Timeline of the early Universe, showing the Dark Ages and the First Light, when reionization allows starlight to travel long distances. (credit: ESO)

Early in the Universe’s history, something ionized most of the diffuse hydrogen gas that’s spread between galaxies. But until now, the source responsible for this ionization has been largely mysterious—a conundrum so persistent, the authors of a new paper call it “one of the key questions in observational cosmology.”

First, a little background: The Universe’s hydrogen started out ionized because the early Universe was too hot and energetic for electrons to settle down and pair with protons. This situation persisted for about 375,000 years after the Big Bang, at which time the Universe had cooled enough for neutral hydrogen to exist. Then, any light produced by interactions among these hydrogen atoms was at a wavelength where it was quickly re-absorbed by other hydrogen atoms—the Universe was opaque.

It wasn’t until a few hundred million years later that some of the hydrogen in the intergalactic medium (IGM) began to be ionized again by an unknown source of energy. This event is known as the epoch of reionization, and it’s the last major phase transition in the history of the Universe. It returned the Universe to a state where light could travel long distances.