Computer simulations of intergalactic
hydrogen in a "dimly lit" universe (left) and a "brightly lit" universe
(right) that has five times more of the energetic photons that destroy
neutral hydrogen atoms. Hubble Space Telescope observations of hydrogen
absorption match the picture on the right, but using only the known
astronomical sources of ultraviolet light produces the much thicker
structures on the left, and a severe mismatch with the observations.
Image is credited to Ben Oppenheimer and Juna Kollmeier. A larger
version is available here.
Pasadena, CA—Something is amiss in the Universe. There appears to be an enormous deficit of ultraviolet light in the cosmic budget.
The vast reaches of empty space between galaxies are bridged by
tendrils of hydrogen and helium, which can be used as a precise “light
meter.” In a recent study published in The Astrophysical Journal Letters,
a team of scientists finds that the light from known populations of
galaxies and quasars is not nearly enough to explain observations of
intergalactic hydrogen. The difference is a stunning 400 percent.
"It's as if you're in a big, brightly-lit room, but you look around
and see only a few 40-watt lightbulbs," noted Carnegie’s Juna Kollmeier,
lead author of the study. "Where is all that light coming from? It’s
missing from our census."
Strangely, this mismatch only appears in the nearby, relatively
well-studied cosmos. When telescopes focus on galaxies billions of light
years away (and therefore are viewing the universe billions of years in
its past), everything seems to add up. The fact that this accounting
works in the early universe but falls apart locally has scientists
puzzled.
The light in question consists of highly energetic ultraviolet
photons that are able to convert electrically neutral hydrogen atoms
into electrically charged ions. The two known sources for such ionizing
photons are quasars—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 required light.
“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. “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.”
The mismatch emerged from comparing supercomputer simulations of
intergalactic gas to the most recent analysis of observations from
Hubble Space Telescope’s 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,” 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.”
“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.
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.
"You know it's a crisis when you start seriously talking about decaying dark matter!" Katz remarked.
“The great thing about a 400% discrepancy is that you know something
is really wrong,” commented 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.”
Whether the explanation is exotic or not, astronomers will be working hard to shed light on the mystery.
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 University
of California Santa Cruz, Charles Danforth of the University of
Colorado, Amanda Ford of University of Arizona, Molly Peeples of the
Space Telescope Science Institute, and Joseph McEwen of The Ohio State
University.
Source: Carnegie Institution for Science
