-independent data-driven methods to test the late-time expansion history of the universe (z<10) against the incredibly large sets of data that will soon be provided by ongoing and future GW, LSS, and high-energy (HE) surveys.

To this end, we bring together a group of international experts belonging to 3 scientific communities: theorists and experts on model-independent cosmological inference; experts on GW and multi-messenger (MM) astrophysics with high-energy (HE) γ-ray observations; experts on LSS probes of cosmology.