Abac, AdrianAbramo, RaulAlbanesi, SimoneLlanes Estrada, Felipe JoséLope Oter, EvaNavarro Moreno, PabloRubiera García, DiegoRubio Peña, JavierBlázquez Salcedo, José LuisZmija, Andreas2026-05-292026-05-292026Abac A, Abramo R, Albanesi S, Albertini A, Agapito A, Agathos M, Albertus C, Andersson N, Andrade T, Andreoni I, Angeloni F, Antonelli M, Antoniadis J, Antonini F, Sedda M A, Celeste Artale M, Ascenzi S, Auclair P, Bachetti M, Badger C, Banerjee B, Barba-González D, Barta D, Bartolo N, Bauswein A, Begnoni A, Beirnaert F, Bejger M, Belgacem E, Bellomo N, Bernard L, Bernardini M G, Bernuzzi S, Berry C P L, Berti E, Bertone G, Bettoni D, Bezares M, Bhagwat S, Bisero S, Bizouard M A, Blanco-Pillado J J, Blasi S, Bonino A, Borghese A, Borghi N, Borhanian S, Bortolas E, Botticella M T, Branchesi M, Breschi M, Brito R, Brocato E, Broekgaarden F S, Bulik T, Buonanno A, Burgio F, Burrows A, Calcagni G, Canevarolo S, Cappellaro E, Capurri G, Carbone C, Casadio R, Cayuso R, Cerdá-Durán P, Char P, Chaty S, Chiarusi T, Chruslinska M, Cireddu F, Cole P, Colombo A, Colpi M, Compère G, Contaldi C, Corman M, Crescimbeni F, Cristallo S, Cuoco E, Cusin G, Canton T D, Dálya G, D’Avanzo P, Davari N, De Luca V, De Renzis V, Della Valle M, Del Pozzo W, De Santi F, De Santis A L, Dietrich T, Dimastrogiovanni E, Domenech G, Doneva D, Drago M, Dupletsa U, Duval H, et al 2026 The Science of the Einstein Telescope J. Cosmol. Astropart. Phys. 2026 08110.1088/1475-7516/2026/03/081https://hdl.handle.net/20.500.14352/137020Artículo firmado por 489 autores. ©2026TheAuthor(s). RYC2020-028870-I PROMETEO/2021/083 CIPROM/2022/69 IT-1628-22 RYC2021-033786-I 2019-T1/TIC-13177 MSCA-101065736 MSCA-101105915 MSCA-2022-PF-01-01 CA16104 CA16214 MSCA-RISE-2017 MSCA-2021-SE-01 RED2022-134411-TEinstein 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.engAttribution 4.0 Internationalhttp://creativecommons.org/licenses/by/4.0/journal article1475-7516https://doi.org/10.1088/1475-7516/2026/03/081https://iopscience.iop.org/article/10.1088/1475-7516/2026/03/081open access524.834:530.12520.2Gravitational wavesExperiments, gravitational wavesSources, gravitational wavesTheory, gravitational wave detectorsAstronomía (Física)Teoría de los quanta2101 Cosmología y Cosmogonía2212.14 Teoría de la Relatividad2212.05 Gravitación2103.02 Telescopios