The Universe Is a Whole Lot Brighter than It Should Be

The universe is becoming a darker place every minute as every bit of energy and matter contained within it races away from every other bit of energy and matter. The progression from the Big Bang and subsequent inflation to the final cosmic resting state is from perfect light to perfect dark; the transition progresses faster every moment. We’re fortunate to be here now, in the great inbetween, when all we have are so many explosions in the darkness and a sky that’s almost supernaturally blue. It’s bright around these parts, and a new analysis of data collected by the Hubble Space Telescope's Cosmic Origins Spectrograph suggests that it may even be too bright.

This conclusion, published in the The Astrophysical Journal Letters, comes courtesy of a team based at the Carnegie Institution for Science and new supercomputer simulations of intergalactic gases, particularly hydrogen and helium. The hydrogen portion of these clouds is called the Lyman-alpha forest, and is the source of our current mystery. This gas, which forms tendrils bridging the vast voids between galaxies, can be viewed as something of a cosmic ultraviolet light meter.

UV photons (light particles) emitted by quasars and very young stars within galaxies travel outward into the void, where they collide with gas particles, leaving them as electrically charged ions. And using the Hubble telescope, we can observe these ions and then calculate how much UV light is traveling through the universe's intergalactic voids.

An obvious question is, how do we even know how much light there should be in the very first place? This is easier than it looks. Astronomers know well enough that most of the UV light emitted by hot young stars gets soaked up by the gases in their local galaxies, while the number of quasars, the objects providing most of the ionizing UV light, is not sufficient to provide the light needed to match our intergalactic gas observations. It would appear that there’s some third UV light source out there, but more on that in a minute.

Hubble being the marvel that it is can look quite deeply into the universe, collecting data going back to nearly the Big Bang. It’s here that a twist is added to the missing light problem. In regions of the universe very far from us, billions of light years away and billions of years in the past, everything seems to add up right. It’s only when we get to our local patch of the cosmos that the imbalance manifests.

Of course, the imbalance could have to do with our gaseous light meters rather than the light itself, but Juna Kollmeier, the study’s lead investigator, is doubtful. “In fact, it's possible that our simulations are predicting the wrong hydrogen distribution relative to reality—i.e. our "light meter" is broken,” she told Motherboard. “But we think this is very unlikely. The hydrogen is one of the most well-understood aspects of our calculations. It is used as a tool for ‘precision cosmology’—one of the primary techniques for anchoring the very properties of the universe itself—for example the properties of the dark matter. In order to resolve this crisis, we would need the local hydrogen distribution to be much smoother than our standard predictions.”

In other words, for the hydrogen distributions the Carnegie team is basing its findings off of to be the “wrong” aspect of the missing-light observation, a lot of very basic stuff we know about the universe would have to be wrong as well. “If we put in the missing photons,” Kollmeier said, “everything seems to match the real universe beautifully. But the problem is that we don't know where those photons are coming from!”

One intriguing suggestion as to this source is not quite what one might expect for extra light: dark matter. Perhaps as dark matter, the as yet undetected (directly) gravitational glue that holds galaxies together, decays, it radiates away energetic photons, in essence generating light from the universe’s most pure darkness. As we're looking at a much younger universe at great distances, this decay might not be as far along as it is in the local, current universe: older dark matter, more decay and more light. Makes sense.

"In our local universe there is more ionized hydrogen than in the past, that's true in all models," Kollmeier said. "But it looks like there is even more than we expected when we accounted for all of the light sources we know about."

A dark matter solution wouldn’t account for the local/distant discrepancy in the missing light observations, however. We’re left mostly with a deep wrongness, which happens to be one of the best things a scientist in any field can discover. Wrongness is the pathway to newness and pure discovery, while confirmation is just that.

“The great thing about a 400 percent discrepancy is that you know something is really wrong," added co-author David Weinberg separately in a statement from the Carnegie Institution. "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."

There's no shortage of next steps in unraveling this mystery. Dark matter is only one of several possible resolutions mentioned in the study, which include incorrect estimates of the "escape fraction" of ionizing photons from galaxies into the intergalactic medium (IGM, the proper name for our voids), changing estimates of quasar brightness, unaccounted-for heat sources in the IGM, and, finally, just plain new sources of ionizing photons.

Kollmeier is unphased: "We really need to investigate all of the potential solutions we describe in the article, in addition to any other possible solutions that others come up with," she said. "If we investigate all of these and none end up being viable, we'll keep working until we figure it out!"