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A simulation of the era of reionization. When the Universe cooled down after the Big Bang, about 13.7 billion years ago, electrons and protons combined to form neutral hydrogen gas. This cool dark gas was the main constituent of the Universe during the so-called Dark Ages, when there were no luminous objects. This phase eventually ended when the first stars formed and their intense ultraviolet radiation slowly made the hydrogen fog transparent again by splitting the hydrogen atoms back into electrons and protons, a process known as reionisation. This epoch in the Universe's early history lasted from about 150 million to 800 million years after the Big Bang. This visualization shows the progress of reionization. Ionized regions are blue and translucent, ionization fronts are red and white, and neutral regions are dark and opaque. See Alvarez et al. (2009) for more details on the simulations. credit: M. Alvarez ( ~malvarez ), R. Kaehler, and T. Abel source:
