Strange dark stuff is making the universe too bright

17 July 2014 by Lisa Grossman

LIGHT is in crisis. The universe is far brighter than it should be based on the number of light-emitting objects we can find, a cosmic accounting problem that has astronomers baffled.

"Something is very wrong," says Juna Kollmeier at the Observatories of the Carnegie Institution of Washington in Pasadena, California.

Solving the mystery could show us novel ways to hunt for dark matter, or reveal the presence of another unknown "dark" component to the cosmos.

"It's such a big discrepancy that whatever we find is going to be amazing, and it will overturn something we currently think is true," says Kollmeier.

The trouble stems from the most recent census of objects that produce high-energy ultraviolet light. Some of the biggest known sources are quasars – galaxies with actively feeding black holes at their centres. These behemoths spit out plenty of UV light as matter falling into them is heated and compressed. Young galaxies filled with hot, bright stars are also contributors.

Ultraviolet light from these objects ionises the gas that permeates intergalactic space, stripping hydrogen atoms of their electrons. Observations of the gas can tell us how much of it has been ionised, helping astronomers to estimate the amount of UV light that must be flying about.

But as our images of the cosmos became sharper, astronomers found that these measurements don't seem to tally with the number of sources found.

Kollmeier started worrying in 2012, when Francesco Haardt at the University of Insubria in Como, Italy, and Piero Madau at the University of California, Santa Cruz, compiled the results of several sky surveys and found far fewer UV sources than previously suggested.

Then in February, Charles Danforth at the University of Colorado, Boulder, and his colleagues released the latest observations of intergalactic hydrogen by the Hubble Space Telescope. That work confirmed the large amount of gas being ionised. "It could have been that there was much more neutral hydrogen than we thought, and therefore there would be no light crisis," says Kollmeier. "But that loophole has been shut."

Now Kollmeier and her colleagues have run computer simulations of intergalactic gas and compared them with the Hubble data, just to be sure. They found that there is five times too much ionised gas for the number of known UV sources in the modern, nearby universe.

Strangely, their simulations also show that, for the early, more distant universe, UV sources and ionised gas match up perfectly, suggesting something has changed with time (Astrophysical Journal Letters, doi.org/tqm).

This could be down to dark matter, the mysterious stuff thought to make up more than 80 per cent of the matter in the universe.

The leading theoretical candidates for dark matter are weakly interacting massive particles, or WIMPs. There are many proposed versions of WIMPs, including some non-standard varieties that would decay and release UV photons.

Knowing that dark matter in the early universe worked like a scaffold to create the cosmic structure we see today, we have a good idea how much must have existed in the past. That suggests dark matter particles are stable for billions of years before they begin to decay.

Theorists can now consider the UV problem in their calculations and see if any of the proposed particles start to decay at the right time to account for the extra light, says Kathryn Zurek, a dark matter expert at the University of Michigan in Ann Arbor. If so, that could explain why the excess only shows up in the modern cosmos.

If WIMPS aren't the answer, the possible explanations become even more bizarre, such as mysterious "dark" objects that can emit UV light but remain shrouded from view. And if all else fails, there's even a chance something is wrong with our basic understanding of hydrogen.

"We don't know what it is, or we would be reporting discovery instead of crisis," says Kollmeier. "The point is to bring this to everyone's attention so we can figure it out as a community."