The intergalactic medium in the young universe
(Phys.org) —In its earliest years, the universe was so hot that electrons and protons could not bind together in neutral atoms: all of the gas in the cosmos was ionized. Then, after 380,000 years of expansion, the universe cooled enough for hydrogen atoms and some helium (about 25%) to form. Much later in cosmic history—the precise dating is an active area of current research but perhaps after a few hundred million years—the first generation of stars emerged from the vast expanses of atomic gas, and these stars emitted enough strong ultraviolet light to re-ionize the neutral hydrogen in their vicinity. As the universe continued to expand and evolve, newer generations of stars continued to re-ionize the hydrogen until at some time most gas between galaxies (the intergalactic medium) was ionized once again. The epoch of re-ionization is an important diagnostic tool because it traces when the first generations of stars...