Astronomers studying the motions of galaxies and the character of the cosmic microwave background radiation came to realize in the last century that most of the matter in the universe was not visible. About 84 percent of the matter in the cosmos is dark matter, much of it located in halos around galaxies. It was dubbed dark matter because it does not emit light, but it is also mysterious: it is not composed of atoms or their usual constituents like electrons and protons.
Meanwhile, astronomers have observed the effects of black holes and recently even detected gravitational waves from a pair of merging black holes. Black holes usually are formed in the explosive death of massive stars, a process that can take many hundreds of millions of years as a star coalesces from ambient gas, evolves and finally dies.
“Deeply Compelling” –Primordial Back Holes as Dark Matter
Some black holes are inferred to exist in the early universe, but there is probably not enough time in the early universe for the normal formation process to occur. Some alternative methods have been proposed, like the direct collapse of primordial gas or processes associated with cosmic inflation, and many of these primordial black holes could have been made.
“The prospect that the mysterious dark matter might consist of primordial black holes is deeply compelling,” astrophysicist Eric Gawiser at Rutgers University told The Daily Galaxy. “We know that dark matter doesn’t emit light and instead interacts only through gravity. And we know that even a few minutes after the Big Bang when the first atomic nuclei were formed, there wasn’t enough normal matter around to make up today’s dark matter, which has led us to imagine that dark matter is made up of a new type of particle that nobody has been able to find yet.
Primordial Black Holes as Ordinary Matter?
“The really intriguing aspect,” continues Gawiser, “is that small black holes could actually be made out of normal matter, as long as it collapsed into black holes in the first minute after the Big Bang. These “primordial” black holes would check all of these boxes; they wouldn’t emit light, they would interact through gravity, they would not have acted like normal matter one minute after the Big Bang – and they wouldn’t require the existence of a made-up particle that physicists seem unable to find!
“So why aren’t primordial black holes the preferred theory for the nature of dark matter?” writes Gawiser. “Because we don’t have a good model for what would cause them to form in the first few seconds after the Big Bang. Also, there are searches within our Milky Way galaxy for gravitational lensing caused by small black holes of this type, called microlensing, and they have not yet found evidence for the type of planet-mass black holes that would be required. That leaves us without a good current theory for the nature of dark matter, but I wouldn’t bet against the black holes.”
Harvard-Smithsonian Center for Astrophysics astronomer Qirong Zhu, who studies the co-evolution of massive black holes and host galaxies from cosmic dawn to present day, led a group of four scientists investigating the possibility that today’s dark matter is composed of primordial black holes, following up on previously published suggestions. If galaxy halos are made of black holes, they should have a different density distribution than halos made of exotic particles. There are some other differences as well—black hole halos are expected to form earlier in a galaxy’s evolution than do some other kinds of halos.
The scientists suggest that looking at the stars in the halos of faint dwarf galaxies can probe these effects because dwarf galaxies are small and faint (they shine with a mere few thousand solar luminosities) where slight effects can be more easily spotted.
The team ran a set of computer simulations to test whether dwarf galaxy halos might reveal the presence of primordial black holes, and they found that they could: interactions between stars and primordial halo black holes should slightly alter the sizes of the stellar distributions.
The astronomers also conclude that such black holes would need to have masses between about two and fourteen solar masses, right in the expected range for these exotic objects (although smaller than the black holes recently spotted by gravitational wave detectors) and comparable to the conclusions of other studies.
The team emphasizes, however, that all the models are still inconclusive and the nature of dark matter remains elusive.
LIGO Observations Do Not Exclude the Possibility
“The idea that dark matter is composed of primordial black holes (PBH) is very intriguing,” Distinguished Lecturer in Physics at the University of New Haven, Nikodem Poplawski. wrote in an email to The Daily Galaxy. “However, if they existed,” he continued, “they would have been creating binary pairs and merging at rates thousand times larger than the rates observed by LIGO: However, a later research refined those calculations and argued that LIGO observations do not exclude the possibility that the entire dark matter could be PBH (with most of them having mass on the order of one solar mass):
“Also,” Poplawski points out, “Hawking’s evaporation puts a lower limit on the mass of a black hole on the order of 10^11 kg (smaller black holes would have evaporated by now).
“I wrote an article back in 2010,” he continued, “considering the Einstein-Cartan theory of gravity (the simplest extension of general relativity in which quantum-mechanical spin produces spacetime torsion that eliminates singularities and allows black holes to create new nonsingular universes). That theory puts a lower limit on black holes on the order of 10^16 kg (the mass of a typical planetoid):This could be consistent with a study arguing that dark matter could be asteroid-mass PBH: Yet, PBHs are still hypothetical and have not been directly detected. The smallest discovered black hole is about 3 solar masses (10^30 kg)”
Image credit: NASA/JPL