The Mystery of the Missing Light

Astronomers have discovered that there is not nearly enough ultraviolet light from stars and quasars in the local universe to account for observations of intergalactic gas.

We appear to be missing something from our local universe, and a whole lot of it too: ultraviolet photons. A recent study of the local universe’s ultraviolet background finds a “photon underproduction crisis” that indicates a serious miscounting of nearby ultraviolet sources, a failure to understand the intergalactic medium, or possibly both.


Artist's impression of a quasar, one of the two primary contributors of UV light to the local universe.

The ultraviolet background pervading the universe comes from two main sources: quasars and young, hot stars. Their UV photons interact with the sparse gas that permeates intergalactic space, converting neutral hydrogen atoms into electrically charged ions. Quasars probably account for most of the extragalactic UV background because stars’ ultraviolet light is usually absorbed by gas in their host galaxies before it can interact with intergalactic hydrogen.

Because the UV background is too dim for astronomers to measure directly, they instead add up all the possible sources of ultraviolet radiation and account for how much is absorbed and reemitted by intergalactic gas.

Observations of the distant cosmos show near-perfect agreement between the number of ultraviolet sources and the ionization rate of intergalactic gas. But a study conducted by Juna Kollmeier (Carnegie Observatories) and colleagues shows a far more peculiar result for the nearby universe.

Kollmeier’s team simulated the ionization rate required to reproduce the Hubble Space Telescope’s observations of nearby intergalactic hydrogen. But to ionize hydrogen at that rate, the nearby universe would need five times more UV radiation than predicted by supercomputer simulations of the ultraviolet background.

Simulations of intergalactic hydrogen

Simulations of intergalactic hydrogen in a "dimly lit" universe (left) compared to a "brightly lit" universe (right) with five times more of the UV photons that destroy neutral hydrogen atoms. Hubble observations of the intergalactic medium match the picture on the right, but simulations using only the known cosmic UV sources produced the much thicker structures on the left. Ben Oppenheimer and Juna Kollmeier

The result puts astronomers in a bind. On the one hand, it’s difficult to believe that they have miscounted UV sources in the local universe by such an extreme amount, explains Romeel Davé (University of the Western Cape, South Africa and University of Arizona). On the other hand, it would be surprising if astronomers’ understanding of intergalactic hydrogen is way off the mark, since current models have explained observations so well for the past two decades.

Kollmeier’s team proposes a number of possible explanations for this crisis, though none of them seems to be totally satisfactory. Astronomers may need to completely reevaluate how much ultraviolet radiation comes from quasars and young stars, as well as how much stellar radiation escapes the stars’ host galaxies.

A more exciting alternative is that hitherto undiscovered photon sources dominate the local universe’s UV background. Possibly the most radical solution is that decaying dark matter in galactic halos produces the missing ultraviolet radiation. Kollmeier’s team estimates that if this were the case, then only a tiny amount of dark matter has decayed over the lifetime of the universe. But this explanation would radically alter our perception of dark matter properties.

“You know it’s a crisis when you start seriously talking about decaying dark matter!” Neal Katz (University of Massachusetts, Amherst) stated in the Carnegie Institution press release.

Speculations aside, the crisis shows a significant discrepancy between our current models and our observations of the present-day universe, says Joe Ribaudo (Utica College), who was not involved with the study. “Of course, anytime your theoretical models disagree with observation, it is an exciting opportunity to learn something new about the universe.”

Learn more about the mysteries of our universe with our special issue, Astronomy's 60 Greatest Mysteries!

3 thoughts on “The Mystery of the Missing Light

    1. Anthony BarreiroAnthony Barreiro

      Here’s what I was trying to say:

      ” ‘You know it’s a crisis when you start seriously talking about decaying dark matter!’ Neal Katz (University of Massachusetts, Amherst) stated in the Carnegie Institution press release.”

      I had a similar thought. Recently it seems that every unexplained observation is hypothesized to be evidence of dark matter.

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