A basic fact of the universe is that space is a dark, black expanse that's only broken up by the lights of stars and galaxies. But that wasn't always the case — space was once a dense, opaque fog.
Just how the early universe went from such a dense conglomeration to the wide open expanse we observe today has been an open question. We at least understand the basic mechanism. Originally, the universe was full of hydrogen atoms, which made up the thick intergalactic medium. At some point, these neutral, uncharged atoms became charged hydrogen plasma, which gave the universe the transparent look that it retains to this day.
But what caused this massive realignment and destroyed the cosmic fog? University of Michigan researchers think they have the answer. They examined the galaxy NGC 5253, which is a dwarf starburst galaxy. Starburst galaxies are pretty much exactly what their name implies, which are galaxies in the middle of massive star formation. There aren't many such galaxies around these days, but astronomers believe they were everywhere in the early universe.
Since stars give off ultraviolet light, a starbust galaxy collectively emits gargantuan amounts of UV radiation. Michigan researchers Jordan Zastrow and Sally Oey found that the galaxy was venting the radiation into intergalactic space, evaporating all gas it came into contact with. The galaxy's stars actually created a "superwind" that cleared away gas within the galaxy and gave the UV radiation a passageway to escape to open space. This passageway took the form of a narrow cone, which was difficult to detect and might explain why we haven't observed this phenomenon before.
The point is that we now know that galaxies can vent huge supplies of ultraviolet radiation into the wider cosmos, and this radiation is able to affect the gas it comes into contact with. This provides a mechanism for how the early universe swept away the hydrogen fog and replaced it with the clear plasma we see (or don't see) today. And because starburst galaxies are rare now but extremely common way back then, it makes sense that they would hold the key to a mystery that has previously baffled astronomers.
Zastrow explains how the team made the find:
"We are not directly seeing the ultraviolet light. We are seeing its signature in the gas around the galaxy. This feature is relatively narrow. The opening that is letting the UV light out is small, which makes this light challenging to detect. We can think of it as a lighthouse. If the lamp is pointed toward you, you can see the light. If it's pointed away from you, you can't see it. We believe the orientation of the galaxy is important as to whether we can detect escaping UV radiation."
Via arXiv. Image by Jordan Zastrow.