The Big Bang Theory is the dominant theory in cosmology about the early development of the universe. According to this theory, the universe started sometime between 10 billion and 20 billion years ago. The universe was initially microscopic, and almost uniformly filled with energy. As the universe rapidly grew, the temperature dropped, leading to the creation of the known forces of physics, elementary particles, and eventually hydrogen and helium atoms. Over time, these (almost) uniformly-distributed atoms were pulled together by gravity into clumps, forming stars, galaxies, and the other structures seen today.
The Big Bang was not an explosion of matter moving outward to fill an empty universe. Instead, it was a rapid growth of the universe itself, whose contents became less uniformly distributed over time. Because it involves the universe itself expanding, distant galaxies can actually move apart faster than the speed of light.
In 1927, the Belgian priest [Georges Lemaître]? was the first to propose that the universe began with the explosion of a primeval atom. His proposal came after observing the red shift in distant nebulae by astronomers to a model of the universe based on relativity. Years later, [Edwin Hubble]? found experimental evidence to help justify Lemaître's theory. He found that distant galaxies in every direction are going away from us with speeds proportional to their distance.
The big bang was initially suggested because it explains why distant galaxies are traveling away from us at great speeds. The theory also predicts the existence of Cosmic Background Radiation (the glow left over from the original energy distribution). The Big Bang Theory received its strongest confirmation when this radiation was discovered in 1964 by [Arno Penzias]? and [Robert Wilson]?, who later won the Nobel Prize for this discovery.
The big bang model is the basis for more sophisticated cosmological models which include [cosmic inflation]?. Also within the big bang model, you can make predictions for how the universe ought to look assuming that it consists of [cold dark matter]? or [warm dark matter]?.
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