Chalk up a potential third win for
hypothetical particles called axions.
If the subatomic particles exist, they
could solve two pressing puzzles of particle physics: the source of the dark
matter that fills galaxies with invisible mass, and the reason why interactions
between quarks — the particles that make up protons and neutrons — adhere to a
certain symmetry of nature, called CP symmetry, that other types of particle interactions
Now, two researchers say that axions could
solve a third thorny problem: why the universe is made mostly of matter, while
antimatter is rare. In the early universe, the axion could have behaved in a manner that produces
an excess of matter, particle physicists
Raymond Co and Keisuke Harigaya suggest in the March 20 Physical Review Letters.
“They have an idea which has all the
right ingredients to do some interesting things,” says physicist Michael Dine
of the University of California, Santa Cruz. But it remains to be seen whether
the idea can fully reproduce the properties of the cosmos, he says. “This is
one of those cases where the devil is in the details.”
Scientists think that 13.8 billion years
ago, the Big Bang birthed equal parts matter and antimatter. Since matter and
antimatter particles annihilate when they meet, that would have left a universe
filled with pure energy. So, as the universe evolved, some process must have
favored matter over antimatter, but scientists still don’t know for sure how it
happened. Some researchers think, for example, that neutrinos played a role (SN: 11/25/19).
Now Co, of the University of Michigan in
Ann Arbor, and Harigaya, of the Institute for Advanced Study in Princeton,
N.J., propose a new idea for how matter gained the upper hand. It’s based on the
evolution of the axion field, a hypothetical ethereal blanket that permeates
space, similar to how an electric field extends around an electric charge.
Oscillations in the strength of the
axion field correspond to axion particles. Picture a marble in the bottom of a
plastic soda bottle: Jiggle the bottle, and the marble will wiggle back and
forth within one of the divots at the bottle’s base. Similar wiggles in an
axion field beget axions. In the early universe, the axion field would have had
a lot of energy, before settling into the lowest energy state possible — akin
to a marble high up the wall of the soda bottle sliding down into the divot.
What Co and Harigaya propose is that, instead
of rolling straight down the bottle’s side, the axion field would have instead
spiraled around the bottle to the bottom. Through a sequence of interactions
involving the strong force, which binds quarks together, and the weak force,
which produces certain radioactive decays, this spiraling, Co says, “is going
to lead to a production of more matter than antimatter in the early universe.”
Scientists are currently searching for
axions, for example with the Axion Dark Matter Experiment in Seattle (SN:3/6/20). Co
and Harigaya’s theory predicts, however, that axions would be a bit more
massive than what ADMX is searching for. Future experiments, such as the International Axion Observatory, could look for those bulkier particles.
However, the numbers don’t quite work out yet: The study predicts about 100 times as much dark matter as is needed to explain observations of the universe. But some additional considerations might be able to rectify that mismatch, Co and Harigaya say, for example, if another massive particle exists that scientists have yet to discover.