A magnet with only one pole — north, but no south. Split it in half, and instead of two ordinary magnets, you get two more one-pole magnets. In our world, no such particle has been found, although many theories predict their existence. These hypothetical objects are called magnetic monopoles.
The authors of the study noticed a curious connection. In some models that explain why different types of quarks (which make up protons and neutrons) have such different masses, additional hidden interactions appear. These interactions operate at energies thousands of times higher than those available at the Large Hadron Collider, and they could produce light magnetic monopoles. Their mass is set not by the gigantic scales of grand unification, but by much more modest ones — about 10 million billion electron volts.
Such monopoles would have filled the universe, but we don't see them. So something must have removed them. Calculations show that to avoid overpopulating the cosmos, a short phase of superfast expansion, inflation, was needed to dilute the monopoles to an acceptable level. And after inflation, the reheating of the universe had to occur at a temperature below the threshold for producing these particles — otherwise they would have reappeared. Thus, the puzzle of quark masses becomes intertwined with the thermal history of the birth of the universe.
Invisible magnetic charges become a bridge between two distant worlds: the microscopic differences among particle families and the global picture of how our world cooled in the first moments after its appearance. And a magnet with only one pole turns out to be the key to this connection.
If you pass a magnetic monopole through a coil of wire, the current in it will change by exactly the same amount, regardless of the monopole's speed, a quantum effect predicted by Dirac.