We may have already seen black holes from the dawn of the universe, called primordial black holes. The Laser Interferometer Gravitational-Wave Observatory (LIGO) in the US and the Virgo observatory in Italy have detected 47 pairs of black holes slamming into one another, and a statistical study suggests that nearly one-third of them may be primordial.
Black holes can form through several different mechanisms. The main way is by a huge star collapsing in on itself, forming what is called an astrophysical black hole. Some black holes are too large to have formed that way directly, so they probably come from the mergers of smaller black holes. And primordial black holes may have formed in the early universe from dense clouds of plasma, but as yet we have no direct evidence for their existence.
鈥淲hen we get a black hole observation from LIGO, it does not come with a label that tells us how it was formed 鈥� it just comes with a mass and a spin,鈥� says at the Massachusetts Institute of Technology. Vitale and his colleagues performed a statistical analysis of the data from LIGO and Virgo that was informed by data from three of the leading formation models for astrophysical black holes, as well as a model of primordial black holes.
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Their analysis concluded that the observatories have collected so much gravitational wave data that all of the formation models are likely to be correct. This includes the idea that some of the data comes from primordial black holes.
鈥淭ypically in this kind of analysis, you鈥檙e punished because of Occam鈥檚 razor for making things more complex and adding more models,鈥� says at the Nice Observatory in France. 鈥淪o the fact that they added primordial black holes and that had the highest probability is interesting.鈥�
The analysis suggested that about 27 per cent of the LIGO and Virgo black holes could be primordial. 鈥淲hen I started this, I was expecting that we would not find any significant level of support for primordial black holes, and instead I got surprised,鈥� says Vitale.
However, because this result relies on theoretical models, it isn鈥檛 proof that primordial black holes exist. Those models are the best we have right now, but they aren鈥檛 guaranteed to be correct.
鈥淭he result is not definitive: it is not a 鈥榟ard proof鈥�, it is more of a change in our expectations in light of the new data,鈥� says at the University of the Basque Country in Spain. 鈥淏ut such a change is important.鈥� It could lead astrophysicists and cosmologists to build more sophisticated models for black hole formation, both for astrophysical and primordial black holes, she says.
If some of these black holes are primordial, they could be a crucial window into our early universe and may even make up part of the mysterious dark matter that holds galaxies together. But either way, this result is a hint that our understanding of black holes is incomplete. 鈥淓ven if these are only astrophysical black holes, there鈥檚 clearly something involved in their formation beyond what has been assumed so far,鈥� says at the University of North Florida.
The next step is to build better models and get more data from LIGO and Virgo. The observatories, along with the Kamioka Gravitational Wave Detector in Japan, are expected to turn on again in 2022. 鈥淲e鈥檒l potentially be getting a new binary black hole every day for 18 months,鈥� says Christensen. 鈥淲e need more, and we will get more.鈥�
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