The neutron, one of the building blocks of the atomic nucleus, could, according to the equations, have a noticeable electric dipole moment: that is, behave like a tiny magnet. In reality, however, this effect is almost absent, creating a puzzle known as the strong CP problem. One elegant explanation is the Nelson–Barr mechanism: a special scalar field spontaneously breaks CP symmetry (the symmetry between matter and antimatter in strong interactions) and "hides" the unwanted magnetic moment. But this solution has a side effect: when the symmetry breaks spontaneously, so-called domain walls form — boundaries between regions of space with different vacuum states. If these walls existed, they would contribute a huge amount of energy to the universe, contradicting observations.

The authors of the new work found a way to get rid of these cosmological defects. They showed that the same field that solves the strong CP problem could have driven inflation — the phase of rapid exponential expansion of the universe in the first moments after the Big Bang. The potential energy of this field resembles a double well with two minima (valleys). During inflation, the field slowly "rolls" down one of the valleys, and after inflation ends, the system lacks the energy to jump into the other valley. As a result, domain walls simply never get a chance to form — the universe remains homogeneous.

This field is a true all-rounder in theoretical physics. It solves the strong CP problem, drives inflation, and then transfers its energy to ordinary particles, "reheating" the universe (a process called reheating). If you add right-handed neutrinos, hypothetical partners of the known left-handed neutrinos, the field also breaks CP symmetry in the lepton sector, creating an excess of matter over antimatter. Thus, one mechanism unites cosmology and particle physics, eliminating the main difficulty of the original Nelson–Barr model — the domain wall problem.