Cosmic Reionization of Hydrogen and Helium

Cosmic Reionization of Hydrogen and Helium

Cosmic reionization is a period in the Universe history when it switched from being predominantly neutral to mostly ionized. We still haven’t quite pinned down the source(s) that caused this transition, but we have our suspicions. It could be quasars. It could be galaxies. Or could it be something else?

[Figure from universe-review.ca/F05-galaxy06.htm]