Tiny primordial black holes created during the first fraction of a second after the Big Bang may have had company, in the form of even smaller "supercharged" black holes with the mass of a rhino that rapidly evaporated.
A team of researchers has theorized that these tiny "rhino" black holes, which would represent an entirely new state of matter, would have been packed to the brim with "color charge." This is a property of fundamental particles called quarks and gluons that's related to their strong force interactions with each other, and it is not related to "color" in the everyday sense.
These supercharged black holes would have been created with primordial black holes when microscopic regions of ultradense matter collapsed in the first quintillionth of a second following the Big Bang.
Though these newly theorized black holes would have evaporated a mere fraction of a second after they spawned, they may have influenced a key cosmological transition: the forging of the first atomic nuclei. That means they may have left a signature that is detectable today.
Related: If the Big Bang created miniature black holes, where are they?
The research team thinks that super-color-charged black holes may have impacted the balance of fusing nuclei in the infant universe. Though the exotic objects ceased to exist in the first moments of the cosmos, future astronomers could potentially still detect this influence.
“Even though these short-lived, exotic creatures are not around today, they could have affected cosmic history in ways that could show up in subtle signals today," study co-author David Kaiser, a professor of physics at the Massachusetts Institute of Technology (MIT), said in a statement.
"Within the idea that all dark matter could be accounted for by black holes, this gives us new things to look for," he added, referring the the mysterious substance that makes up about 85% of the material universe.
Not all black holes are created equally
When picturing a black hole, the immediate image that may spring to mind is cosmic titan-like supermassive black holes with masses millions or even billions of times that of the sun. These black holes sit at the heart of galaxies, dominating their surroundings, and are created by a chain of mergers of progressively larger pairs of black holes.
More common in the universe are stellar-mass black holes with masses tens or hundreds of times that of the sun, which are born when massive stars run out of fuel for nuclear fusion and collapse.
These two types of black holes, as well as elusive intermediate black holes between these two mass ranges, are classed as "astrophysical black holes." Scientists have long hypothesized that there may have once been non-astrophysical black holes born just after the Big Bang, with masses between those of Earth and that of a large asteroid.
Rather than forming from the collapse of a star, these primordial black holes could have formed from much smaller patches of collapsing matter before the first stars or even the simplest atoms had even emerged.
The more massive a black hole is, the wider its outer boundary or "event horizon" is. If a primordial black hole had a mass around that of Earth, it would have been no wider than a dime. If it had the mass of a large asteroid, it would have been smaller than an atom.
The reason we use past tense when describing these black holes is because current theories suggest that these primordial black holes would have been so small they rapidly lost mass through the "leaking" of a type of thermal radiation called Hawking radiation. This would have resulted in their evaporation, meaning they wouldn't be around in the universe today.
Some scientists have proposed "rescue mechanisms" that could allow primordial black holes to persist into the modern epoch of the cosmos. If these mechanisms are valid, then primordial black holes could actually account for dark matter.
Dark matter is so mysterious because, despite representing around 85% of the matter in the universe, it doesn't interact with light and thus can't be the same as the other 15% of "stuff" in the cosmos that includes stars, planets, moons, our bodies, and the cat next door.
Primordial black holes could be a good fit for dark matter because, like all black holes, they would be bounded by event horizons. These are light-trapping surfaces that also mean black holes, like dark matter, don't emit or reflect light.
To better explore the dark matter/primordial black hole connection, Kaiser and MIT graduate student Elba Alonso-Monsalve set out to discover what these tiny and early black holes are (or were) made of.
"People have studied what the distribution of black hole masses would be during this early-universe production but never tied it to what kinds of stuff would have fallen into those black holes at the time when they were forming," Kaiser explained.
Primordial black hole companions were supercharged rhinos
The first step for the two researchers was to look at pre-existing primordial black hole theories and how their mass would have been distributed during the formation of the universe.
"Our realization was, there’s a direct correlation between when a primordial black hole forms and what mass it forms with," Alonso-Monsalve explained. "And that window of time is absurdly early."
In this case, "absurdly early" means within a quintillionth of a second following the Big Bang. This brief period would have seen the birth of "standard" primordial black holes with masses around that of large asteroids and widths smaller than an atom.
Yet Alonso-Monsalve and Kaiser predict that this brief spell would have also seen the birth of a small fraction of exponentially smaller black holes, with masses around that of a rhino and sizes much smaller than a single proton, the particles that (along with neutrons) compose the nuclei at the heart of atoms.
Both these sizes of black holes in the early universe would have been surrounded by a dense sea of quarks and gluons. These elementary particles are not found freely in the universe during its current era, being bound up in particles like protons and neutrons. However, in the dense early universe, there was a "hot soup" or plasma of free quarks and gluons that had yet to combine.
Not only would any black holes formed in the early universe feed on this plasma soup, but they would also absorb a property of free unbound quarks and gluons called color charge.
"Once we figured out that these black holes form in a quark-gluon plasma, the most important thing we had to figure out was, how much color charge is contained in the blob of matter that will end up in a primordial black hole?" Alonso-Monsalve said.
Turning to a theory called "quantum chromodynamics," which describes the action of the strong force between quarks and gluons, the duo calculated the distribution of color charge that should have existed throughout the early universe's hot, dense plasma. They then compared this distribution to the size of a region that would be able to collapse and birth a black hole in just the first quintillionth of a second of the cosmos.
This revealed that the "typical" primordial black hole wouldn't have soaked up a great deal of color charge. This is because the larger region of the quark-gluon plasma they consumed would have contained a mix of color charges, adding up to a neutral charge.
Rhino-mass black holes forming from a smaller patch of quark-gluon plasma, however, would have been packed with color charge, the duo found. In fact, they would have contained the maximum amount of any type of charge allowed for a black hole, according to the fundamental laws of physics.
This isn't the first time such "extremal" black holes have been hypothesized, but Alonso-Monsalve and Kaiser are the first scientists to lay out a realistic process by which such cosmic oddities actually could have formed in our universe.
Though the rhino-supercharged black holes would have quickly evaporated, they could have still been around about one second after the Big Bang when the first atomic nuclei began to form. This means rhino black holes would have had plenty of time to throw conditions in the cosmos out of equilibrium. Those disturbances could have affected matter in a way that can still be observed today.
"These objects might have left some exciting observational imprints," Alonso-Monsalve concluded. "They could have changed the balance of this versus that, and that’s the kind of thing that one can begin to wonder about."
The team's research was published on Thursday (June 6) in the journal Physical Review Letters.
