THE REIONIZATION OF COSMIC HYDROGEN BY THE FIRST GALAXIES
5.1 Ionization Scars by the First Stars
The cosmic microwave background (CMB) indicates that hydrogen atoms formed 400 thousand years after the Big Bang, as soon as the gas cooled below 3,000K as a result of cosmological expansion. Observations of the spectra of early galaxies, quasars, and gamma-ray bursts indicate that less than a billion years later the same gas underwent a wrenching transition from atoms back to their constituent protons and electrons in a process known as reionization. Indeed, the bulk of the Universe’s ordinary matter today is in the form of free electrons and protons, located deep in intergalactic space. The free electrons have other side effects; for example, ...