Black holes are strange. Unlike anywhere else in the universe, their gravitational pull is so strong that nothing escapes them, not even light.
You might think that they’re an interesting theoretical toy, not something that really exists – and that, in any case, there couldn’t be that many of them.
In fact, black holes are everywhere. Astronomers have detected multiple collisions between them, and we think that almost every galaxy has a supermassive black hole lurking at its centre.
Although black holes might seem like uniquely destructive forces, we’re coming to understand that they’re helping to shape our Universe. For example, supermassive black holes may play a role in galaxy evolution. Now researchers have proposed that a different and even more ancient kind of black hole may be behind heavy metals and even dark matter.
For all their importance, we don’t know when the first black holes formed. We know that black holes can form when massive stars run out of fuel and collapse in on themselves, so perhaps there were no black holes until the first stars collapsed. Alternatively, black holes may have formed in the first second after the Big Bang – researchers have dubbed these primordial black holes.
In a paper published in Physical Review Letters, UC San Diego professor George Fuller, UCLA professor Alexander Kusenko and UCLA postdoc Volodymyr Takhistov propose that primordial black holes might be important for the formation of heavy metals such as gold and uranium.
“Scientists know that these heavy elements exist, but they’re not sure where these elements are being formed,” Kusenko says. “This has been really embarrassing.”
They suggest that heavy metals might be formed if a neutron star – the dense, city-sized remnant of a star left behind after a supernova explosion – captured a primordial black hole. As the primordial black hole consumed the neutron star, the neutron star would spin faster and faster, ejecting material into space. Heavy metals would then be able to form in the neutron-rich material.
The study predicts that such collisions are rare, potentially explaining why some galaxies are richer in heavy metals than others. It is also consistent with how much dark matter there is in our Galaxy and how it’s distributed.
A separate study also published in Physical Review Letters, by Kusenko and UCLA graduate student Eric Cotner, aims to explain how these primordial black holes formed.
Cosmologists describe the early Universe using scalar fields. Essentially, this is like going around the early Universe and labelling different points in space and time with different numbers. The fields describe how energy is distributed in the universe, and where this energy goes determines how the universe changes over time.
At some points in space and time, the field can be in special configurations with the lowest energy possible. Because these parts of the field are associated with a charge, which physicists write as Q, and because they’re roughly spherical, they’re known as Q-balls. The Q-balls cause fluctuations in the density of the universe, which may then collapse to form primordial black holes.
A computer simulation shows two black holes colliding to merge into one, producing gravitational waves, or ripples in space-time. Image Credit: SXS, the Simulating eXtreme Spacetimes (SXS) project
What sets this scenario apart from others is that it is more general. It doesn’t rely on what Kusenko calls the “unlikely coincidences” other scenarios require. Interestingly, their paper predicts that primordial black holes could account for 100 per cent of the dark matter in the universe.
Currently, scientists are working on simulating collisions between primordial black holes and neutron stars so that astronomers know what to look for. In another paper, Cotner and Kusenko suggest that the collapse of the fluctuations into primordial black holes may leave a gravitational-wave signal. The primordial black holes may themselves collide, producing other gravitational-wave signals which could be detected by instruments such as LIGO. The authors plan to calculate the gravitational-wave spectrum in a further paper.
If we do find evidence of primordial black holes and evidence that they play a part in the formation of heavy metals, we’ll shed light on some of the most mysterious aspects of our universe. Even if we don’t, that gives researchers room to work out new scenarios and even new physics.
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