Can primordial black holes fill in the dark matter blank?
For something that hasn't actually been found yet, there are a lot of theories around about dark matter and where this elusive stuff could be hiding. Now an astrophysicist at NASA's Goddard Space Flight Center has weighed-in with a new idea that black holes created in the milliseconds after the Big Bang may be where all that missing matter ended up. Bizarrely, the hypothesis also suggests that all of the galaxies in the universe may be embedded in an ocean of millions of these black holes.
Because the number of visible objects such as stars, gas, and dust are insufficient to explain the total gravitational effects observed in the universe, where around 85 to 95 percent of all matter and energy is unaccounted for, the nature and location of dark matter is one of the big unresolved problems of astrophysics today. To try to account for this, a range of theories have been put forward, including the idea that it could be made of (yet undetectable) electrically-charged particles and the notion that it might be found as "hairs" surrounding planets.
One of the favorite theoretical models currently being expounded is that dark matter exists as an exotic massive particle, however on-going studies, including NASA's Fermi Gamma-ray Space Telescope missions, have so far been unsuccessful in turning up any proof that these theoretical particles even exist.
Nevertheless, observations using the Spitzer Space Telescope back in 2005 led astrophysicist Alexander Kashlinsky, to another conclusion, particularly when it was observed that light seemed to be "clumping" in various spots around the universe. After a similar X-ray study by the Chandra Observatory in 2013 confirmed that the uneven glow of the cosmic X-ray background (CXB) aligned with much of the unevenness of the cosmic infrared background (CIB), Kashlinsky surmised that the only things that could be luminous enough across the observed energy range would be black holes.
An image from NASA's Spitzer Space Telescope shows an infrared view of a sky area in the constellation Ursa Major
"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 Kashlinsky. "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 scientists have theorized that primordial black holes must have been plentiful among the first stars formed just after the Big Bang where, by their estimates, they probably made up at least about one out of every five of the energy sources contributing to the CIB. According to the researchers, the boiling-hot, rapidly-expanding early universe could have created primordial black holes just milliseconds after the Big Bang. As such, given that the time-frame for their generation was within a minuscule fraction of the very first second of existence, the researchers also expect that primordial black holes would be within a specified, narrow range of masses that they could look for.
As it turned out, the detection of gravitational waves by LIGO generated by two merging black holes showed that the masses of each of these objects were somewhere between 29 and 36 times the sun's mass, which meant that they were not only surprisingly large, but that their sizes were remarkably similar to the masses expected of primordial black holes.
"Depending on the mechanism at work, primordial black holes could have properties very similar to what LIGO detected," said Kashlinsky. "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."
An artist's interpretation of two black holes merging to produce gravity waves captured by the LIGO project
Much of the conjecture raised in the research centers on what might be if dark matter was actually a huge population of black holes distributed throughout the universe, and similar in size to those detected by LIGO. According to the scientists, the distortion caused by the black holes would have altered the spreading-out of mass in the early universe, which then showed its effects hundreds of millions of years afterwards when the very first stars began to coalesce.
According to accepted theory, for the first 500 million years of the universe after the Big Bang, ordinary matter was too hot to create stars as atomic excitation did not allow the electrostatic and nuclear force effects to take hold. Dark matter, on the other hand, was purportedly unworried by high temperatures when aggregating, because its primary motivator to formation is supposedly wholly gravity-driven. As such, the researchers posit the notion that dark matter formation was by mutual gravitational attraction, followed by coalescing into clumps known as "minihaloes" that then enabled normal matter to accumulate around them.
After this, hot gases fell in toward the minihaloes, which eventually became dense enough to collapse and create stars. In this way, if black holes are formed initially from dark matter then, as far as Kashlinsky is concerned, this would account for the "lumpiness" in the CIB, even if just a tiny proportion of minihaloes collapsed sufficiently to create stars. If this is the case, many of the stars in our universe could have been created in this way, and all of time and space could effectively be surrounded by an enormous ocean of black holes.
One more supporting factor to this theory comes from the idea that if cosmic gas was drawn into the minihaloes, then the resulting black holes would ultimately produce X-rays as these were consumed. By matching infrared light captured by Spitzer with X-rays from gas falling into black holes observed by Chandra, the researchers believe the alignment of the patchy nature of the CIB and the CXB can be explained.
"Future LIGO observing runs will tell us much more about the universe's population of black holes, and it won't be long before we'll know if the scenario I outline is either supported or ruled out," said Kashlinsky.
The video below is an animation depicting the merging of two black holes, as detected by the LIGO project.