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For 300,000 years, protons and atomic nuclei continued to roam about in a almost totally opaque sea of photons, electrons and neutrinos--opaque because the photons couldn't travel far without bumping into another particle. Any electron that combined with a proton or atomic nucleus was immediately knocked off by a traveling photon. Matter and radiation were intimately linked.
But after about 300,000 years, the opaque soup of nuclear matter and radiation began to clear. By this time, the temperature of the universe dropped to 3,000 K. Photons no longer had enough energy to knock electrons free from atomic nuclei and protons. Now the photons were free to travel through the universe, at last decoupled from matter. This era, called recombination, actually lasted about a million years.
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The vast sea of photons created during the earliest epochs prior to recombination persist to this day, in the form of cosmic microwave background that pervades the universe. No longer so energetic after being stretched by the expansion of the universe for roughly 20 billion years, this radiation has cooled to a chilly 2.73 K (minus 270.43 degrees Celsius!). It's nonetheless considered by cosmologists to be one of the clearest signatures of the Big Bang. The uniformity of this radiation--to within a few parts in 100,000--indicates that the universe was extremely smooth around the time of recombination.
Tiny variations have recently been found in this background radiation, indicating minute fluctuations in density of matter and energy at recombination. These fluctuations were eventually amplified by gravity to form the objects which make up our hierarchical universe: stars, galaxies, galaxy clusters and superclusters.
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