Dark Ages of the Universe

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I hope you enjoy as much as I did.<br /><br />Yesterday I was reading an article from Abraham Loeb, The Dark Ages of the Universe, and later as I was reading a thread "Shadowy question" I remembered an article in which both shadows and Abraham Loeb are mentioned.<br /><br />Probably the less known period in universe is the one between Recombination and Reionization.<br /><br />Recombination occurred ~300K yrs after the BB when neutral atoms were formed and during this process CMBR was released.<br /><br />Reionization occurred between ~100M yrs after the BB (when first stars formed and reionized the intergalactic gas) and ~1G yrs after the BB (when large galaxies were formed.<br /><br />But what is Recombination and what is reionization?<br /><br />Recombination from chaos to order at the atomic level:<br /><br />The first ionization period occurred during the first 300K yrs after the BB. The temperature was high enough that all of the hydrogen was ionized (electrons were free and separate from the protons). The free electrons caused photons to scatter around in all directions and could not travel far before changing their direction. This interaction between electrons and protons is called Thomson scattering. Matter and radiation were coupled together and the universe was "opaque" during this period.<br /><br />As the universe expanded and the gas cooled down (~3000 Kelvin) the hydrogen ionization rate (the ionization rate increases with increasing temperature) became lower than the recombination rate. Protons and electrons were able to combine to form neutral hydrogen atoms. <br /><br />After recombination, photons were able to travel through the universe relativel
 
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