Cosmology

Cosmology is the branch of physics dealing with the origin and evolution of the universe -- the formation of stars, galaxies, and similar large-scale structure. The book anchors the topic in a specific recent discovery: in 2015, the existence of gravitational waves was directly detected, and the Nobel Prize was awarded for this discovery in 2017. Gravitational waves are disturbances in the curvature of space-time itself, and they travel at the speed of light. Just as an accelerated electric charge emits an electromagnetic wave, an accelerated mass emits a gravitational wave -- but because gravity is such a weak force compared to electromagnetism, these waves are extremely weak, even for a mass as large as the Earth's. The strongest known sources of gravitational waves are black holes, and it is precisely because black holes are such strong sources that their detection made it possible to study black-hole structure directly; the recent discoveries in fact come from gravitational waves emitted when two black holes merge into a single, larger black hole. The idea itself is not new: Albert Einstein theoretically proposed the existence of gravitational waves back in 1915, and it took 100 years -- until the 2015 detection -- for his prediction to be experimentally confirmed. Black holes themselves are described as the end stage of highly dense, massive stars, with masses ranging from about 20 times the mass of the Sun up to about 1 million times the mass of the Sun. Their gravitational pull is so strong that no particle, not even light, can escape from them, and their existence is inferred indirectly, from the way stars orbiting a black hole behave differently from other stars nearby. Every galaxy is understood to have a black hole at its centre; Sagittarius A* is specifically the black hole at the centre of our own Milky Way galaxy. The physicist Stephen Hawking is credited with foundational work in the field of black holes. And in a striking, very recent confirmation, the Event Horizon Te …