What is dark matter — the mysterious stuff that makes up 85 percent of the total mass of the universe we live in? For years, scientists have been looking for particles that dark matter may consist of, and yet, for all their efforts, they have come up empty.
There are currently two hypothetical dark matter particle candidates — the Weakly Interacting Massive Particle (WIMP), which, as their name suggests, are heavy particles that interact with normal matter only through gravity and the weak nuclear force, and axions, which are much lighter.
The hunt for WIMPs has so far been fruitless, and even the most powerful particle accelerators and underground detectors have failed to detect them. The problem is compounded by the fact that the exact nature of these particles is not known, and the Standard Model of particle physics — a framework that governs our understanding of the three of the universe’s four known fundamental forces — does not predict its existence.
The existence of axions, meanwhile, is predicted by an extension of quantum chromodynamics, which is a theory that lies within the ambit of the Standard Model and explains how the strong nuclear force — one of the four fundamental forces — works.
Now, in a study published Wednesday in the journal Nature, a team of scientists have presented their calculations of the estimated mass of axions. The calculation, made using the JUQUEEN supercomputer at the Forschungszentrum Jülich research centre in Germany, reveals that if axions do make up the bulk of dark matter, they would have a mass of between 50 and 1,500 microelectronvolts — up to 10 billion times lighter than electrons.
This means that every cubic centimetre of the universe should contain an average of 10 million axions, and regions having larger concentration of dark matter, such as our local region of the Milky Way, should contain roughly 1 trillion axions per cubic centimetre.
“The results we are presenting will probably lead to a race to discover these particles,” lead author Zoltán Fodor from the University of Wuppertal, Germany, said in a statement.
If axions are indeed discovered, it would, in addition to solving the longstanding dark matter mystery, provide clues to an equally vexing quantum physics puzzle — why is the strong force time symmetric, when the theory that describes the force indicates it shouldn’t be? According to the researchers, axions, if they exist, could be preventing the strong force from violating time symmetry by neutralizing a term in the quantum chromodynamics equations that causes the violation.