Astronomers May Have To Add Dark Light To Their List Of Open Problems

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Two long-standing, unsolved problems in astrophysics are the discrepancies between different ways of calculating mass and energy in the universe. Even though we don’t know how to reconcile the different measurements, these unknown solutions already have cool names (that no doubt add to their popular appeal), dark matter and dark energy. Now it looks like astronomers may have another discrepancy to add to the list: dark light.

“It’s as if you’re in a big, brightly lit room, but you look around and see only a few 40-watt lightbulbs,” says the Carnegie Institution for Science’s Juna Kollmeier, lead author of a new study on the missing light published in The Astrophysical Journal Letters. “Where is all that light coming from? It’s missing from our census.”

Dark Light: A 400% discrepancy in the number of hydrogen ions

When hydrogen gas is hit by particularly energetic ultraviolet light some of the neutral hydrogen is turned into hydrogen ions. The catch is that, as far as anyone knows, the right kind of light is only produced by extremely two different sources: young stars, whose radiation is almost completely absorbed in the host galaxy, and quasars, which are powered by supermassive black holes and aren’t exactly commonplace.

When Kollmeier and her fellow researchers used the $70 million Cosmic Origins Spectrograph installed on the Hubble Space Telescope to measure hydrogen ions in the hydrogen gas clouds between nearby galaxies, there were five times more ions than they could explain. When they looked at distant galaxies, the effect went away, meaning that something important has changed since the early days of the universe.

Dark Light: “Something is really wrong”

“The great thing about a 400 percent discrepancy is that you know something is really wrong,” said co-author David Weinberg of Ohio State University. “We still don’t know for sure what it is, but at least one thing we thought we knew about the present day universe isn’t true.”

As with dark matter and dark energy, there are a number of possible reasons for the discrepancy. The most obvious is that there are sources of ultraviolet light that weren’t present in the early universe, and which have somehow escaped notice until now. It could also mean that there is something wrong with the simulations that the team’s observations are being measured against, but this would be just as interesting because it implies a more fundamental misunderstanding of what’s happening in the universe.

“It’s possible the simulations do not reflect reality, which by itself would be a surprise, because intergalactic hydrogen is the component of the universe that we think we understand the best,” says Benjamin Oppenheimer of CU-Boulder’s Center for Astrophysics and Space Astronomy.

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