The first 5 years of gravitational-wave astrophysics.

BACKGROUND Gravitational waves are ripples in spacetime produced by accelerating masses, as predicted by the general theory of relativity. They have been directly detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Virgo detector. Gravitational waves encode several physical properties of their sources, such as the masses, spins, equation of state of nuclear matter, and distances. Because they are emitted in regions where gravity is extremely strong, gravitational waves also enable tests of the general theory of relativity. Some astrophysical phenomena are expected to emit both gravitational and electromagnetic waves, including the mergers of binary neutron stars, a neutron star merger with a black hole, or core-collapse supernovae within the Milky Way. This potentially enables multimessenger studies of these objects. ADVANCES More than 50 gravitational-wave events have been detected, emitted by the inspiral and merger of compact objects (i.e., neutron stars and black holes) in binary systems. The gravitational-wave event GW170817 was emitted by a binary neutron star merger 40 million parsecs from Earth. The collision also generated a highly energetic flash of gamma rays, which yielded the first multimessenger observation of a gravitational-wave source. These measurements showed that binary neutron star mergers are the progenitors of at least some gamma-ray bursts, confirming a hypothesis made decades earlier. The discovery of electromagnetic emission at lower energies—from x-ray to radio frequencies—has enabled an extensive study of the source and has shown that binary neutron stars can produce many of the elements heavier than iron. Analysis of GW170817 and its electromagnetic counterparts has constrained the equation of state of nuclear matter, the relation between density and pressure in the core of neutron stars; has measured the Hubble constant, which quantifies the local expansion rate of the Universe; and has confirmed that the speed of gravitational waves is equal to the speed of light, within one part in ~1015. A second binary neutron star gravitational-wave signal, GW190525, has neutron star masses outside the range measured in the Milky Way using x-ray observations. Dozens of gravitational-wave events have been detected from binary black hole mergers. These have shown that the mass distributions of black holes cannot be a single power law, like the mass distribution of the parent stars. Instead, the preferred model has both a power law component and a Gaussian component, centered at 33.5 − 5.5 + 4.5 solar masses. This could indicate that gravitational-wave events arise from more than one astrophysical population. This two-component distribution might be a result of the physical processes involved in the explosions of stars of more than ~100 solar masses, which predict a maximum mass for black holes formed in supernovae. The highest mass source detected, GW190521, has component black holes that are more massive than expected from stellar evolution theory. This could indicate that one or both component black holes were formed in a previous merger event. Meanwhile, the binary system GW190814 hosts an ~2.6–solar masses compact object, which makes it either the least massive black hole or the most massive neutron star yet observed. The spins of black holes in all of these mergers are consistent with being preferentially small, unlike Galactic black holes observed in x-ray binaries. This could indicate that most black holes are born with small spin and are later spun-up by accretion. Tests of the general theory of relativity using gravitational-wave data have found no departure from its predictions. Within the current precision, general relativity correctly describes the behavior of compact astrophysical objects moving in extreme gravitational fields. OUTLOOK Existing gravitational-wave detectors are undergoing upgrades to their sensitivities, and additional detectors are under construction. These are expected to detect multiple neutron star binary mergers and ~100 binary black hole mergers every year. The growing dataset should provide a better understanding of the astrophysical formation pathways of compact objects over the mass range between ~1 and a few hundred solar masses. Independent measurements using pulsar timing arrays could detect the lower-frequency gravitational waves produced by supermassive black hole binaries, which are expected to form when galaxies merge.