July 9, 2014

Something is amiss in the universe. There appears to be an enormous deficit of ultraviolet light in the cosmic budget.

Observations made by the Cosmic Origins Spectrograph, a $70 million instrument designed by the University of Colorado Boulder and installed on the Hubble Space Telescope, have revealed that the universe is “missing” a large amount of 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,” said 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.”

The research team—which includes Benjamin Oppenheimer and Charles Danforth of CU-Boulder’s Center for Astrophysics and Space Astronomy—analyzed the tendrils of hydrogen that bridge the vast reaches of empty space between galaxies. When hydrogen atoms are struck by highly energetic ultraviolet light, they are transformed from electrically neutral atoms to charged ions.

The astronomers were surprised when they found far more hydrogen ions than could be explained with the known ultraviolet light in the universe, which comes primarily from quasars. The difference is a stunning 400 percent.

Strangely, this mismatch only appears in the nearby, relatively well-studied cosmos. When telescopes focus on galaxies billions of light years away—which shows astronomers what was happening when the universe was young—everything seems to add up. The fact that the accounting of light needed to ionize hydrogen works in the early universe but falls apart locally has scientists puzzled.

The mismatch emerged from comparing supercomputer simulations of intergalactic gas to the most recent analysis of observations from the Cosmic Origins Spectrograph.

“The simulations fit the data beautifully in the early universe, and they fit the local data beautifully if we’re allowed to assume that this extra light is really there,” said CU-Boulder’s Oppenheimer. “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.”

The type of light that is energetic enough to turn neutral hydrogen into hydrogen ions is called “ionizing photons” and is known to come from only two sources in the universe: quasars, which are powered by hot gas falling onto supermassive black holes over a million times the mass of the sun, and the hottest young stars. Observations indicate that the ionizing photons from young stars are almost always absorbed by gas in their host galaxy, so they never escape to affect intergalactic hydrogen. But the number of known quasars is far lower than needed to produce the amount of light necessary to create the quantity of hydrogen ions measured by the research team.

“If we count up the known sources of ultraviolet ionizing photons, we come up five times too short,” Oppenheimer said. “We are missing 80 percent of the ionizing photons, and the question is where are they coming from? The most fascinating possibility is that an exotic new source, not quasars or galaxies, is responsible for the missing photons.”  

For example, the mysterious dark matter, which holds galaxies together but has never been seen directly, could itself decay and ultimately be responsible for this extra light.  

“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.”

Other co-authors on the study are Francesco Haardt of the Università dell’Insubria, Romeel Davé of the University of the Western Cape, Mark Fardal of University of Massachusetts Amherst, Piero Madau of the University of California, Santa Cruz, Amanda Ford of the University of Arizona, Molly Peeples of the Space Telescope Science Institute, and Joseph McEwen of Ohio State University.

The study was funded in part by NASA, the National Science Foundation and the Ahmanson Foundation.