All the light in the observable universe provides about as much illumination as a 60-watt bulb seen from 2.5 miles away. And all the energy ever radiated by all the stars that ever existed is still with us, filling the universe with a sort of fog, a sea of photons known as the extragalactic background light. And yet, a discovery in 2014 suggested that the source of light in the universe from known populations of galaxies and quasars is not nearly enough to explain observations of intergalactic hydrogen. The filaments of hydrogen and helium that bridge the vast reaches of empty space between galaxies that astronomers use as a “light meter” yielded a stunning 400 percent discrepancy.
“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 of the University of Massachusetts at Amherst about the discovery that the source of light in the Universe from known populations of galaxies and quasars is not nearly enough to explain observations of intergalactic hydrogen. Filaments of hydrogen and helium that bridge the vast reaches of empty space between galaxies that astronomers use as a precise “light meter” yield a stunning 400 percent discrepancy.
Exotic Dark Matter
“Astronomers estimate that the observable universe — a bubble 14 billion light-years in radius, which represents how far we have been able to see since its beginning — contains at least two trillion galaxies and a trillion trillion stars,” writes Dennis Overbye for “Out There” in New York Times. “Most of these stars and galaxies are too far and too faint to be seen with any telescope known to humans.”
In a 2014 study, The Photon Underproduction Crisis, published in The Astrophysical Journal, 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.”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,” noted Carnegie Institute’s Juna Kollmeier. “Where is all that light coming from? It’s missing from our census.”
Arriving at a number on the amount of starlight ever produced has variables that make it difficult to quantify. But according to the new measurement, the number of photons (particles of visible light) that escaped into space after being emitted by stars translates to 4×10^84, or a lot.
Enter mysterious dark matter, which, suggests Katz, holds galaxies together but has never been seen directly, and 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!” he quips.
“The great thing about a 400% discrepancy is that you know something is really wrong,” commented co-author and astrophysicist, 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.”
It All Adds Up in the Early Universe
Strangely, this mismatch only appears in the nearby, relatively well-studied cosmos. When telescopes focus on galaxies billions of light years away and 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 baffled.
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.
Missing Source of Ionizing photons
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 new version of Hubble’s deep image is shown at the top of the page In dark grey you can see the new light that has been found around the galaxies in this field–the brightness of more than one hundred billion suns. The image tookresearchers at the Instituto de Astrofísica de Canarias almost three years to produce this deepest image of the Universe ever taken from space, by recovering a large quantity of ‘lost’ light around the largest galaxies in the iconic Hubble Ultra-Deep Field.