Antimatter and dark matter are two of modern science's biggest mysteries, and now scientists are probing whether they could be linked in order to explain why and how our universe exists at all.
Physicists currently don’t understand why, at the origin of the universe or just after, matter and antimatter were seemingly not produced in equal quantities. If this had happened—as would be the expected result, assuming symmetry in the universe—opposing matter and antimatter would have been cancelled out, leaving nothing at all. The universe would never have formed.
For the first time, an experiment led by the Fundamental Symmetries Laboratory at Japanese research institution Riken has explored whether dark matter, itself a mystery in cosmology, could be the cause of the dominance of matter over antimatter.
Dark matter is a form of mass and energy that doesn’t interact with light, making it undetectable in astronomical observations. Observations of unexpected effects of gravity imply there is far more matter in the universe than the matter we can see. Hypothetically, if dark matter interacts differently with matter and antimatter, it could produce the imbalance between the two, creating the right conditions for matter to exist without being annihilated by antimatter.
To initially test this theory, collaborators from the international BASE project (Baryon Antibaryon Symmetry Experiment) designed an experiment to detect interactions between antimatter and a hypothetical axion particle, which is one of many proposed candidates for what dark matter is made of. Their findings were reported in a paper published on Wednesday in Nature.
Using a specially designed device, researchers trapped a single antiproton (the antiparticle of the proton) and kept it isolated to avoid annihilation through interacting with a photon. They measured a property of the antiproton which should be constant, postulating that observed fluctuations could be the result of dark matter axions.
In this experiment, the axion effect on the antiproton was not observed, but Christian Smorra, lead author of the study and researcher at Riken Fundamental Symmetries Laboratory, said the experiment nonetheless made progress in determining what dark matter-antimatter interactions might look like.
"For the first time, we have explicitly searched for an interaction between dark matter and antimatter, and though we did not find a difference [between effects on matter and antimatter], we set a new upper limit for the potential interaction between dark matter and antimatter," Smorra said in a statement.
Not everyone believes that axions are the best candidate for dark matter, however, but the potential for a specific dark matter theory to explain other mysteries in physics, like antimatter/matter asymmetry, has a large part to play in which theory of dark matter scientists choose to explore.
“I don't think anybody in the field really believes that a specific candidate for dark matter is correct,” says David Morrissey, who researches antimatter/matter asymmetry at Canada’s particle accelerator centre, TRIUMF. “It's more a matter of looking for the candidates that seem to have the best theoretical motivation.”
Axions were originally proposed to explain a different asymmetry problem in particle physics, and later also suggested as a potential candidate for dark matter. But if axions do indeed interact differently with matter and antimatter, Morrissey said, it would violate an important law of physics called CPT symmetry (charge, parity, and time reversal symmetry).
An alternative theory is “Asymmetric Dark Matter," Morrissey said, which preserves CPT symmetry and also explains the dominance of matter over antimatter as a consequence of dark matter interactions. But instead of the same dark matter having different effects on matter and antimatter, it proposes two types of dark matter: dark matter and anti-dark matter. The theory also predicts an excess dark matter over anti-dark matter in the universe.
Nonetheless, Morrissey says the Riken experiment is world-leading irrespective of whether it reveals anything about axion-antimatter interactions.
“The actual measurement they did was very difficult and world-leading in terms of its precision,” he says. “The limits derived here on CPT-violating axion interactions with matter are also better than have been obtained previously.”
Correction: An earlier version of this article misstated that an antiproton is the antiparticle of a photon, when it is in fact the antiparticle of a proton. Motherboard regrets the error.