Constraining the Self-Interacting Neutrino Interpretation of the Hubble Tension

Large, nonstandard neutrino self-interactions have been shown to resolve the $\sim 4\sigma$ 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 $\tau$-flavored neutrinos. Furthermore, we find that the light neutrinos must be Majorana particles, and that a UV-complete model requires a nonminimal mechanism to simultaneously generate neutrino masses and appreciable self-interactions.

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 $\mathrm{MI}\nu$ and $\mathrm{SI}\nu$ are the preferred regions from Eq. (2) [22] translated into the $g_{\varphi }\text{−}{m}_{\varphi }$ plane. Also shown are constraints from $\tau$ and rare meson decays [54, 55, 56, 57], double-beta decay experiments [58, 59, 60] (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 $\mathrm{SI}\nu$ ($\mathrm{MI}\nu$) 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.