The Big Bang Theory
The Big Bang model postulates that about 15 to 20 billion years ago, the universe violently exploded into being, in an event called the Big Bang. Before the Big Bang, all of the matter and radiation of our present universe were packed together in the primeval fireball-an extremely hot dense state from which the universe rapidly expanded.1 The Big Bang was the start of time and space. The matter and radiation of that early stage rapidly expanded and cooled. Several million years later, it condensed into galaxies.
The universe has continued to expand, and the galaxies have continued moving away from each other ever since. Today the universe is still expanding, as astronomers have observed.The Steady State model says that the universe does not evolve or change in time. There was no beginning in the past, nor will there be change in the future. This model assumes the perfect cosmological principle. This principle says that the universe is the same everywhere on the large scale, at all times.2 It maintains the same average density of matter forever.
There are observational evidences found that can prove the Big Bang model is more reasonable than the Steady State model. First, the redshifts of distant galaxies. Redshift is a Doppler effect which states that if a galaxy is moving away, the spectral line of that galaxy observed will have a shift to the red end. The faster the galaxy moves, the more shift it has. If the galaxy is moving closer, the spectral line will show a blue shift. If the galaxy is not moving, there is no shift at all. However, as astronomers observed, the more distance a galaxy is located from Earth, the more redshift it shows on the spectrum.
This means the further a galaxy is, the faster it moves. Therefore, the universe is expanding, and the Big Bang model seems more reasonable than the Steady State model.The second observational evidence is the radiation produced by the Big Bang. The Big Bang model predicts that the universe should still be filled with a small remnant of radiation left over from the original violent explosion of the primeval fireball in the past. The primeval fireball would have sent strong shortwave radiation in all directions into space. In time, that radiation would spread out, cool, and fill the expanding universe uniformly.
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