Gravitational waves are not simple conceptually, as they require one to think of the fabric of the Universe in terms of spacetime. Spacetime is a four-dimensional quantity, described by Einstein’s general theory of relativity, which fuses three-dimensional space with time. Mass warps spacetime, and gravity is actually the result of spacetime’s being curved by an object’s mass. Ripples through spacetime are created by the movement of any object with mass, and these are known as gravitational waves. Gravitational waves are constantly passing unnoticed through the Earth. Only the waves created by extremely intense events — events that result in very rapid changes in the velocity of very massive objects — can be detected by present-day instruments. The gravitational waves that are currently detectable on Earth are generated by incredibly dramatic collision events, such as when two black holes or neutron stars merge.
In 2017 Hubble observed for the first time the source of a gravitational wave, created by the merger of two neutron stars. This discovery was the first glimpse of multi messenger astronomy (in which coordinated observations and interpretations are undertaken of different astronomical signals), bringing together both gravitational waves and electromagnetic radiation.
Also in 2017, an international team of astronomers using Hubble uncovered a supermassive black hole that had been propelled out of the center of the distant galaxy 3C186. They concluded that it was likely ejected by the powerful gravitational waves unleashed by the merger of two massive black holes at the center of its host galaxy. This was the first time that astronomers found a supermassive black hole at such a large distance from the center of its host galaxy.
