The first 5 years of gravitational wave astrophysics
Gravitational waves are ripples in space-time produced by the acceleration of masses, as predicted by the general theory of relativity. They were 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 masses, spins, the equation of state of nuclear matter, and distances. Because they are emitted in regions where gravity is extremely strong, gravitational waves also test the theory of general relativity.
Some astrophysical phenomena are expected to emit both gravitational and electromagnetic waves, including binary neutron star fusions, neutron star fusion with a black hole, or core collapsing supernovae in the Milky Way. This potentially allows for multi-messenger studies of these objects.
Over 50 gravitational wave events have been detected, emitted by the inspiration and fusion of compact objects (i.e. neutron stars and black holes) in binary systems. The gravitational wave event GW170817 was emitted by a merger of binary neutron stars 40 million parsecs from Earth. The collision also generated a highly energetic flash of gamma rays, which resulted in the first multi-messenger sighting of a gravitational wave source. These measurements showed that the fusions of binary neutron stars are at the origin of at least some gamma-ray bursts, confirming a hypothesis formulated decades earlier. The discovery of electromagnetic emissions at lower energies, from x-rays to radio frequencies, allowed for an in-depth study of the source and showed that binary neutron stars can produce many elements that are heavier than iron.
Analysis of GW170817 and its electromagnetic counterparts constrained the nuclear equation of state of matter, the relationship between density and pressure in the cores of neutron stars; measured the Hubble constant, which quantifies the rate of local expansion of the Universe; and confirmed that the speed of gravitational waves is equal to the speed of light, in a part in ~ 1015. A second gravitational wave signal from binary neutron stars, GW190525, has masses of neutron stars outside the range measured in the Milky Way using x-ray observations.
Dozens of gravitational wave events have been detected from binary black hole fusions. These showed that the mass distributions of black holes cannot be a single power law, like the mass distribution of mother stars. Instead, the preferred model has both a power law component and a Gaussian component, centered at
solar masses. This could indicate that gravitational wave events originate from more than one astrophysical population. This two-component distribution could be the result of physical processes involved in 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 black holes that are more massive than predicted by the theory of stellar evolution. This could indicate that one or both of the constituent black holes formed during a previous merger event. Meanwhile, the GW190814 binary system harbors a compact object of about 2.6 solar masses, making it either the least massive black hole or the most massive neutron star ever observed.
The spins of black holes in all of these mergers are consistent with the fact that they are preferentially small, unlike galactic black holes seen in x-ray binaries. This could indicate that most black holes are born with a small spin and are then generated by accretion.
General relativity theory tests using gravitational wave data found no deviation from his predictions. In current accuracy, general relativity correctly describes the behavior of compact astrophysical objects moving in extreme gravitational fields.
Existing gravitational wave detectors are being improved in sensitivity and additional detectors are under construction. These are expected to detect several binary neutron star mergers and around 100 black hole binary mergers each year. The growing dataset is expected to provide a better understanding of the pathways of astrophysical formation of compact objects over the mass range from ~ 1 to a few hundred solar masses. Independent measurements using pulsar synchronization networks could detect low-frequency gravitational waves produced by supermassive black hole binaries, which are expected to form when galaxies merge.