Kaluza–Klein freeze-in as the origin of the JWST high-redshift galaxy excess

Abstract. We show that the ultraviolet freeze-in mechanism of Kaluza–Klein (KK)moduli dark matter, characterised by a production rate R(T ) ∝ T 8, naturally explainsthe excess of massive galaxies at z > 10 observed by the James Webb Space Telescope(JWST). The extreme temperature sensitivity of the rate (index n = 8) convertsstandard adiabatic perturbations into a correlated cold dark matter isocurvaturemode of amplitude SKK = (5/3) ζ, where ζ ≈ 4.6 × 10−5 is the primordial curvatureperturbation. This enhances the matter power spectrum at galactic scales by (8/3)2 ≈7.1, boosting the halo number density at z = 10–14 by factors of ∼ 9–157 over ΛCDMfor M⋆ ∼ 1010–1011 M⊙ — consistent with JWST observations across all redshiftsand mass bins, without adjusting any free parameter. The amplitude SKK = (5/3)ζis fixed by the single topological integer h1,1 = 40, the same Hodge number thatdetermines the dark matter relic density, dark energy equation of state, and fifth-forcephenomenology in the same framework. We identify a falsifiable prediction testablewith existing data: the bright-end slope of the UV luminosity function at z > 12should be d log Φ/dMUV ≈ −0.9, compared to ≈ −1.5 in ΛCDM.