Q.What are black holes?

Cosmology is the branch of physics dealing with the origin and evolution of the universe, including how stars and galaxies form. The unit anchors the topic in the 2015 direct detection of gravitational waves, for which the Nobel Prize was awarded in 2017. Gravitational waves are disturbances in the curvature of space-time itself, travelling at the speed of light; just as an accelerated electric charge radiates an electromagnetic wave, an accelerated mass radiates a gravitational wave, but because gravity is such a weak force these waves are extremely faint, even for a mass as large as the Earth's. Black holes are the strongest known source of gravitational waves, which is exactly why detecting the waves made it possible to study black-hole structure directly -- the detections made so far actually come from the merger of two black holes into one larger black hole. None of this is a new idea: Albert Einstein theoretically proposed the existence of gravitational waves in 1915, and it took a full century -- until the 2015 detection -- for his prediction to be experimentally confirmed. Black holes themselves are the end stage of highly dense, massive stars, with masses ranging from roughly 20 times the mass of the Sun up to about 1 million solar masses; their gravitational pull is so strong that nothing, not even light, can escape, and their presence is inferred from how stars orbiting them behave differently from other nearby stars. Every gala …