According to our current understanding of the universe, equal amounts of matter and antimatter should have been produced in the Big Bang, then both annihilated. However, the universe is dominated by matter, and most of the antimatter is missing! Where did this antimatter go? The matter-antimatter asymmetry problem is one of the greatest challenges in physics today. A new theory may lead us closer to solving this fundamental mystery.
Nikodem Poplawski, a Polish theoretical physicist of the University of New Haven, theorizes that tiny primordial black holes created during the Big Bang consumed vast amounts of antimatter, leading to the asymmetry between matter and antimatter. Antimatter can be interpreted as the “opposite” of regular matter, made up of antiparticles like antiprotons and antielectrons, with signs opposite to those of regular protons and electrons.
“Primordial black holes are hypothetical black holes that formed soon after the Big Bang because of extreme, high-density fluctuations in the early universe. They are good candidates for being the seeds of supermassive black holes at the centers of massive galaxies, as well as of intermediate-mass black holes at the centers of globular clusters,” Poplawski said. “There are other models of elimination of antimatter, but they all assume some physics beyond the Standard Model of particle physics.
According to Poplawski, because antimatter particles are more massive than matter particles, when matter-antimatter pairs were produced in the early universe, the antimatter particles were slower than the corresponding matter particles. Since a black hole is more likely to capture massive particles due to their slow speed, the primordial black holes gobbled up more antimatter than matter, and the rest of the antimatter was annihilated when it met with the surviving matter.
“Because the probability for gravitational capture of a massive particle by a black hole increases as its speed decreases, the antimatter particles were captured by black holes at larger rates than the corresponding matter particles,” Poplawski explained. “The missing antimatter fell into primordial black holes, and what did not fall was annihilated by matter.”
This theory could explain another problem in cosmology, in which the James Webb Space Telescope (JWST) began spotting supermassive black holes around 500 million years after the Big Bang. However, the process of formation and growth of these supermassive black holes was predicted to take at least 1 billion years. Poplawski thinks that by gobbling up antimatter, primordial black holes could have gotten a head start on these growth processes.
“Primordial black holes consumed more antimatter than matter, and because antimatter was much heavier than matter, primordial black holes enormously increased their masses,” he said. “This could possibly explain how supermassive black holes recently observed in the early universe have grown so quickly.”
To confirm this theory, observational evidence of the existence of primordial black holes could be obtained. However, these have remained hypothetical since they were first proposed by Stephen Hawking in the 1970s.
Another theory proposed by physicists at the University of Massachusetts Amherst relates a mysterious 2023 event to these primordial black holes.
In 2023, scientists detected a subatomic particle called a neutrino hitting Earth with an energy level so extreme that it seemed impossible. In a study published in Physical Review Letters, researchers at the University of Massachusetts Amherst suggested that this event may have originated from an exploding primordial black hole.
“Primordial black holes would have existed in the very early universe, which is currently very difficult to probe. I hope that gravitational waves and neutrinos could be used in the future to test this hypothesis,” Poplawski said. “Also, there could be future experiments testing if matter and antimatter particles may have slightly different masses at higher densities or smaller distances compared to those currently probed.”
