For every kilogram of matter we can see, from the computer on our desk to distant stars and galaxies, there are 5 kilograms of invisible matter filling our environment. This “dark matter” is a mysterious entity that escapes any direct observation, but makes its presence felt through the invisible effect on visible objects.

Fifty years ago, physicist Stephen Hawking proposed an idea for what dark matter might be: a population of black holes that may have formed soon after the Big Bang.

Such “primordial” black holes may not be the goliaths we see today, but rather microscopic regions of superdense matter that formed in the first quintillionth of a second after the Big Bang, then collapsed and dispersed into space, engulfing the surrounding space. -in ways that could explain the dark matter we know today.

Now MIT physicists have discovered that this primordial process may have also created some unexpected companions: smaller black holes with unprecedented amounts of nuclear-physical properties known as “color charge.”

These smallest, “supercharged” black holes could be an entirely new state of matter, evaporating from their formation in less than a second. But they could still influence an important cosmological transition: the time when the first atomic nucleus was created.

Physicists think that color-charged black holes may have affected the stability of merging nuclei in ways that astronomers may one day detect in future measurements. Such an observation would conclusively indicate that primordial black holes are the root of all present-day dark matter.

“Even if these short-lived, exotic creatures disappear today, they may have influenced the history of the cosmos in ways that can be detected today by subtle signals,” says David Kaiser, Hermeshausen Professor of the History of Science and Professor of Physics. at MIT. “The idea that all dark matter can be explained by black holes gives us new things to look for.”

Kaiser and his co-author, MIT graduate student Elba Alonso-Monsalve, published their work in the journal Physical Examination Letters.

It’s time to go to the stars

The black holes we know and detect today are the product of stellar collapse, where the center of a massive star collapses in on itself, creating a region so dense that it can bend space-time and allow anything, including light, to penetrate. . Such “astrophysical” black holes can be several times larger than the Sun to billions of times larger.

In contrast, “primordial” black holes can be much smaller and are thought to have formed before stars. Scientists believe that even before forming the basic elements of the universe, let alone stars, extremely dense pockets of primordial matter could have accumulated and collapsed to form microscopic black holes that could be dense enough to squeeze the mass of an asteroid into a region. The gravitational pull of these tiny, invisible objects, the size of a single atom, scattered throughout the Universe could explain all the dark matter we can’t see today.

If so, what might these primordial black holes be made of? This is the question that Kaiser and Alonso-Monsalve pose in their new study.

“People have studied what the mass distribution of black holes would have been during this early formation of the universe, but they never attributed this to the material that fell into these black holes during their formation,” Kaiser explains.

Supercharged Rhinos

MIT physicists first looked at existing theories about the possible mass distribution of black holes when they first formed in the early universe.

“We noticed a direct correlation between the time of formation of the primordial black hole and its mass,” says Alonso-Monsalve. “And that time frame is ridiculously early.”

He and Kaiser calculated that primordial black holes should have formed within the first quintillionth of a second after the Big Bang. This time burst would produce “typical” microscopic black holes as large as an asteroid and as small as an atom. It would also reveal a small fraction of exponentially smaller black holes, with rhinoceros mass and size much smaller than a single proton.

What were these primitive black holes made of? To do this, they turned to studies examining the composition of the early universe, specifically the theory of quantum chromodynamics (QCD), which studies how quarks and gluons interact.

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

The researchers found that for the first quintillionth of a second, the universe will still be a soup of uncombined free quarks and gluons. Any black holes formed at this time would have also absorbed unbound particles, along with an exotic property known as “color charge”, a charge state possessed only by unbound quarks and gluons.

“Once we realized that these black holes were formed in quark-gluon plasma, the most important thing we needed to understand was how much color charge was in the clump of matter that would eventually become primordial black holes?” – says Alonso-Monsalve.

Using QCD theory, they calculated the color charge distribution expected to exist in the hot early plasma. They then compared this to the size of the region that would collapse to form a black hole in the first quintillionth of a second. It turns out that most typical black holes at the time didn’t have much color charge; because they were formed by absorbing multiple regions with the charge mixture, eventually resulting in a “neutral” charge.

But the smallest black holes will be filled with a colored charge. In fact, according to the fundamental laws of physics, they contain the maximum amount of charge of any type allowed for a black hole. While such “extreme” black holes have been hypothesized for decades, no one has yet discovered a realistic process by which such oddities could form in our universe.

Supercharged black holes would quickly evaporate, but perhaps only after the first atomic nuclei began to form. Scientists estimate that this process began about one second after the Big Bang; This would give extreme black holes enough time to disrupt the equilibrium conditions that existed when the first nuclei began to form. Such perturbations could affect how these oldest nuclei form in ways that could potentially one day be observed.

“These objects may have left fascinating observational traces,” Alonso-Monsalve speculates. “They can change the balance between this and that, and that’s something you can start to wonder about.”