Their answer is that a hypothetical particle, called the axion, was not as heavy as previously thought. Lighter axions would act as less catalyst, allowing more matter to survive the early universe. "The reason we have matter in the universe has to do with some exotic decay of this axion-like particle," said Peter Graham, assistant professor of physics at The University of Texas at Austin. "Our calculation was that the axion was just light enough to produce a little bit of this decay and allow enough matter to survive."
Axions are hypothetical elementary particles that were predicted to exist as a solution to the strong CP problem, which is a theoretical puzzle about why there is no electric dipole moment in neutrons. The Peccei-Quinn theory offers an answer, suggesting that axions exist and their interactions cancel out the neutron's electric dipole moment.
The existence of axions has been actively pursued by physicists, and their masses are thought to range from 10^-36 to 10^-26 electron volts. The mass of the axion determines its impact on the evolution of matter in the early universe. Heavy axions would lead to fast neutron-antineutron oscillations, which would quickly deplete matter. Lighter axions would allow more protons to survive, resulting in the matter-dominated universe we observe today.
To probe the axion's mass and its interaction with photons, the team of researchers performed simulations with supercomputers at the Texas Advanced Computing Center (TACC). They explored a wide range of axion masses and calculated the probability of axion-photon interactions.
Their calculations showed that for an axion mass of around 10^-28 electron volts, the axion-photon coupling was strong enough to induce a slow enough evolution of the neutron-antineutron system, preserving more matter in the early universe.
This finding opens up new possibilities for axion searches, suggesting that experiments using optical and X-ray cavities may be able to probe axion masses close to this range.
The study was published in the journal Physical Review Letters and involved a collaboration with David Moore and Gordan Krnjaic of the Kavli Institute for Cosmological Physics at the University of Chicago. The work was supported by the Department of Energy, the National Science Foundation, and the Alfred P. Sloan Foundation.
