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# : 18 Sep 2007 21:17 | ویرایش بوسیله: unbekannt
The History of the Universe
Big Bang: The most accepted theory of the origin of the universe is the "Big Bang" theory. This suggests, at the start of time the universe was a single point, a singularity. The universe is expanding, we know this because the light from distant galaxies is shifted towards the red end of the spectrum. This shows that it is moving away from us. In general the further away the object is, the greater the red shift. This implies that all the matter originated from a single point.
Two dimensional diagram of the expansion of the universe.
Stages of the universe: The universe went through many stages and different states to get to the current one we know today. Due to studies of very distant objects and cosmic microwave background radiation a well educated theory explains the stages of the Big Bang. Before the Big Bang and up to 10-43s after it, it is unknown what events occurred. There are rough theories, like the big bang occurred in the singularity of a black hole, but nothing substantial. At about 10-6s the formation of subatomic particles, like quarks, and electrons occurred. This lead on to collisions between these particles, annihilating each other at about one second. Between one second and three minutes nuclear fusion occurred and light elements were formed.
After about 38,000 years photons no longer are able to react strongly with atoms, and hydrogen nuclei capture electrons and form stable atoms. Cosmic microwave background radiation also forms. After about 108years the first star begins to shine, leading on to the formation of protogalaxies after 109years and eventually galaxies after 3*109years. Finally after 1.37*1010years we reach the universe today.
Diagram of the stages of the big bang leading to the modern day universe
The Present State of the Universe
The universe is expanding at an accelerated rate. For this to be possible many theories suggest the existence of dark energy and dark matter. It is believed that matter as we know it only accounts for about 4% of the universe. Dark matter accounts for about 22% and dark energy about 74%.
Dark matter is named "dark", because it cannot be detected by electromagnetic radiation. It is only detected through its gravitational effect on other matter. It accounts for a number of effects that occur in the large scale universe, such as gravitational lensing by galaxy clusters, rotation speed of galaxies, orbiting velocity of galaxy clusters and other effects in galaxies and galaxy clusters. Evidence suggests that dark matter is made up of new elementary particles, called nonbaryonic dark matter.
Dark energy is a form of energy with negative pressure, which covers all of space. It is believed to exist, in order to explain the accelerated expansion of the universe. Measurements of the cosmic microwave background radiation imply that the universe is very nearly flat. It also suggests that the total matter in the universe, baryonic and dark matter, accounts only for about 26% of the density that would be needed for this to be true. Therefore an additional form of energy (i.e. dark energy) must account for the remaining 74%. Dark energy has a density of 10-26Kgm-3, is very homogenous and is only known to interact through gravity.
The Future
The future of the universe has many theories associated with it. They don't all agree whether the universe will have and end, or be infinite. The main theories are:
The Big Rip: The big rip is a theory that occurs following general relativity, it states the universe will continue to expand at an accelerated rate, eventually reaching a point where life cannot exist. The accelerated expansion occurs due to dark energy, leading to everything being ripped apart into elementary particles. As the acceleration becomes infinite, the universe becomes a singularity.
The Big Crunch: The big crunch assumes that the average density of the universe will stop it expanding and it will begin to contract. This may cause it to collapse into a singularity, which may then lead to a big bang creating another universe. If this is repeated then the universe is an infinite cycle of universes starting in big bangs and ending in big crunches.
The Big Freeze: The big freeze theory suggests that the universe will continue to expand until it becomes too cold to support life. This can only occur for certain shapes of the universe, which allow it to expand forever. The heat death theory is related to this and will occur for all three models of space, and requires a minimum temperature to be reached. It suggests everything is evenly distributed and and life cannot be supported.
Other theories: consist of the universe being one of many, and the false vacuum theory, in which the universe isn't currently at its lowest energy state. This transition to the lowest energy state would cause a drastic alteration in the structure and laws of the universe.
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