"The authors have performed a careful and thorough analysis of the problem," said theoretical astrophysicist Abraham Loeb, chairman of the astronomy department at Harvard University, who did not take part in this research. [See amazing images of galaxies]
The most exciting possibility these findings raise is that the missing photons are coming from some exotic new source, not galaxies or quasars at all, Kollmeier said. For example, dark matter, the invisible and intangible substance thought to make up five-sixths of all matter in the universe, might be capable of decay and generating this extra light.
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"You know it's a crisis when you start seriously talking about decaying dark matter," study co-author Neal Katz at the University of Massachusetts at Amherst said in a statement.
There still may be a simpler explanation for this missing light, however. Astronomers could be underestimating the fraction of ultraviolet light that escapes from galaxies in the nearby universe. "All that one needs is an average escape probability on the order of 15 percent to relieve the discrepancy," Loeb told Space.com.
Nearby, recent "low-redshift" galaxies have less gas to absorb ultraviolet rays that more distant, early "high-redshift" galaxies, Loeb noted.
"The more I think about it, the more plausible it appears that the escape fraction of ultraviolet photons is higher in local galaxies than in high-redshift galaxies," Loeb said.
On the other hand, "the biggest problem with this possible solution is that there are measurements of local galaxies that indicate the average escape fraction is significantly lower than 15 percent - more like 5 percent," Kollmeier said."In principle, it is possible that these galaxies are not representative and therefore we need to do more such measurements, but we cannot just dismiss the data."
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Another potential explanation is ionization of intergalactic hydrogen by x-rays and cosmic rays, Loeb said. Although he noted this radiation does not play a major role in ionizing intergalactic hydrogen in the most distant corners and earliest times in the universe, astronomers may want to see how much of a role x-rays and cosmic rays play in the nearby universe, "where they are produced more vigorously," he said.
The scientists detailed their findings in the July 10 issue of the Astrophysical Journal Letters.
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