Small 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 rhinoceros, which quickly evaporated.
A team of researchers theorized that these tiny “rhino” black holes, which would represent an entirely new state of matter, would be filled to the brim with “color charge.” This is a property of fundamental particles called quarks and gluons that is related to their strong interactions with each other and is not related to “color” in the everyday sense.
These supercharged black holes would have been created with the primordial black holes when microscopic regions of ultradense matter collapsed in the first quintillionth of a second after the Big Bang.
Although these newly theorized black holes would have evaporated just a fraction of a second after they nucleated, they may have influenced a key cosmological transition: the forging of the first atomic nuclei. This means they may have left a signature that can be detected today.
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The research team believes that black holes with a supercolor charge may have affected the balance of merging nuclei in the infant universe. Although exotic objects ceased to exist in the first moments of space, future astronomers could still detect this influence.
“Even though these short-lived, exotic creatures aren’t around today, they could have influenced cosmic history in ways that could show up in subtle signals today,” study co-author David Kaiser, a professor of physics at MIT ( MIT), said in a statement.
“Within the idea that all dark matter can be explained by black holes, it gives us new things to look for,” he added, referring to the mysterious substance that makes up about 85 percent of the material universe.
Not all black holes are created equal
When you imagine a black hole, the immediate image that may pop into your mind is cosmic titan-like supermassive black holes with masses millions or even billions of times that of the sun. These black holes are located at the heart of the dominant galaxies around them 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 tens or hundreds of times that of the sun, which are born when massive stars run out of nuclear fusion fuel and collapse.
These two types of black holes, as well as the elusive intermediate black holes between these two mass ranges, are classified as “astrophysical black holes”. Scientists have long suspected that there may once have been non-astrophysical black holes, born just after the Big Bang, with masses between those of Earth and those of a large asteroid.
Rather than forming from the collapse of a star, these primordial black holes could have formed from much smaller blobs of collapsing matter before the first stars or even the simplest atoms had even formed.
The more massive a black hole, the wider its outer boundary, or “event horizon.” If a primordial black hole had a mass about that of Earth, it would be no wider than a dime. If it had the mass of a large asteroid, it would be smaller than an atom.
The reason we use the past tense when describing these black holes is that current theories suggest that these primordial black holes would have been so small that they rapidly lost mass through the “leakage” of a type of thermal radiation called Hawking radiation. This would cause them to evaporate, meaning they would not be in the universe today.
Some scientists have proposed “rescue mechanisms” that could allow primordial black holes to persist in the modern space age. If these mechanisms are valid, then primordial black holes could actually be responsible for dark matter.
Dark matter is so mysterious because, although it makes up about 85% of the matter in the universe, it does not interact with light and therefore cannot be the same as the other 15% of “stuff” in the cosmos, which includes stars, planets, moons, our bodies and the neighbor’s cat.
Primordial black holes could be suitable for dark matter because, like all black holes, they would be bounded by event horizons. These are light-trapping surfaces, which also means that black holes, like dark matter, do not emit or reflect light.
To better investigate the relationship between dark matter and the primordial black hole, Kaiser and MIT graduate student Elba Alonso-Monsalve set out to discover what these small and early black holes are (or were) made of.
“People have studied what the mass distribution of black holes would be during this production of the early universe, but they’ve never related it to what kinds of things would have gone into these black holes at the time they formed,” Kaiser explained .
The primary companions of black holes were supercharged rhinos
The first step for the two researchers was to look at existing theories about primordial black holes and how their mass would have been distributed during the formation of the universe.
“Our realization was that there is a direct relationship between when a primordial black hole forms and with what mass it forms,” explained Alonso-Monsalve. “And that time period is ridiculously early.”
In this case, “absurdly early” means within a quintillionth of a second after the Big Bang. This short period would see the birth of “standard” primordial black holes with masses around those of large asteroids and sub-atomic widths.
Yet Alonso-Monsalve and Kaiser predict that this brief spell would also have resulted in the birth of a small fraction of exponentially smaller black holes, with masses around that of a rhinoceros and sizes much smaller than a proton, the particles that (along with neutrons) make up the nuclei at the heart of atoms.
Black holes of both sizes in the early universe would have been surrounded by a dense sea of quarks and gluons. These elementary particles do not exist freely in the universe during its present era, being bound into particles such as 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.
Any black holes formed in the early universe would not only feed on this plasma soup, but also absorb a property of free unbound quarks and gluons called color charge.
“Once we understood 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?” said Alonso- Monsalve.
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 in the hot, dense plasma of the early universe. They then compared this distribution to the size of a region that could collapse and give birth to a black hole in just the first quintillionth of a second of space.
This revealed that a “typical” primordial black hole would not absorb a large amount of color charge. This is because the larger region of the quark-gluon plasma they consumed would contain a mixture of colored charges, adding a neutral charge.
Black holes with the mass of a rhinoceros formed from a smaller patch of quark-gluon plasma, however, would be full of colorful charge, the duo found. In fact, they would contain the maximum amount of any kind of charge allowed for a black hole, according to the basic laws of physics.
This isn’t the first time such “extreme” black holes have been hypothesized, but Alonso-Monsalve and Kaiser are the first scientists to posit a realistic process by which such cosmic oddities could actually form in our universe.
Although rhino-charged black holes would quickly evaporate, they could still exist for about a second after the Big Bang, when the first atomic nuclei began to form. This means that rhinoceros black holes would have enough time to throw the conditions in space out of equilibrium. These perturbations could affect matter in ways that can still be observed today.
“These objects may have left some exciting observational imprints,” Alonso-Monsalve concluded. “They could change the balance between this and that, and that’s something you can start to wonder about.”
The team’s research was published Thursday (June 6) in the journal Physical Review Letters.
