What specific cosmic events, involving the violent merging of two very heavy compact objects like black holes or neutron stars, are the main sources we detect gravitational waves from?
The main sources of detected gravitational waves stemming from the violent merging of two very heavy compact objects are Binary Black Hole (BBH) mergers, Binary Neutron Star (BNS) mergers, and Neutron Star-Black Hole (NSBH) mergers. These compact objects, black holes and neutron stars, are extremely dense celestial bodies, remnants of massive stars that have exhausted their nuclear fuel and undergone gravitational collapse. Gravitational waves themselves are ripples in the fabric of spacetime, caused by the acceleration of massive objects, traveling at the speed of light. They distort the distances between objects as they pass.
In a Binary Black Hole (BBH) merger, two black holes orbit each other in a close binary system. A black hole is a region of spacetime where gravity is so strong that nothing, not even light, can escape, formed from the complete gravitational collapse of a very massive star. As these two black holes spiral inward, a process known as inspiral, they continuously emit gravitational waves, which causes their orbits to shrink and them to accelerate closer together. The inspiral culminates in a violent merger, where the two black holes collide and coalesce to form a single, more massive black hole, releasing the strongest burst of gravitational waves. Following the merger, the newly formed black hole settles into a stable configuration in a phase known as ringdown, emitting diminishing gravitational waves as any initial distortions smooth out. BBH mergers were the first type of gravitational wave event ever detected and remain the most frequently observed.
Binary Neutron Star (BNS) mergers involve two neutron stars orbiting each other. A neutron star is an incredibly dense remnant core of a massive star after a supernova explosion, composed primarily of neutrons, making it one of the densest known objects in the universe. Similar to black hole binaries, these two neutron stars undergo an inspiral phase, emitting gravitational waves as their orbits decay and they draw closer. The ultimate collision, or merger, of two neutron stars is an extraordinarily energetic event that generates powerful gravitational waves. This merger can result in the formation of a more massive neutron star or a black hole, and is often accompanied by the ejection of neutron-rich material that can produce electromagnetic emissions, such as short gamma-ray bursts and kilonovae, making them crucial for multimessenger astronomy, where both gravitational waves and light are observed.
Finally, Neutron Star-Black Hole (NSBH) mergers occur when a single neutron star orbits and then collides with a black hole. In this scenario, the neutron star spirals inward towards the black hole, emitting gravitational waves. Depending on the masses and spins of the objects involved, the black hole can either tidally disrupt the neutron star, tearing it apart into a disk of material before swallowing its remnants, or simply engulf it whole without significant disruption. Both outcomes produce strong gravitational wave signals. The tidal disruption can also eject material, potentially leading to detectable electromagnetic counterparts similar to those seen in BNS mergers, making NSBH mergers another important source for multimessenger studies. These three types of compact binary mergers are the primary cosmic events from which gravitational waves have been directly detected.