Degenerate Sub-keV Fermion Dark Matter from a Solution to the Hubble Tension

TL;DR

The paper presents a hidden Abelian dark sector with anomaly-free chiral fermions that yields a decaying sub-keV fermion DM capable of addressing both the Hubble tension and the core-cusp problem. It develops a non-thermal production history in which DM decays into a dark photon and a light radiation, shaping late-time cosmology, and it maps viable parameter space onto gauge-coupling and symmetry-breaking scales. The analysis identifies a window where m_DM ≈ 100–300 eV with V1 ≈ 10^12–10^13 GeV and V6 ≈ 10–100 GeV satisfies H0 tension, cusp-core, and free-streaming bounds, while remaining consistent with BBN and Lyman-α constraints. The work highlights a fully dark-sector framework that links inflation scale to late-time cosmology and demonstrates how a degenerate, non-thermally produced DM can simultaneously address multiple small-scale and expansion-rate anomalies, albeit with caveats related to momentum distributions and baryonic effects.

Abstract

We present a dark sector model addressing both the Hubble tension and the core-cusp problem. The model is based on a hidden Abelian gauge symmetry group with some chiral fermions required by the anomaly cancellation conditions, producing a candidate for the decaying fermion dark matter as a solution to the Hubble tension. Moreover, the sub-keV mass regime and the thermal history of the dark sector help the dark matter candidate resolve the core-cusp problem occurring in the standard $Λ$CDM cosmology.

Figures (2)

• Figure 1: Constraints on $(g,V_{1})$ space where $g$ is the gauge coupling of $U(1)_{X}$. The right panel is magnification of the left panel for the relatively smaller coupling regime. The yellow shaded region between the yellow solid and dashed line is mapping onto $(g,V_{1})$ space of constraints on the life of DDM and $\epsilon$ parameter in DDM solution to the Hubble tension Vattis:2019efj at 1$\sigma$ level. (On the right panel, we showed the same mapping at 2$\sigma$ level with the green solid line and green shaded region.) The blue shaded region corresponds to $(g,V_{1})$ values that produce sub-keV decaying fermion DM as a solution to not only the Hubble tension, but the core-cusp problem, satisfying 1$\sigma$ level constraints.
• Figure 2: $(m_{{\rm DM}},V_{6})$ plane which is constrained by (1) $T_{{\rm DS}}(a_{{\rm FS}})\!\!<\!\!m_{6}$ (region above the green dashed line), (2) $\Delta N_{{\rm eff}}^{{\rm BBN}}\!\!\lesssim\!\!1$ (region below the black dashed line), (3) $0.3{\rm Mpc}\!\!<\!\!\lambda_{{\rm FS}}\!<\!0.5{\rm Mpc}$ (region between the solid and dotdashed red lines). The eventual intersection of these lies in the red colored shaded region.