The Universe Is Missing Some Light

New data from the Hubble Space Telescope and computer simulations have revealed that the universe has much less ultraviolet light than previously thought.

Credit: Ben Oppenheimer and Juna Kollmeier

An extraordinary amount of ultraviolet light appears to be missing from the universe, scientists have found.

One potential source of this missing light might be the mysterious dark matter that makes up most of the mass in the cosmos but a simpler explanation could be that ultra violet light escapes from galaxies more easily than is currently thought, according to the new research.

This puzzle begins with hydrogen, the most common element in the universe, which makes up about 75 percent of known matter. High-energy ultraviolet light can convert electrically neutral hydrogen atoms into electrically charged ions.

The two known sources for such ionizing rays are hot young stars and quasars, which are supermassive black holes more than a million times the mass of the sun that release extraordinarily large amounts of light as they rip apart stars and gobble matter.

Astronomers previously found that ionizing rays from hot young stars are nearly always absorbed by gas in their home galaxies. As such, they virtually never escape to affect intergalactic hydrogen.

Space Telescope Imaging Spectrograph

However, when scientists performed supercomputer simulations of the amount of intergalactic hydrogen that should exist and compared their results with observations from the Hubble Space Telescope's Cosmic Origins Spectrograph, they found the amount of light from known quasars is five times lower than what is needed to explain the amount of electrically neutral intergalactic hydrogen observed.

"It's as if you're in a big, brightly-lit room, but you look around and see only a few 40-watt lightbulbs," lead study author Juna Kollmeier, a theoretical astrophysicist at the Observatories of the Carnegie Institution of Washington in Pasadena, Calif., said in a statement.

"Where is all that light coming from? It's missing."

The researchers are calling this giant deficit of ultraviolet light "the photon underproduction crisis."

"In modern astrophysics, you very rarely find large mismatches like the one we are talking about here," Kollmeier told reporters.

"When you see one, you know that there is an opportunity to learn something new about the universe, and that's amazing."

"The great thing about a 400 percent discrepancy is that you know something is really wrong," study co-author David Weinberg at Ohio State University said in a statement.

"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."

Strangely, this missing light only appears in the nearby, relatively well-studied cosmos.

When telescopes focus on light from galaxies billions of light years away, and therefore from billions of years in the past, no problem is seen.

In other words, the amount of ultraviolet light in the early universe makes sense, but the amount of ultraviolet light in the nearby universe does not.

"The authors have performed a careful and thorough analysis of the problem," said theoretical astrophysicist Abraham Loeb, chairman of the astronomy department at Harvard University, who did not take part in this research.

The most exciting possibility these findings raise is that the missing photons are coming from some exotic new source, not galaxies or quasars at all, Kollmeier said.

For example, dark matter, the invisible and intangible substance thought to make up five-sixths of all matter in the universe, might be capable of decay and generating this extra light.

"You know it's a crisis when you start seriously talking about decaying dark matter," study co-author Neal Katz at the University of Massachusetts at Amherst said in a statement.