Gravitational wave experiments: achievements and plans

TL;DR

The chapter surveys gravitational-wave experiments, detailing detector principles (LIGO/Virgo/KAGRA), data acquisition, and signal extraction, with emphasis on CBCs, source properties, and multimessenger synergy. It covers the scientific impact across astrophysics, nuclear physics, GR tests, and cosmology, highlighting standard sirens, EoS constraints, and stochastic backgrounds. The text also outlines the complementarity of GW observations with EM/neutrino channels and reviews plans for next-generation detectors (ET/CE) and space missions (LISA, DECIGO, TianQin), including Moon-based concepts. Together, these elements illuminate how the GW field has evolved into a mature, multi-messenger discipline with profound implications for our understanding of the universe and fundamental physics. The material emphasizes that future networks will dramatically improve event localization, enable high-precision cosmology, and probe physics beyond the Standard Model through dark matter and early-Universe signals.

Abstract

Gravitational wave (GW) experiments have transformed our understanding of the Universe by enabling direct observations of compact object mergers and other astrophysical phenomena. This chapter reviews the concepts of GW detectors, such as LIGO, Virgo, and KAGRA, and describes their operating principles, data acquisition and analysis techniques, and some of the methods used to extract source properties. The scientific impact of GW observations is discussed as well, including contributions to astrophysics, tests of general relativity, and cosmology. We also examine the role of multimessenger astronomy and the complementarity between different GW detectors and with other astroparticle experiments. Finally, we outline future prospects with next-generation detectors, like the Einstein Telescope and Cosmic Explorer, and space-based missions.

Figures (15)

• Figure 1: The four currently active detectors in the current gravitational wave detector network.
• Figure 2: View of the inside of the AURIGA detector, an example of the early GW detectors, located at the INFN Laboratori Nazionali di Legnaro, Italy. The resonant bar antenna itself is the silver cylinder at the forefront of the image. During operations, the antenna was enclosed in a cryostat. Credits: aurigafig.
• Figure 3: Timeline of the LIGO-Virgo-KAGRA (LVK) operations. Vertical grey bands depict downtime for upgrades and commissioning. The BNS range, i.e., the maximum distance over which each detector can detect a BNS merger with a fixed intensity threshold, is also reported, in different colors. Image from the $\,$https://observing.docs.ligo.org/plan/.
• Figure 4: Map of GW observatories across the globe: in yellow are depicted the current (as of 2025) operating facilities, in orange, the planned project of the third LIGO observatory, to be sited in India (Section \ref{['future:ground']}). Image from: https://www.ligo.caltech.edu/.
• Figure 5: (a) Simplified optical layout of the Advanced Virgo detector. The laser beam is split by a beam splitter (BS). Each 3 km-long arm cavity is formed by an input mirror (IM) and an end mirror (EM). The power recycling mirror (PRM) and the signal recycling mirror (SRM) are shown along with auxiliary optical components Credits: aVirgo. (b) Representative design of the Advanced Virgo sensitivity and noise limitations. The reference sensitivity (solid black) is given by the square root of the power spectral density of the detector and consists of the sum of various contributions affecting different frequency ranges (solid and dashed, colored). Credits: ASD.