Dark Matter May Be Made Up of Black Holes

Monday, 13 June 2016 - 5:32PM
Space
Black Holes
Monday, 13 June 2016 - 5:32PM
Dark Matter May Be Made Up of Black Holes
We all know about dark matter - the elusive, mysterious substance that composes most of the material universe, and is widely thought to be some sort of massive exotic particle. Now, some astronomers are beginning to think that dark matter is actually made up of ancient black holes formed during the first second of our universe's existence. 

Alexander Kashlinsky, an astronomer at the NASA Goddard Space Flight Center in Maryland, thinks that these primordial black holes could perfectly explain the observations of gravitational waves made by LIGO. "This stud 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," says Kashlinsky.

If Kashlinsky's theory hold true, then dark matter might be composed of these primordial black holes, and all galaxies are embedded within a vast sphere of black holes, each measuring over 30 times the sun's mass. 

In 2005, Kashlinsky and his colleagues used NASA's Spitzer Space Telescope to explore the background infrared glow in order to explore the early universe. Since light from cosmic objects takes a finite amount of time to travel through space, examining light helps astronomers see the way distant objects looked in the past. "Suppose you look at New York from afar. You cannot see individual lampposts or buildings, but you can see the light they produce," explains Kashlinsky. When researchers removed all the light from the known galaxies, they could still detect excess light - the background glow from the first sources to illuminate the universe billions of years ago.

In 2013, another study compared how this cosmic infrared background (CIB) compared to the cosmic X-ray background, CBX in the same area of the sky. The first stars emitted mainly optical and ultraviolet light, which means that there should have been relatively little overlap between the CIB and the CBX. Yet the irregular glow of low-energy X-rays in the CBX matched the CIB patches quite well. The only objects scientists know of that that can produce this kind of light in both infrared and X ray are black holes, but Kashlinsky says, "It never crossed my mind at that time that these could be primordial black holes."  

Then, in 2015, there was LIGO, the observatory that made the first ever direct detection of gravitational waves, and subsequently, the first direct detection of black holes. 

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"Depending on the mechanisms at work, primordial black holes could have properties very similar to what LIGO detected," says Kashlinsky.
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In his new paper, the NASA astronomer analyzes what might have happened if dark matter consisted of a population of black holes similar to those detected by LIGO.  For the first 500 million years of the universe's history, dark matter collapsed into clumps called halos, which provided the gravitational seeds that would later enable matter to accumulate and form the first stars and galaxies. But if that dark matter was comprised of primordial black holes, that process would have created many more halos, thus explaining the infrared glow.  

The gas that created those stars would also have fallen onto the primordial black holes, heating up to high enough temperatures that it would have sparked X-rays. While the cosmic infrared background can be explained, albeit to a lesser extent, without the black hole theory, the cosmic X-ray background cannot be. Kashlinsky's theory connects these two observations together, but more research on black holes is needed to confirm.
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Black Holes

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