Mysterious black holes might be clues to dark matter

Depiction of a primordial black hole forming amid a sea of hot, color-charged quarks and gluons, a tiny fraction of a second after the Big Bang. Credit: Kaća Bradonjić.

Did you know that for every kilogram of visible matter in the universe, there are 5 kilograms of invisible matter?

This mysterious “dark matter” can’t be seen directly, but we know it’s there because of its gravitational pull on visible objects.

Fifty years ago, physicist Stephen Hawking had a bold idea: dark matter might be made up of tiny black holes that formed soon after the Big Bang.

These “primordial” black holes would be much smaller than the massive ones we know today. They would have formed in the first quintillionth of a second after the Big Bang and then spread out across the universe.

Now, scientists from MIT have discovered that these tiny primordial black holes might have had some surprising companions: even smaller black holes with an unusual property called “color charge.”

These super-charged black holes would have been a new state of matter, likely evaporating a fraction of a second after they formed. However, they could still have influenced key events in the early universe, such as the formation of the first atomic nuclei.

“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,” says David Kaiser, a professor of physics at MIT.

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

A Time Before Stars

The black holes we know today are formed when massive stars collapse, creating regions so dense that even light can’t escape. These “astrophysical” black holes can be many times the mass of our sun.

In contrast, primordial black holes are thought to have formed before stars existed. Scientists believe that pockets of ultradense matter in the early universe could have collapsed to form these tiny black holes. These microscopic black holes could be as dense as an asteroid squeezed into a space as small as an atom. Their gravitational pull might explain the dark matter we can’t see today.

But what were these primordial black holes made of? Kaiser and Alonso-Monsalve tackled this question in their new study.

“People have studied the distribution of black hole masses in the early universe, but no one had connected it to what kinds of stuff would have fallen into those black holes at the time they were forming,” Kaiser explains.

Super-Charged Black Holes

The MIT physicists first looked at theories about the likely distribution of black hole masses in the early universe.

“Our realization was, there’s a direct correlation between when a primordial black hole forms and what mass it forms with,” Alonso-Monsalve says. “And that window of time is absurdly early.”

They calculated that primordial black holes must have formed within the first quintillionth of a second after the Big Bang. This brief moment would have produced typical microscopic black holes, as well as smaller ones with the mass of a rhino but much smaller than a single proton.

What would these primordial black holes have been made of? To find out, the researchers looked at the early universe’s composition, focusing on the theory of quantum chromodynamics (QCD). QCD studies how quarks and gluons—the building blocks of protons and neutrons—interact.

Right after the Big Bang, the universe was a hot plasma of quarks and gluons. Any black holes that formed would have swallowed these particles, along with their “color charge”—a unique property of quarks and gluons.

Using QCD theory, the researchers worked out the distribution of color charge in the early plasma. They found that most typical black holes would have neutral color charge because they absorbed a mix of regions.

But the smallest black holes would have been packed with color charge, containing the maximum amount allowed by physics.

These super-charged black holes would have evaporated quickly but might have existed long enough to influence the formation of the first atomic nuclei, which started about one second after the Big Bang. This influence could have left detectable signals in the early universe.

“These objects might have left some exciting observational imprints,” Alonso-Monsalve muses. “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 findings open up new possibilities for understanding the mysterious dark matter and the early universe.

Source: MIT.