Probing Cosmic Expansion and Early Universe with Einstein Telescope

TL;DR

Probing Cosmic Expansion and Early Universe with Einstein Telescope outlines how a third-generation GW observatory can address key cosmological questions inaccessible to electromagnetic probes. It argues that ET's enhanced sensitivity and broad frequency coverage, especially with CE detectors and in synergy with LISA and large-scale structure surveys, will enable precise standard-siren cosmography, high-$z$ GW detections, and tests of gravity through GW propagation. The study highlights opportunities to probe the early Universe via the primordial GW background, first-order phase transitions, cosmic strings, and scalar-induced GWs, and to constrain the ratio $d_L^{\rm gw}(z)/d_L^{\rm em}(z)$, testing deviations from General Relativity. It also outlines ESO-ET synergy requirements for rapid follow-up, spectroscopic redshift programs, and data infrastructures to enable GW–LSS cross-correlations, paving the way for sub-percent $H_0$ and percent-level $\Omega_m$ constraints.

Abstract

Over the next two decades, gravitational-wave (GW) observations are expected to evolve from a discovery-driven endeavour into a precision tool for astrophysics, cosmology, and fundamental physics. Current second-generation ground-based detectors have established the existence of compact-binary mergers and enabled GW multi-messenger astronomy, but they remain limited in sensitivity, redshift reach, frequency coverage, and duty cycle. These limitations prevent them from addressing many fundamental open questions in cosmology. By the 2040s, wide-field electromagnetic surveys will have mapped the luminous Universe with unprecedented depth and accuracy. Nevertheless, key problems including the nature of dark matter, the physical origin of cosmic acceleration, the properties of gravity on cosmological scales, and the physical conditions of the earliest moments after the Big Bang will remain only partially constrained by electromagnetic observations alone. Progress on these fronts requires access to physical processes and epochs that do not emit light. Gravitational waves provide a unique and complementary observational channel: they propagate over cosmological distances largely unaffected by intervening matter, probe extreme astrophysical environments, and respond directly to the geometry of spacetime. In this context, next-generation GW observatories such as the Einstein Telescope (ET) will be transformative for European astronomy. Operating at sensitivities and frequencies beyond existing detectors, ET will observe binary black holes and neutron stars out to previously inaccessible redshifts, enable continuous high signal-to-noise monitoring of compact sources, and detect gravitational-wave backgrounds of astrophysical and cosmological origin. Together with space-based detectors, ET will play a central role in advancing our understanding of cosmic evolution and fundamental physics.