The Creation of The Universe
Most astronomers today believe that the universe began with a cosmic explosion, the Big Bang, that occurred throughout all space at the beginning of time, approximately 15 billion years ago.
In the very beginning, all matter in the universe was concentrated in a state of infinite density. At the Big Bang, when the temperature of the universe reached an incredible 10 billion degreee Kelvin, the universe began to expand, marking the creation of the universe. In the next 3 minutes, when the temperature began to drop to less than 1 billion degree Kelvin, the nucleosynthesis began, filling the universe completely with high-energy photons colliding vigorously with protons and electrons. At this time, the universe was in its "primordial fireball" state that it was opaque. 1 million years after the Big Bang, when the energy of the photons became too weak to keep the protons and electrons apart, the protons and the electrons started to combine to form hydrogen atoms as temperature dropped below 3000 K. At this time, atoms became the most stable form, and since they are much smaller than visible light that they do not block the photons, the universe became transparent as we know it today.
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