A Geometric Radion as Coupled Dark-Sector Quintessence: Confronting DESI's Hints of Evolving Dark Energy

Recent DESI baryon-acoustic-oscillation data have strengthened a preference for dynamical dark energy, with an equation of state that appears to evolve with redshift rather than remaining fixed at w = −1. We propose that this evolving component is a geometric modulus—the radion Φ obtained from Kaluza–Klein reduction of a six-dimensional bulk. We make one focused, falsifiable claim and follow it honestly. (i) The radion’s late-time potential, generated by non-perturbative bulk effects, is a periodic (thawing) quintessence potential whose canonically normalized decay constant lands near the Planck scale, F ≈ 0.95 MP l, as a consequence of the reduction geometry rather than of tuning. (ii) As a minimally coupled field it predicts dark energy at the edge of the DESI-preferred (w0, wa) region—consistent with current data but unable to reproduce the phantom-divide crossing that the central values suggest. (iii) Because the radion is a geometric modulus, it carries a natural non-minimal coupling ξΦ2R, which permits a benign, ghost-free effective phantom crossing and can bend the predicted track into the interior of the DESI region. (iv) Solar-system bounds forbid this if the radion couples to baryons, but are evaded if it couples predominantly to the dark sector—which is precisely the coupling already required for the radion to mediate Branon dark- matter production. We state clearly what is derived, what is assumed, and what remains to be established by a full numerical background-and-perturbation analysis. More speculative elements (a cyclic bulk background; an interpretive “temporal volume” reading of quantum indefiniteness) are retained only in a clearly demarcated section and carry no load.