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.
Since galaxies were moving away from us, this suggested two possibilities. One is that the which was suggested by Gamow? was that the universe begin at a finite time and has been expanding ever since. The other was Fred Hoyle's [steady state model]? in which new matter would be created as the galaxies moved away from each other and that the universe at one point in time would look roughly like any other point in time.
The observation which decided between the two was the detection of Cosmic Background Radiation. If you further back away, you are looking back in time. The big bang model predicts that if you look far enough away, you will eventually see a time in which the temperature of the universe exceeded 3000 K. At that temperature, the protons and electrons in the universe began to combine to form hydrogen atoms. This produced a burst of radiation which we should be able to see as in radio waves because the radiation is [red shifted]?. 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.
Although there are a few cosmologists who support [non-standard cosmologies]?, 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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