Galaxies are bright places thanks to the fact that they're home to the giant spotlights we call stars. The space in between galaxies, though, tends to be a bit darker. Still, even these less dazzling expanses of space do contain their share of wispy light, produced when hydrogen gas is molecularly excited by ultraviolet photons.
Now, using computer simulations, researchers at the Carnegie Institution for Science have determined that this area between galaxies actually has more light than they can account for -- 400 percent more light to be exact.
"It's as if you're in a big, brightly lit room, but you look around and see only a few 40-watt light bulbs," said Carnegie's Juna Kollmeier, lead author of a study recently published in The Astrophysical Journal Letters. "Where is all that light coming from? It's missing from our census."
Two primary sources of the photons cause hydrogen to light up in intergalactic space: quasars and hot young stars. The photons from young stars, however, rarely escape their host galaxy, so that leaves the quasars -- supermassive black holes fed by hot gas.
Yet when the researchers look at the number of known quasars and compare it to the light they see in intergalactic space using supercomputer simulations and data from the Hubble Space Telescope's Cosmic Origins Spectrograph, there is a mismatch.
"Either our accounting of the light from galaxies and quasars is very far off, or there's some other major source of ionizing photons that we've never recognized," Kollmeier said in a statement. "We are calling this missing light the photon underproduction crisis. But it's the astronomers who are in crisis -- somehow or other, the universe is getting along just fine."
Puzzlingly, when the researchers focus their telescopes at extremely distant galaxies -- those billions of light years away -- the discrepancy vanishes. It only appears when observing the more recent light from closer galaxies.
"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," explained Ben Oppenheimer, a co-author from the University of Colorado. "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."
So where is all the extra light coming from, and why the discrepancy between the distant universe and the one closer to us? At this point, astronomers don't really know but posit that the source could be decaying dark matter -- the enigmatic material that NASA estimates makes up about 27 percent of the universe.
"The most exciting possibility is that the missing photons are coming from some exotic new source, not galaxies or quasars at all," said Neal Katz, a co-author from the University of Massachusetts at Amherst. "You know it's a crisis when you start seriously talking about decaying dark matter."
Whether from dark matter or some other source, Katz isn't the only one who's lit up about "the photon underproduction crisis."
"The great thing about a 400 percent discrepancy is that you know something is really wrong," said co-author David Weinberg of The 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."