Natural Phantom Crossing from Axion-WIMP Interactions

Abstract

We construct a technically natural model in which thermal dark matter (DM) interacts with axion dark energy (DE) and produces an apparent late-time crossing of the phantom divide. A direct axion coupling to weakly-interacting massive particles (WIMPs) would ordinarily radiatively destabilize the ultralight axion potential. We avoid this issue through $N$ fermion species related by a cyclic $\mathbb{Z}_N$ symmetry, which projects the leading Coleman-Weinberg potential onto the exponentially suppressed $N$th harmonic. Although the microscopic theory preserves $\mathbb{Z}_N$, the axion-dependent WIMP masses generate unequal equilibrium abundances that freeze-out imprints on the cosmological relic state, thereby breaking the symmetry spontaneously. The resulting relic distribution retains a memory of the initial axion value and generates an unsuppressed finite-density potential that holds the field fixed at early times. As the WIMP density dilutes, the axion rolls toward the minimum of its confining potential, transferring energy from DE to DM at late times. An observer assuming separately conserved components then infers an effective equation of state that crosses below $-1$, without ghosts or violation of the null-energy condition. We present an illustrative cosmological solution with a DESI-like phantom crossing and percent-level suppression of structure growth, and discuss the implications of the WIMP multiplicity and relic distribution for DM searches.