Exotic black holes may be a byproduct of dark matter

Fifty years ago, physicist Stephen Hawking suggested that dark matter might be a population of black holes, which may have formed shortly after the Big Bang. They also existed for a short time but affected the parameters of the universe

For every kilogram of matter we can see - from the computer on the desktop to distant stars and galaxies - there are 5 kilograms of invisible matter filling our environment. This "dark matter" is a mysterious entity that eludes any form of direct observation but its presence is evident through its invisible pull on visible objects.

Fifty years ago, physicist Stephen Hawking suggested that dark matter might be a population of black holes, which may have formed shortly after the Big Bang.

Such "primordial" black holes were not the giants we recognize today, but rather microscopic regions of extremely dense matter that formed in the first fifth of a second after the Big Bang and then collapsed and scattered throughout the cosmos, affecting the space-time around them in ways that could explain the dark matter we see. Meet today.

Now, physicists at MIT have discovered that this primordial process could also have produced unexpected secondary phenomena: smaller black holes with unprecedented amounts of a nuclear physical property known as "color charge."

These small "supercharged" black holes were in a state of brand new matter, having probably evaporated a fraction of a second after they were formed. However, they could still affect a key cosmological phase: the time when the first atomic nuclei were formed.

The physicists hypothesize that charged blacks could affect the balance of fusing nuclei, in a way that astronomers could discover in the future through measurements. Such an observation would convincingly point to primordial black holes as the root of all dark matter today.

"Although (these exotic, short-lived blackheads) do not exist today, they could have affected cosmic history in ways that may show up in subtle signals today," says David Kaiser, professor of the history of science and professor of physics at MIT. "Within the idea that all dark matter can be explained by black holes, it gives us direction on new things we can look for."

Kaiser and his co-author, graduate student Alba Alonso-Monsalba of MIT, published their study in the journal Physical Review Letters.

before the stars were formed

The black holes we know and discover today are the product of stellar collapse, when the center of a massive star collapses in on itself and creates a region so dense that it can bend space-time so that everything, even light, is trapped inside. Such "astrophysical" black holes can be several times more massive than the Sun and up to billions of times more massive.

In contrast, "primordial" black holes can be much smaller and are believed to have formed in a time before the appearance of stars. Before the universe formed even the basic elements, let alone stars. Scientists believe that pockets of extremely dense primordial material could have accumulated and collapsed to form microscopic black holes that could have been so dense that they compressed the mass of an asteroid into an area as small as a single atom. The gravitational pull from those tiny, invisible objects scattered throughout the universe could explain all the dark matter we can't see today.

If that were the case, what were those primordial black holes made of? This is the question that Kaiser and Alonso-Monsalva investigated in their new study.

"People have studied what the distribution of black hole masses was at the time of their formation in the early universe but never related it to the types of material that fell into the black holes at the time they formed," explains Kaiser.

Supercharged black holes

The MIT physicists first examined existing theories for the likely distribution of black hole masses as they formed in the early universe.

"Our understanding was that there was a direct correlation between when a primordial black hole formed and at what mass it formed," says Alonso-Monsalaba. "And that window of time is absurdly early."

She and Kaiser calculated that primordial black holes must have formed within the first fifth of a second after the Big Bang. This time period would have produced "typical" microscopic black holes that were as massive as an asteroid and as small as an atom. It would also produce a small fraction of exponentially small black holes, with a mass of a rhinoceros and a size much smaller than a single proton.

What were these ancient black holes made of? To this end, they examined studies that investigated the composition of the early universe, and in particular, the theory of quantum chromodynamics (QCD) - the study of the interactions between quarks and gluons.

Quarks and gluons are the basic building blocks of protons and neutrons - elementary particles that have combined to form the basic elements of the periodic table. Immediately after the Big Bang, physicists estimate, based on QCD, that the universe was a very hot plasma of quarks and gluons that cooled rapidly and combined to form protons and neutrons.

The researchers speculate that within a fifth of the first second, the universe was still a soup of quarks and free gluons that had not yet connected. Any black hole formed at this time would have swallowed the unbound particles, along with an exotic property known as "color charge" – a state of charge that only unbound quarks and gluons carry.

"Once we realized that black holes form in quark-gluon plasma, the most important thing we had to figure out was, how much color charge does the mass of matter that will end up in a primordial black hole contain?" says Alonso-Monsalba.

Using QCD theory, they calculated the distribution of the color charge that was present in the primordial hot plasma. They then compared this to the size of the region that would collapse to form a black hole in the first fifth of a second. It turns out that most typical black holes at the time did not have much color charge, as they were formed by the absorption of a large number of regions that had a mixture of charges, which were ultimately "neutral" in charge.

But the smallest black holes were loaded with color charge. In fact, they would contain the maximum amount of any type of charge allowed for a black hole, according to the basic laws of physics. Although for decades these anomalous black holes have been hypothesized to exist, until now no one has discovered a realistic process by which such strange black holes could form in our universe.

The supercharged black holes would evaporate quickly, but perhaps only after the first atomic nuclei had begun to form. Scientists estimate that this process began around a second after the Big Bang, which would have given these anomalous black holes plenty of time to break the equilibrium conditions that existed when the first nuclei began to form. Such perturbations could affect how the first nuclei were formed, in a way that might be expected in the future.

"These objects could have left some exciting observational imprints," muses Alonso-Monsalba. "They could change the balance of one against the other, and that's the kind of thing you start to wonder about."

for the scientific article

More of the topic in Hayadan:

• The quantum mechanics of black holes
• Measure the heat of the dark matter
• Researchers in Tel Aviv shed light on the period when the black holes grew
• At least ten thousand Milky Way black holes surround the central black hole
• A method that significantly improves the measurement of the mass of black holes in the centers of galaxies