Science Case for the Einstein Telescope

TL;DR

The paper argues that the Einstein Telescope will transform gravitational-wave astronomy by delivering orders-of-magnitude improvements in sensitivity and frequency coverage, enabling a census of black hole populations from the early universe to the present and unprecedented constraints on neutron-star equations of state. It details ET's science program across astrophysics, fundamental physics, and cosmology, including multi-messenger and multi-band opportunities with LISA and EM/neutrino observatories. The work highlights potential groundbreaking discoveries, such as primordial black holes, exotic compact objects, and new tests of gravity and dark-energy physics, while also addressing practical aspects like detector networks and data-analysis challenges for stochastic backgrounds. Overall, ET is presented as a comprehensive, high-impact platform for exploring the origin and evolution of the universe through gravitational waves.

Abstract

The Einstein Telescope (ET), a proposed European ground-based gravitational-wave detector of third-generation, is an evolution of second-generation detectors such as Advanced LIGO, Advanced Virgo, and KAGRA which could be operating in the mid 2030s. ET will explore the universe with gravitational waves up to cosmological distances. We discuss its main scientific objectives and its potential for discoveries in astrophysics, cosmology and fundamental physics.

Figures (14)

• Figure 1: Left panel: the sensitivity of the ET-B (red, dot-dashed) and ET-D (yellow, dashed) configurations, compared to the official target sensitivity of advanced Virgo (blue solid line). Right panel: the separate contributions from the LF and HF instruments to the sensitivity of ET-D.
• Figure 2: Left: astrophysical reach for equal-mass, nonspinning binaries for Advanced LIGO, Einstein Telescope and Cosmic Explorer (from ref. Sathyaprakash:2019nnu3GScienceBook). Right: lines of constant signal-to-noise ratio in the (total mass, redshift) plane, for a network of one ET and two CE detectors. The curves shown assume equal-mass binary components (figure courtesy by M. Colpi and A. Mangiagli).
• Figure 3: Masses of the LIGO/Virgo BHs detected during the O1 and O2 runs (light blue), of BHs discovered through X-ray binaries (purple), NSs known electromagnetically (yellow) and the two initial NS in the binary GW170817 (orange). Image taken from https://media.ligo.northwestern.edu.
• Figure 4: Left: Conjectured interior structure of a neutron star. Right: Matter encountered in neutron stars and binary mergers explores a large part of the QCD phase diagram in regimes that are inaccessible to terrestrial collider experiments.
• Figure 5: Gravitational wave signal from a NS-NS merger at a distance 100 Mpc, as it sweeps across the detector-accessible frequency range. From Maggiore:2018zz (figure courtesy of Jocelyn Read, based on results presented in Read:2013zra).