Constraining the Self-Interacting Neutrino Interpretation of the Hubble Tension

TL;DR

The paper investigates whether large neutrino self-interactions can alleviate the $H_0$ tension by delaying neutrino free-streaming during recombination. It shows that achieving the required $G_{ m eff}$ demands a light mediator with mass $m_\phi$ in the keV–100 MeV range and a sizable coupling $g_\phi$, making the scenario subject to stringent bounds from BBN, $\Delta N_{ m eff}$, laboratory decays, and astrophysical observations. All flavor-universal and most flavor-specific (except a potential $\nu_\tau$-only MI$\nu$ window) realizations are excluded, and Dirac neutrino scenarios face strong constraints due to RHN thermalization. UV completions within minimal seesaw frameworks generally fail to reproduce the needed interaction strength while generating neutrino masses, signaling a need for non-minimal model-building or reconsideration of mediator mass ranges. Overall, while a highly constrained, flavor-specific MI$\nu$ scenario remains marginally viable, a robust UV-complete realization of strong neutrino self-interactions compatible with all data remains an open challenge.

Abstract

Large, non-standard neutrino self-interactions have been shown to resolve the $\sim 4σ$ tension in Hubble constant measurements and a milder tension in the amplitude of matter fluctuations. We demonstrate that interactions of the necessary size imply the existence of a force-carrier with a large neutrino coupling ($> 10^{-4}$) and mass in the keV -- 100 MeV range. This mediator is subject to stringent cosmological and laboratory bounds, and we find that nearly all realizations of such a particle are excluded by existing data unless it carries spin 0 and couples almost exclusively to $τ$-flavored neutrinos. Furthermore, we find that the light neutrinos must be Majorana, and that a UV-complete model requires a non-minimal mechanism to simultaneously generate neutrino masses and appreciable self-interactions.

Figures (1)

• Figure 1: Bounds (shaded regions) on light neutrino-coupled mediators with flavor-universal couplings (top-left), and flavor-specific couplings to $\nu_{e}$ (top-right), $\nu_\mu$ (bottom-left), and $\nu_\tau$ (bottom-right). The bands labeled MI$\nu$ and SI$\nu$ are the preferred regions from Eq. (\ref{['H0-favored']}) Kreisch:2019yzn translated into the $g_\phi$-$m_\phi$ plane. Also shown are constraints from $\tau$ and rare meson decays Blum:2014ewaBerryman:2018ogkKelly:2019wowKrnjaic:2019rsv, double-beta decay experiments Agostini:2015nwaBlum:2018ljvBrune:2018sab (purple), and BBN (red). We combine the $\tau$/meson decay and double-beta decay constraints as "Lab Constraints" in the upper-left panel. BBN yields depend on the baryon density $\eta_b$; thick (thin) lines correspond to the SI$\nu$ (MI$\nu$) preferred values of $\eta_b$. Nucleosynthesis constraints are stronger for complex scalar mediators (dashed red) than for real scalars (solid red). If neutrinos are Dirac, their right-handed components equilibrate before BBN above the dashed black line.