Francesco Iacovelli

Einstein Telescope (ET) is the European project for a gravitational-wave (GW) observatory of third-generation. In this paper we present a comprehensive discussion of its science objectives, providing state-of-the-art predictions for the capabilities of ET in both geometries currently under consideration, a single-site triangular configuration or two L-shaped detectors. We discuss the impact that ET will have on domains as broad and diverse as fundamental physics, cosmology, early Universe, astrophysics of compact objects, physics of matter in extreme conditions, and dynamics of stellar collapse. We discuss how the study of extreme astrophysical events will be enhanced by multi-messenger observations. We highlight the ET synergies with ground-based and space-borne GW observatories, including multi-band investigations of the same sources, improved parameter estimation, and complementary information on astrophysical or cosmological mechanisms obtained combining observations from different frequency bands. We present advancements in waveform modeling dedicated to third-generation observatories, along with open tools developed within the ET Collaboration for assessing the scientific potentials of different detector configurations. We finally discuss the data analysis challenges posed by third-generation observatories, which will enable access to large populations of sources and provide unprecedented precision.

This document summarizes talks and discussions from the workshop "deci-Hz Gravitational Wave Observations on the Moon and Beyond" that took place at Johns Hopkins University between September 1 and September 3, 2025. The workshop focused on experimental proposals to observe gravitational waves in the deci-Hz band, including lunar detectors, laser interferometers in space, and atom interferometry; gravitational wave sources in the deci-Hz frequency band; and the multi-messenger and multi-band astronomy that would be enabled by these observations.

The Einstein Telescope (ET), a proposed next-generation gravitational wave (GW) observatory, will expand the reach of GW astronomy of stellar-mass compact object binaries to unprecedented distances, enhancing opportunities for multi-messenger observations. Here we investigate multi-messenger emission properties of binary neutron star (NSNS) and black hole-neutron star (BHNS) mergers detectable by ET and provide projections to optimise observational strategies and maximise scientific insights from these sources. Using a synthetic population of compact binary mergers, we modelled each source’s GW signal-to-noise ratio, sky localization uncertainty, kilonova light curves (in optical and near-infrared bands), and the fluence of the relativistic jet gamma-ray burst prompt emission and afterglow light curves across radio, optical, X-ray, and very high energy wavelengths. We analysed multi-messenger detectability prospects for ET as a standalone observatory with two different configurations and within a network of next-generation GW detectors. ET will detect over 10 ⁴ NSNS mergers annually, enabling the potential observation of tens to hundreds of electromagnetic (EM) counterparts. In contrast, BHNS mergers have more limited multi-messenger prospects, but joint GW-EM rates will increase by an order of magnitude compared to current-generation instruments. We quantified the uncertainties due to the NS equation of state (EoS) and mass distribution of NSNSs as well as the NS EoS and BH spin for BHNSs. While a single ET will achieve an impressive GW detection rate, the fraction of well-localised events (< 100 deg ² ) is orders of magnitude lower than in a network with additional detectors. This significantly limits efficient EM follow-up and science cases requiring well-characterized counterparts or early observations. The challenge is even greater for BHNS mergers due to their low EM rate. Thus, multi-messenger astronomy in the next decade will critically depend on a network of at least two detectors.

We study the performances of a world-wide network made by a European third-generation gravitational-wave detector, together with a 40 km Cosmic Explorer detector in the US, considering three scenarios for the European detector: (1) Einstein Telescope (ET) in its 10 km triangle configuration; (2) ET in its configuration featuring two 15 km L-shaped detectors in different sites, still taken to have all other ET characteristics (underground, and with each detector made of a high-frequency interferometer and a cryogenic low-frequency interferometer); (3) A single L-shaped underground interferometer with the ET sensitivity curve, either with 15 km or with 20 km arm length. Overall, we find that, if a configuration with two widely separated L-shaped detectors (‘2 L’) should be retained for ET, the network made by a single-L European underground detector together with CE-40 km could already provide a very interesting intermediate step toward the construction of a full 2 L+CE network, and is in any case superior to a 10 km triangle not inserted in an international network. We also study the performance of a network made by a single L-shaped underground interferometer with the ET sensitivity curve together with a single 40 km CE and with LIGO-India (taken at A # sensitivity), and we find that it also has very interesting performances.