The Beginning of Modern Cosmology
The modern view of cosmology is based on the work of Edwin P. Hubble, an American astronomer. In 1929, he discovered that all the galaxies in the universe are rushing away from one another, which means that the universe as a whole is expanding. Since matter is known to cool as it expands, cosmologists concluded that the universe began in an immensely hot, dense state, often called the primordial fireball.
The theory of the primordial fireball, also called the big bang theory, has become widely accepted because a number of its predictions have been confirmed by observation. The first strong evidence that the universe had a fiery birth came in 1965. That year Arno A. Penzias and Robert W. Wilson, researchers working at Bell Telephone Laboratories in New Jersey, discovered that weak microwave radiation is arriving at Earth from all directions in space. Cosmologists interpreted these microwaves, now called the cosmic background radiation, as the faint glow that survives today from the blazing heat of the primordial fireball. Today we know, based on data obtained from the Cosmic Background Explorer (COBE) satellite in the early 1990's, that the properties of the background radiation agree precisely with what we would expect from the glow of hot matter in the early universe.
In addition, physicists have studied how the nuclei of atoms would have formed in a fireball as it cooled. They found that the nuclei in the universe would be mainly of the lightest known types: two isotopes (forms) of hydrogen, two isotopes of helium, and an isotope of lithium. The amounts of these elements that astronomers have measured in our universe agree well with what the theorists calculated. Cosmologists believe that heavier elements formed much later, in the interior of stars, so they do not provide a test for the big bang theory.