**1. Introduction**

Since their discovery in the late 1960s [1], the attention given by the scientific community to the gamma-ray burst (GRB) phenomenon has continuously increased. Nowadays, GRBs represent one of the most important sources in the Universe for observational cosmology, multi-messenger astronomy as well as extreme physics.

Their brightness in gamma-rays during the prompt phase allows us to detect the socalled "long-duration" GRBs, the GRBs generated by the gravitational collapse of rapidly rotating and massive stars, up to distances where the very first stars and galaxies form. The

**Citation:** Stratta, G.; Amati, L.; Branchesi, M.; Ciolfi, R.; Tanvir, N.; Bozzo, E.; Götz, D.; O'Brien, P.; Santangelo, A. Breakthrough Multi-Messenger Astrophysics with the THESEUS Space Mission † . *Galaxies* **2022**, *10*, 60. https:// doi.org/10.3390/galaxies10030060

Academic Editor: Elena Moretti, Francesco Longo and Yosuke Mizuno

Received: 28 February 2022 Accepted: 15 April 2022 Published: 21 April 2022

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**Copyright:** © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

census of high-redshift GRBs allows us to shed light on the main processes responsible for Universe reionization after Dark Ages.

In the context of multi-messenger astronomy, "short-duration" GRBs are today playing a major role after the confirmation of their association with binary neutron star merger (BNS) systems, in particular for the case of GRB 170817A [2,3]. Electromagnetic counterparts of gravitational wave (GW) sources, are fundamental for accurate sky localization that allows us to improve GW parameter estimation accuracy and to measure the cosmological redshift of the GW sources. The latter has crucial implications for the measurement of cosmological parameters and, being an independent method, can potentially solve current tensions over the value of the Hubble constant [4].

Finally, GRBs provide a unique and extremely powerful benchmark for performing tests of fundamental physics. For instance, the vast photon flux emitted during the prompt emission phase of these phenomena over several orders of magnitude of energy, combined with their extreme cosmological distances, make these phenomena powerful probes for testing the Lorentz Invariance Violation (LIV), which is predicted by different families of Quantum Gravity theories [5].

In all these respects, future GRB missions (such as the proposed THESEUS and Gamow Explorer mission concepts) will provide an ideal synergy with the large electromagnetic facilities of the future like the VRO/LSST, ELT, TMT, SKA, CTA and ATHENA in the electromagnetic domain, and advanced second generation (2G) and third generation (3G) GW detectors and future large neutrino detectors (e.g., KM3NeT) in the non-photonic domain.
