Gravitational Waves
Gravitational waves are ripples in spacetime produced by the acceleration of massive objects, a prediction of Einstein’s general theory of relativity from 1916. They propagate at the speed of light and carry information about violent cosmic events such as merging black holes and neutron stars. For approximately a century, gravitational waves remained theoretical until their direct detection by the Laser Interferometer Gravitational-Wave Observatory (LIGO) in 2015, confirming a major prediction of general relativity.
Detection and Sources
Modern gravitational wave detectors use laser interferometry to measure minute distortions in spacetime caused by passing waves. Since the first confirmed detection, observatories including LIGO, Virgo, and KAGRA have identified numerous events, predominantly from the merger of compact objects. These observations have revealed populations of black holes across a range of masses, including systems with components lighter than the sun—objects classified as sub-solar mass black holes that challenge existing models of stellar evolution and black hole formation.
Primordial Black Holes
Gravitational wave observations have provided evidence for black holes that may have formed in the early universe rather than through stellar collapse. Sub-solar mass black holes detected through gravitational wave signatures are candidates for primordial black holes, which could have originated from density fluctuations in the primordial plasma. The detection of such objects through gravitational waves offers a novel method for constraining the properties and abundance of primordial black holes, with implications for understanding the universe’s earliest moments and the nature of dark matter.