Year: 2026
Type: preprint
Abstract: Dynamical dark energy offers an alternative to a cosmological constant with distinct observational signatures. However, the small energy density scale, Hubble-sized mass, and Planckian excursions make simple models fine-tuned and unnatural. In this work, we show that a weak version of the axion, identified with the phase field of the anomalous $U(1)_{B+L}$ of the Standard Model, can generate the scale hierarchies expected for dark energy. The axion potential is controlled by sources of explicit baryon and lepton number violation and is radiatively stable. We show that the leading contribution comes from two inequivalent Weinberg operators, one $B+L$-conserving and one $B+L$-violating, which generate the axion potential. We propose a flavor selection rule in the lepton sector that simultaneously removes the quadratic divergence and dominant temperature-dependent contributions to the effective potential. The resulting potential first appears at quartic order in the axion-dependent neutrino masses and, depending on the flavor model and PMNS parameters, its amplitude is parametrically close to the dark-energy density. Since the scale of dark energy is sensitive to changes in the neutrino parameters, improved measurements of $m_{ββ}$, the sum of neutrino masses, $δ_{\rm CP}$ and $θ_{23}$ can favor certain models. Cosmologically, the field behaves as thawing quintessence, stays frozen by Hubble friction until late times and never enters the adiabatic regime.
Source: arXiv (Cornell University)
Authors: Bittar, Pedro, Wagner, Carlos E. M.
Institution: Perimeter Institute
Language: English
Subfield: Nuclear and High Energy Physics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
OpenAlex ID: W7169011228