Natural Phantom Dark Energy from a ${\mathbb{Z}}_N$--Axion

Abstract

We present a technically natural microscopic realization of apparent phantom dark energy based on an axion coupled to $N$ copies of two-flavor dark QCD related by a ${\mathbb{Z}}_N$ exchange symmetry. The symmetry exponentially suppresses the axion vacuum potential, naturally generating the dark-energy scale, while reheating into a single dark sector induces a controlled breaking of ${\mathbb{Z}}_N$ that simultaneously restores the physical axion periodicity, sets the dark-matter abundance, and drives the late-time dark-energy dynamics. The selected sector contains dark-pion dark matter, whose finite density initially traps the axion away from its vacuum minimum. As the Universe expands and the dark-pion density redshifts away, the axion is released and rolls on the reheating-induced vacuum potential, generating an effective phantom crossing without tuned cancellations. We identify a viable parameter region that simultaneously reproduces the observed dark-matter relic abundance and dark-energy scale, satisfies cosmological and astrophysical constraints, and qualitatively reproduces the DESI preference for an evolving dark-energy equation of state.

Comments: 17 pages