News FocusAstronomy

What Reionized the Universe?

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Science  01 Jun 2012:
Vol. 336, Issue 6085, pp. 1095-1096
DOI: 10.1126/science.336.6085.1095

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Some 400,000 years after the big bang, protons and electrons had cooled off enough for their mutual attraction to pull them together into atoms of neutral hydrogen. A few hundred million years later, something stripped the electrons off the atoms again. This time, however, the expansion of the universe had dispersed the protons and electrons enough so that the new energy source kept them from recombining. The "particle soup" was also dilute enough so that most photons could pass through it unimpeded. As a result, most of the universe's matter turned into the light-transmitting ionized plasma that it remains today. What caused this cosmic reionization? No one is sure.

  • * Edwin Cartlidge is a science writer in Rome.