When spacetime vibrates: An introduction to gravitational waves

TL;DR

The paper introduces gravitational waves from both theoretical and experimental perspectives, detailing the GR framework, the quadrupole emission mechanism, and the inspiral–merger–ringdown phenomenology of compact binaries. It highlights the pivotal role of giant interferometers (LIGO, Virgo, KAGRA) in inaugurating gravitational-wave astronomy through the GW150914 detection and subsequent multi-messenger events like GW170817. It surveys the current and future GW landscape, including ground-based networks, space-based detectors like LISA, and pulsar timing arrays, as well as the quest to detect primordial backgrounds from the Big Bang. The work emphasizes the scientific impact on astrophysics, cosmology, and fundamental physics, and envisions a future where GW observations complement and extend electromagnetic astronomy across a wide frequency range.

Abstract

This article presents a comprehensive analysis of the physics of gravitational waves, exploring both the theoretical foundations and the most recent experimental advances. After a general introduction to the theory of general relativity and its major implications, the article discusses the history of gravitational waves, from their prediction by Einstein to their actual detection. It then explains what gravitational waves are and how they interact with appropriate detectors. The main mechanisms of gravitational radiation emission are analyzed, with a focus on compact binary systems of compact objects, whose orbits typically evolve in three phases: inspiral, merger, and the final ringdown phase, each of these phases leaving distinct signatures in the emitted waves. The article highlights the fundamental role of the giant interferometers LIGO, Virgo, and KAGRA, true cathedrals of modern science, and revisits the historic event GW150914, the first direct detection of gravitational waves, which confirmed the predictions of general relativity and opened a new era for astronomy. This achievement was recognized with the 2017 Nobel Prize in Physics. Other observed events are also discussed, along with their astrophysical sources, and the possibility of detecting gravitational waves of cosmological origin, originating from the Big Bang itself. Finally, current and future projects are analyzed, including observatories based on increasingly sophisticated interferometers, as well as proposals for alternative detection methods, illustrating how gravitational-wave astronomy is shaping the present and future of our exploration of the universe. In concluding, the detection of gravitational waves is set in a broader context by examining the discoveries across the electromagnetic spectrum, thereby illustrating the complementary perspectives these different observational channels provide.

Figures (18)

• Figure 1: Representation of two black holes inspiraling on a collision course, emitting gravitational waves, with the end point being their merger.
• Figure 2: Two test particles, $A$ and $B$, are separated by a distance $L$ along the $x$-axis. As a gravitational wave passes, the particles will oscillate according to the wave.
• Figure 3: The upper part of the figure shows the deformation $h$ as a function of time when the gravitational wave passes by the test particles placed along the $x$-axis or the $y$-axis. The lower part shows that the particles oscillate in the $x\times y$ plane with $+$ polarization.
• Figure 4: Representation of a source of gravitational waves and the corresponding detector. A small element of the source is located at $r'$, where $r'$ represents the position vector of that element relative to a given origin. The position vector of the detector relative to the origin is $r$. The position vector from the element of the source to the detector is $r-r'$. The distances are obtained by taking the magnitude of the position vectors. Each element of the source is characterized by the quantity $T_{ab}$, which gives the energy and momenta of that element.
• Figure 5: Two compact objects, each of mass $M$, in a tight circular orbit of radius $R$. For the emission of gravitational waves intense enough to be detected, the objects must be either black holes or neutron stars.