The Standard Model of particle physics does a decent enough job of explaining the universe, but it still has some holes in it. Now, a new study outlines how one hypothetical particle, the axion, may be the answer to three separate, massive mysteries of the universe – including why we’re here at all.
If it exists, the axion is said to be incredibly light – billions of times lighter than a proton – and have no electric charge. They should be floating around basically everywhere, but rarely interact with other matter. All of this makes them pretty hard to detect, which may be why we haven’t found direct evidence of them yet.
For the new study, researchers at the University of Michigan and the Institute for Advanced Study suggest that the axion could essentially have saved our universe from annihilating itself in the early days.
By all counts, the Big Bang should have produced matter and antimatter in equal measure. The problem is, if ever the two shall meet they’ll destroy each other in a burst of energy. That means that if there were equal amounts of matter and antimatter, they should basically have cancelled each other out long ago. But here we are today, in a universe filled with regular matter. Where all that antimatter went is one of the biggest mysteries in physics.
According to the researchers on the new study, axions may be the answer. The team suggests that in the early days of the universe, the axion field started to oscillate, and this motion would have created just a fraction more matter than antimatter. Even if the imbalance was one part in 10 billion, that would have left enough matter behind to form everything around us today.
The team named this mechanism “axiogenesis.” And the matter-antimatter asymmetry isn't the only problem that axions have been proposed to solve.
The axion was first hypothesized in 1977 by physicists Roberto Peccei and Helen Quinn, in order to plug a particular hole in the Standard Model. Neutrons don’t interact with electric fields – but they should. That’s because they’re made up of smaller particles called quarks, which are charged and do interact with electric fields. The question of why neutrons don’t interact with electric fields is known as (an extremely abridged version of) the strong CP problem.
Axions are thought to be able to switch off this interaction between neutrons and electric fields, effectively solving the conundrum. But that’s not all. A few years later and scientists have realized axions are also a handy explanation for dark matter.
Decades of astronomical observation has repeatedly shown that there’s more mass out there than just the stuff we can see. Galaxy clusters wouldn’t be able to hold themselves together under the gravity of just visible mass, and calculations suggest as much as 85 percent of matter is unaccounted for. This invisible mass is dubbed dark matter.
What exactly dark matter is has been debated for decades, with many different candidate particles being put forward. And one of the main contenders that keeps coming up is the axion. Get enough of them together in a pervasive field, and their extra mass could account for the odd gravitational effects attributed to dark matter.
Physicists have been hunting for evidence of axions for years, using experimental facilities that search for their rare interactions with neutrons or electromagnetism. So far, no sign of them has been detected, which is narrowing down the range of masses that they could potentially have.
And if they ever are found, it looks like they may be more valuable than we thought. Not only would we have found dark matter, but we could solve the strong CP problem and the matter-antimatter asymmetry in one fell swoop.
“The versatility of the axion in solving the mysteries of fundamental physics is truly amazing,” says Raymond Co, an author of the study. “We are thrilled about the unexplored theoretical possibilities that this new aspect of the axion can bring. More importantly, experiments may soon tell us whether the mysteries of nature truly hint towards the axion.”
The study is due to be published in the journal Physical Review Letters.
Sources: University of Michigan, Institute for Advanced Study
