One of general relativity’s most striking predictions arises if we consider what happens to the universe as a whole.
Shortly after Einstein published his theory, Russian meteorologist and mathematician Alexander Friedmann and Belgian priest Georges Lemaître showed that it predicted that the universe should evolve in response to all the energy it contains. They argued that the universe should start off small and dense, and expand and dilute with time. As a result, galaxies should drift away from each other.
Einstein was initially sceptical of Friedmann and Lemaître’s conclusion, favouring a static universe. But a discovery by the American astronomer Edwin Hubble changed his mind.
Hubble analysed how galaxies recede from the Milky Way. He found that distant galaxies move away faster than those that are relatively nearby. Hubble’s observations showed that the universe was indeed expanding. This model of the cosmos later became known as the big bang.
Over the past 20 years, a plethora of powerful observations by satellites and large telescopes have further firmed up the evidence for an expanding and evolving universe. We have obtained an accurate measure of the expansion rate of the universe and of the temperature of the “relic radiation” left over from the big bang, and we have been able to observe young galaxies when the universe was in its infancy. It is now accepted that the universe is about 13.7 billion years old.