Reionisation, and cosmology explained

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2 Responses to “Reionisation, and cosmology explained”

  1. I’m no astronomer, but having read extensively about the timeline of the universe ( extensively on Wikipedia at least, hehehe) this chart seems to be flawed. It says that the universe was “opaque” during the dark ages and became transparent after the dark ages. But everything I’ve read says the universe became transparent roughly at the START of the dark ages. Photons were “set free” when the primordial plasma cooled to form neutral hydrogen.

    • The “dark ages” are so named since at that time, as you say, the universe was opaque to photons, as they were scattered by neutral hydrogen atoms. These atoms formed only after the universe became cool enough to allow atoms to form. The universe became transparent after ultraviolet light from the first stars began to ionise the hydrogen, allowing photons to propagate without scattering.

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