Primordial black holes (PBHs) are particularly special types of black hole, thought to have been formed from dense pockets of subatomic matter within a second after the Big Bang – and a new study reports on what could be the first direct observation of one.
That might take years to prove, but the possibility is thrilling.
Black holes are usually caused by the collapse of a supernova star, but scientists have long thought that PBHs could have existed in the earliest moments of the Universe as well, with no star required. They’ve always been theoretical, but there’s a growing body of recent evidence hinting at their existence.
Now, astrophysicists Alberto Magaraggia and Nico Cappelluti, from the University of Miami, have caught sight of another potential PBH using the Laser Interferometer Gravitational-Wave Observatory (LIGO), located across two sites in Washington and Louisiana.
The gravitational waves that LIGO detects are ripples across spacetime, and can be triggered by the collision of two black holes. A signal picked up by LIGO and investigated by the researchers indicated a collision in which one of the objects was less than a single solar mass (the mass of the Sun) – potentially a PBH.
“The most common black holes form as the result of a supernova, the death of a massive star,” says Cappelluti. “So, their masses can range from a few times the Sun’s mass to billions of solar masses.”
Primordial black holes, on the other hand, are expected to have much lower masses.
“We believe our study will aid in confirming that [PBHs] actually do exist,” says Cappelluti.
Further study of the signal, known as S251112cm, is needed to know for sure, but the researchers say the existence of a PBH with a subsolar mass is the most likely explanation.
Magaraggia and Cappelluti also ran calculations on the expected frequency of PBHs in space, and from there how often LIGO might be expected to spot them – which matched up with the data from LIGO since it first started detecting gravitational waves in 2015.
“We attempted to estimate how many primordial black holes may exist in the Universe and how many of them LIGO should be able to detect,” says Magaraggia.
“And our results are encouraging. We predict that subsolar black holes like the one LIGO may have observed should indeed be rare, consistent with how infrequently such events have been seen so far.”
Like the more common, regular black holes, PBHs don’t let any light escape from them, making them difficult to detect. They’re also believed to be smaller than other black holes, perhaps down to asteroid size in some cases.
Add in the difficulties of looking back through billions of years of time, and we really are talking about spotting needles in a cosmic haystack. However, if they can be identified and charted, they may help explain another cosmic phenomenon: dark matter.
Like PBHs, dark matter is also hypothetical, but astrophysicists think that it might make up 85 percent of the mass of the Universe, and be responsible for holding everything else together. While we can’t directly see dark matter, clues to its existence can be found in the behavior of space and time around us.
PBHs may be responsible for the majority of dark matter, experts think. There would have been a mind-bogglingly high number of them to begin with, starting from incredibly tiny sizes, and could then have expanded out to fill the vastness of space.
We’re going to need to spot more PBHs to verify their existence, but that should become increasingly likely as installations such as LIGO continue to get upgraded and new instruments come online – like the European Space Agency’s Interferometer Space Antenna (LISA), a gravitational wave detector launching in 2035.
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“LIGO picked up what is very strong evidence that these types of black holes exist, but we’ll need to detect another such signal or even several others to get the smoking-gun confirmation that they are real,” says Cappelluti.
“What is clear is that they cannot be excluded as being real.”
The research will be published in an upcoming issue of The Astrophysical Journal, and is available on arXiv.