Exploding black holes could solve a big cosmic mystery

DENVER, Colo. — Tiny, exploding black holes might solve a major mystery about how the universe as we know it came to be.

In the cosmos today, matter is much more common than antimatter. But scientists don’t know why. Matter makes up the stuff we can see, smell and touch. Yet antimatter — which is mostly identical to matter, just with flipped electric charges — is rarely observed. It can be released in radioactive decay or in particle collisions. But it doesn’t form solid objects.

Some physicists think that matter’s takeover of the universe may have involved tiny black holes. A black hole is a place where matter is packed so densely that nothing can escape it once it falls in, not even light.

The black holes at play here would have been born in the first instants after the Big Bang, when the universe began. If such black holes existed, they would have quickly evaporated and exploded. Those explosions would have sent out shock waves that may have set the stage for matter to take over.

Physicist Alexandra Klipfel shared this idea in March at the American Physical Society’s Global Physics Summit. She and her teammates also described the work in two papers on arXiv.org. (Studies posted to that site have not yet been vetted by other scientists.)

Pop goes the black hole

Scientists believe the universe began with equal amounts of matter and antimatter. But when they meet, matter and antimatter destroy one another. Without something to tip the balance in matter’s favor, the universe would have been boring — and empty.

Tiny black holes could have shifted the balance to produce our matter-rich cosmos, Klipfel’s team says. That could have allowed stars, planets and galaxies to form.

Usually, black holes form when a star dies and collapses. But these black holes would have formed differently. They would have collapsed from fluctuations in the density of energy in the early universe. Each one would typically have had only about as much mass as a small car.

The black holes would have spewed out particles called Hawking radiation. As a result, the black holes would steadily lose mass and eventually explode. All this would have happened within the first tenth of a billionth of a second of the universe’s existence.

Such explosions would have launched out traveling walls of energy called shock waves. Each explosion and shock wave would have heated a black hole’s surroundings. “It’s a really sharp wall,” Klipfel says, with different conditions inside and outside the shock. A wall like that would provide the right conditions to create an excess of matter.

Hitting a wall

Early in the universe, there may have been processes that converted antimatter into matter. But in a universe where everything is smoothly spread out, any process that could do that would work both ways at once. That should keep matter and antimatter in equal amounts.

With the sharp wall of a shock wave, conditions on one side would differ drastically from those on the other. That could give matter a boost.

Inside a thin shell behind a shock wave, temperatures would extremely high. So high that, weirdly, particles would not have mass.

Outside the shock wave, particles would have mass — and inside they wouldn’t. When particles crossed the border, their mass would change. This could be happening at the same time as other weird physics that might have been at work in the early universe. Together, those effects could have caused more and more matter to build up at the boundary of a shock wave. And that extra matter could have then been locked in as the shock expanded.

Matter and antimatter still would have destroyed one another, but the extra matter means that some would have been left over at the end. And that’s what would make up everything we see around us.

For tiny black holes to be responsible for matter’s victory, many, many of them would have had to explode moments after the Big Bang. Instead of being a finale, those fireworks could have been just the beginning.