Black Holes ARE Dark Matter?

Dark matter could be almost anything.  With little data other than how much total dark matter mass exists, we can’t decode much about what individual chunks of dark matter might be made of.  I’ve talked before about Massive Compact Halo Objects (MACHOs) and Weakly Interacting Massive Particles (WIMPs), but these are just two possibilities.  Other theorists have talked about Modified Newtonian Gravity (MNG), where gravity may work differently on the grand scale than it does on our small Earth scales.  Or perhaps it’s something I haven’t seen before.  Maybe what we call dark matter is just a large population of ancient black holes.

The Cosmic Infrared Background. Credit: NASA/JPL-Caltech/A. Kashlinsky (Goddard)

“This study is an effort to bring together a broad set of ideas and observations to test how well they fit, and the fit is surprisingly good,” said Alexander Kashlinsky, an astrophysicist at NASA Goddard. “If this is correct, then all galaxies, including our own, are embedded within a vast sphere of black holes each about 30 times the sun’s mass.”

The Cosmic Infrared Background (CIB) is a diffuse glow of light, thought to be the glow from the first sources of light in the Universe after the big bang.  In 2013, it’s origin came into question at the Cosmic X-ray Background (CXB) was shown to match the variation seen in the CIB.  The first stars in the Universe, like stars today, shone mainly in Infrared and Ultraviolet light, and would not create a matching X-ray pattern.  So if the CIB and CXB are related, what contributed to both of them?

The only object known to be luminous across a wide enough energy range is a black hole, namely a primordial black hole.  In this case, it means that primordial black holes from the big bang must have been abundant among the first stars, making up one in 5 of the sources contributing to the CIB.  The early universe may have been like Swiss cheese.

But many scientists are focusing on dark matter being comprised of WIMPs, and spending big money on particle detectors to look for them.  As they narrow down the potential range of energies for a dark matter particle to have, the window for new ideas slowly opens. “These studies are providing increasingly sensitive results, slowly shrinking the box of parameters where dark matter particles can hide,” Kashlinsky said. “The failure to find them has led to renewed interest in studying how well primordial black holes — black holes formed in the universe’s first fraction of a second — could work as dark matter.”

Because primordial black holes formed quickly in the first fraction of a second after the big bang, there is a very narrow range of masses they can have.  Theoretical models place each of them at around 30 solar masses.  And this is where the fun part comes in.  When gravitational waves were detected earlier this year, it was also the first direct measurement of black holes.  The black holes that merged were 29 and 36 solar masses, plus or minus 4.  This puts them at nearly the same mass, and quite close to the theoretical mass for primordial black holes.  Did LIGO detect primordial black holes that are part of a massive population that resides between galaxies?

“Depending on the mechanism at work, primordial black holes could have properties very similar to what LIGO detected,” Kashlinsky explained. “If we assume this is the case, that LIGO caught a merger of black holes formed in the early universe, we can look at the consequences this has on our understanding of how the cosmos ultimately evolved.”

By continuing to interpret future LIGO data, we can gain incredible insights into the early universe.  If LIGO sees these mergers on a regular basis, they could be common, and so could these black holes.  We’re only working with one data point right now, but as we continue to add to the data, the patterns will emerge, and the answers will reveal themselves.


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