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In the Big Bang theory, the Universe was extremely hot and dense which rapidly expanded. This caused it to cool and is presently still expanding. It was thought to have taken place 13.75 billion years ago--the age of the universe. It expanded from a singularity and when it began to cool, energy was converted to subatomic particles. These combined to form nuclei and eventually, atoms formed. Elements like hydrogen formed and then helium and lithium. These became stars and galaxies. There is evidence that the universe is expanding still--redshifts of galaxies, for instance. Also, there is cosmic microwave background radiation coming from all directions, indicating the universe is expanding. However, will it keep expanding? There is a theory called the Big Freeze which proposes that continued expansion results in a universe asymptotically approaching absolute zero in temperature. This would mean it would stop. Heat death is a different scenario in which the universe goes to maximum entropy where everything is equally distributed and no more gradients exist. Therefore, there is no energy transfer nor any life. Right now, it is not at absolute zero and it is still expanding.
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