The Big Bang is a theory about the creation of the universe. It is the most widely accepted theory on the subject and it is the model of the initial conditions and subsequent development of the universe that is supported by the most comprehensive and accurate explanations from current scientific evidence and observation. Big bang is the idea that the universe has expanded from a primordial hot and dense initial condition beginning around 13.3 to 13.9 billion years ago and continues to expand today.
Georges Lamaitre proposed the big bang theory; however, he called it the hypothesis of the primeval atom. The theory is based on the past works of great mathematicians an physicists. He built his work on theories by Albert Einstien, Edwin Hubble, and Alexander Freidman, among others. Lamaitre started with Einstein’s theory of general relativity and simplified some assumptions. In 1927 he proposed that that the distances to far away galaxies were generally proportional to their redshifts(the appearance that an object is redder than it would appear to a hypothetical observer both closer to it and at rest relative to it ). This proposition was confirmed by Hubble in 1929. This was taken by scientists to indicate that all very distant galaxies and clusters have an apparent velocity going directly away from our vantage point: the farther away, the higher the apparent velocity. If the distance between galaxy clusters is increasing today, everything must have been closer together in the past. This idea has been considered in to extreme densities and temperatures. Large particle accelerators have been built to test such conditions, resulting in significant confirmation of the theory within the scope of the limitations of these accelerators ability to test high energy regimes. The big bang does not attempt to prove the initial condition of the universe, but what happened afterward.
With painstaking care, scientists have developed a time line for the big bang theory. By extrapolating the expansion of the universe backwards in time, general relativity yields an infinite density and temperature at a definite time in the past. This singularity signals the breakdown of general relativity. Many scientist debate how closely we can extrapolate to the singularity, but probably not earlier than the Planck era(the earliest period of time in the history of the universe, from zero to approximately 10-43 seconds Planck time, during which quantum effects of gravity were significant). The earliest, dense hot phase is considered the big bang which is considered the birth of our universe. Based on measurements of the expansion using type 1a super novae(a sub-category of cataclysmic variable stars that result from the violent explosion of a white dwarf star), measurements of temperature fluctuations in the cosmic microwave background, and measurements of the correlation function of galaxies, the universe has a calculated age of 13.73 ± 0.12 billion years.
There are too many theories about what happened during the early seconds of the big bang to list them all. Many scientists agree that the universe was filled homogeneously and isotropically with an incredibly high energy density, huge temperatures and pressures, and was very rapidly expanding and cooling. The overwhelming consensus begins to fall apart there.
Here on Universe Today we have a great article about the Big Bang. There is a good article about the big bang here. Astronomy Cast offers a good episode about the possibility that the big bang spawned other universes.
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