The Axion Survivor

The axion is a hypothetical particle that solves the strong CP problem — why QCD does not violate CP symmetry despite having no apparent reason not to. As a bonus, the axion is a dark matter candidate. But cosmological constraints impose an upper bound on the axion decay constant f_a: if f_a is too large, the axion field carries too much energy density after inflation, overproducing dark matter and overclosing the universe. This upper bound is usually treated as a fundamental constraint on axion models.

Dvali, Fitz, and Komisel (arXiv:2603.28620, March 2026) remove the upper bound entirely. The mechanism: during inflation, if an SU(5) grand unified theory confines (rather than remaining in its Coulomb phase), the confinement generates an early axion potential that traps and dilutes the problematic energy density. After inflation ends and SU(5) deconfines, the early potential vanishes and the axion is left with its standard QCD potential — but the dangerous energy density has already been diluted by the inflationary expansion.

The key surprise is what happens to the axion during inflation when the Peccei-Quinn scalar expectation value vanishes — when the field that normally hosts the axion does not exist. The axion survives as the phase of the fermion ’t Hooft determinant, a topological quantity that persists even when the underlying scalar field is absent. The degree of freedom that becomes the axion at low energies exists during inflation in a different mathematical guise. It is not born after inflation; it was present throughout, wearing a different identity.

The result: the axion works as dark matter at arbitrarily large decay constants. The cosmological upper bound, thought to be a structural constraint, was a consequence of the assumption that no early confinement occurs. If GUT-scale physics includes a confining phase during inflation — which is a natural possibility in SU(5) models — the constraint simply does not apply.

The structural observation: a cosmological bound that constrains a fundamental parameter is removed by including physics at a different epoch. The bound was real but epoch-dependent: it applied only in cosmologies without early confinement. The parameter space of the theory is larger than the standard cosmological bounds suggest, because the bounds assume a specific thermal history that is not the only possibility.