The universe may have once been speckled with tiny black holes that had a strange property called colour charge. These exotic objects, if they existed, would have formed in the instants after the big bang and evaporated just as quickly, but they could have upset the balance of elements in the early universe.
The minuscule black holes that may have formed right at the start of the cosmos are known as primordial black holes. They are nearly impossible to detect because of their size, making them a potential candidate to explain dark matter, the mysterious substance that makes up nearly 70 per cent of the universe but has yet to be directly observed.
鈥淚f they are to account for dark matter, gobs and gobs of black holes must have formed at a very specific time in the early universe,鈥� says at the Massachusetts Institute of Technology. He and , also at MIT, analysed how primordial black holes with a trait known as colour charge could have formed in the soup of particles that composed the early universe.
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Colour charge is a feature of fundamental particles called quarks and gluons. As with the more familiar electric charge, different colour charges attract one another to form charge-neutral objects. Unlike electricity, however, there are three charges: red, green and blue. 鈥淭hey鈥檙e not really colours. If we zoomed in with a microscope we wouldn鈥檛 see colours with our eyes, but it鈥檚 a way of accounting for the fact that nature seems to only allow colour-neutral combinations,鈥� says Kaiser. Protons, for example, are made up of one quark of each colour, so a proton has no net colour charge.
Today, there are no quarks or gluons just floating solo in the universe 鈥� everything has combined to create colour-neutral particles. But in the first hundred-thousandth of a second after the big bang, quarks and gluons of similar colour charges gathered together in patches. Kaiser and Alonso-Monsalve found this is the exact same time period when primordial black holes may have formed.
The patches of similarly charged quarks and gluons were large enough that some of the small black holes would only have access to one type of charge as they swallowed up nearby matter, so they would have become colour-charged themselves.
鈥淭his is the first realistic mechanism for black holes to form in our universe with any kind of charge,鈥� says Alonso-Monsalve. 鈥淸And] if primordial black holes formed, then these little guys would have formed too.鈥�
Even if these black holes did exist, though, we may not ever be able to find proof. 鈥淚t would be very difficult to find observational signatures of these black holes because they most likely evaporate in a tiny fraction of a second, leaving no trace,鈥� says at the Autonomous University of Madrid in Spain. 鈥淚 wouldn鈥檛 say the traces are subtle 鈥� they鈥檙e even less than that.鈥�
But the researchers say colour-charged black holes could potentially have resisted evaporating for up to a second after the big bang because of the crowded, hot conditions in the early universe. If this is the case, they could have shifted the balance of the elements that began to form seconds later. We don鈥檛 yet have powerful enough instruments to measure such a slight shift, but if we are able to detect it someday, it could be the first proof of an exotic new type of black hole and a hint as to the true nature of dark matter.
Physical Review Letters
