Dear Colleagues,

Since the first detection of gravitational waves (GWs) in September 2015, the era of GW astronomy has become a reality [1–3]. However, despite the hundreds of detections made possible via observation in recent years [4], many expected signals have yet to be detected [5], likely because of the insufficient sensitivity of second-generation detectors, among other factors. Perspectives such as these have prompted the scientific community to develop third-generation (3G) GW detectors [6,7], which will aim to explore the universe through gravitational waves up to cosmological ages, providing significant advancements in comprehending warped space–time, the generation of cosmic matter, and much more.

The purpose of this Special Issue is to provide an overview of the technological challenges that need to be addressed and overcome to reduce the contribution of noise that limits the sensitivity band of current ground-based GW detectors, and to demonstrate how the Cosmic Explorer and Einstein Telescope projects will overcome these challenges. Several reviews on the physics of the design of current and future detectors already exist in the literature [8–12]. However, considering GWs as a united scientific community, the aim of this review is to present the solutions implemented by the two 3G detectors to handle fundamental and technical noise sources. These approaches can be both similar and complementary, but their application will lead to the development of the most sensitive GW detectors on Earth.

Moreover, given the strong interest this field of research has received in recent years, there is growing interest in the frequency bands outside those of terrestrial detectors. Therefore, part of this Special Issue will be devoted to space-based detectors, whose sensitivity will extend to the sub-Hz band, and to those projects aimed at detecting signals at frequencies above tens of kHz, the so-called ultra-high-frequency gravitational waves.

References

[1] B.P. Abbot et al., Observation of Gravitational Waves from a Binary Black Hole Merger, Phys. Rev. Lett. 116, 061102 (2016).

[2] B.P. Abbott et al. GW170814: A Three-Detector Observation of Gravitational Waves from a Binary Black Hole Coalescence, Phys. Rev. Lett. 119, 141101 (2017).

[3] B.P. Abbot et al., GW170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral,

Phys. Rev. Lett. 119, 161101 (2017).

[4] B.P. Abbot et al. GWTC-1: A Gravitational-Wave Transient Catalog of Compact Binary Mergers

Observed by LIGO and Virgo during the First and Second Observing Runs, Phys Rev X, 9: 031040 (2019).

[5] A. Abbot et al, Prospects for observing and localizing gravitational-wave transients with Advanced LIGO, Advanced Virgo and KAGRA, Living Rev. Relativ. 23, 3 (2020).

[6] D. Reitze et al., Cosmic Explorer: The US CoNTRibution to Gravitational-Wave Astronomy beyond LIGO, Bull. Am. Astron. Soc., 51, 7:35 (2019).

[7] M. Punturo et al., The Einstein Telescope: A third-generation gravitational wave observatory. Class. Quantum Grav. 27, 194002 (2010).

[8] Bassan, M. Advanced Interferometers and the Search for Gravitational Waves. Springer: London, UK; 2014. ISBN 978-3-319-03791-2; https://doi.org/10.1007/978-3-319-03792-9.

[9] Reitze, D. H.; Saulson, P.; Grote, H. Advanced Interferometric Gravitational-Wave Detectors. World Scientific: Singapore, 2016; https://doi.org/10.1142/10181.

[10] Maggiore, M. Gravitational Waves: Volume 2: Astrophysics and Cosmology. Oxford University Press: Oxford, UK, 2018. ISBN-13: 9780198570899; https://doi.org/10.1093/oso/9780198570899.001.0001.

[11] Gravitational Waves Detectors - Hartmut Grote, Andreas Freise, Oliver Jennrich editors. Galaxies special issue https://www.mdpi.com/journal/galaxies/special_issues/gravi_wave

[12] Present and Future of Gravitational Wave Astronomy - G. Vajente editor. Galaxies special issue https://www.mdpi.com/journal/galaxies/special_issues/pfgwa

Dr. Annalisa Allocca
Dr. Lucia Trozzo
Dr. Valeria Sequino
Guest Editors

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• gravitational waves
• third-generation GW detectors
• space-based GW detectors
• ultra-high-frequency GW detectors