Discordance

Abstract In 1927, Georges Lemaître argued that our universe is expanding, a conclusion rendered more startling by the astronomical data that backed it up, presented two years later by Edwin Hubble. The speed of this expansion is governed by Hubble’s constant, and Discordance tells its troubled history. This begins with the first tentative steps to measure the distances to nearby stars and galaxies. It traces the extraordinary interplay between theory and observation which has given us the standard ‘Big Bang’ theory. It was not all plain sailing. This is a story littered with crises of confidence, astonishing discoveries, and extraordinary personalities. Astronomers found that not all stars are the same, and the Hubble constant was instantly halved. A universe born in a big bang should have left an afterglow, a window on the early universe called the cosmic background. This was found in 1965 before the search for it could begin. The Hubble constant fell further, as astronomers argued intensely about its value in the ‘Hubble Wars’ of the 1970s. A new generation of satellite-based instruments was launched, amid triumph and tragedy. Observations of distant galaxies lit up by supernova explosions led to the discovery, against all expectation, that the expansion of the universe is accelerating. Further satellite missions were expected to connect early-universe predictions for the Hubble constant and late-universe measurements based on distances. But from 2009, the results diverged. This is the Hubble tension. It is not clear if this is another crisis. The story is not over yet.