Fate of the Universe

The theoretical scientific exploration of the ultimate fate of the universe became possible with Albert Einstein's 1915 theory of general relativity. General relativity can be employed to describe the universe on the largest possible scale. There are many possible solutions to the equations of general relativity, and each solution implies a possible ultimate fate of the universe. Alexander Friedmann proposed one such solution in 1921. This solution implies that the universe has been expanding from an initial singularity; this is, essentially, the Big Bang.

An important parameter in fate of the universe theory is the density parameter, Omega (O), defined as the average matter density of the universe divided by a critical value of that density. This creates three possible ultimate fates of the universe, depending on whether O is equal to, less than, or greater than 1. These are called, respectively, the Flat, Open and Closed universes. These three adjectives refer to the overall geometry of the universe, and not to the local curving of spacetime caused by smaller clumps of mass (for example, galaxies and stars).

Observational evidence was not long in coming. In 1929, Edwin Hubble published his conclusion, based on his observations of Cepheid variable stars in distant galaxies, that the universe was expanding. From then on, the beginning of the universe and its possible end have been the subjects of serious scientific investigation. In 1933, Georges-Henri Lemaître set out a theory that has since come to be called the Big Bang theory of the origin of the universe. In 1948, Fred Hoyle set out his opposing theory of a static universe, called the Steady state theory. These two theories were active contenders until the 1965 discovery, by Arno Penzias and Robert Wilson, of the cosmic microwave background radiation, a fact that is a straightforward prediction of the Big Bang theory, and one that the Steady State theory cannot account for. The Big Bang theory immediately became the most widely held view of the origin of the universe.