A solution to the S8 tension through neutrino-dark matter interactions

TL;DR

The paper investigates neutrino–dark matter ($\nu$DM) interactions as a mechanism to alleviate the persistent $S_8$ tension between early and late-universe measurements. It models a temperature-independent $\nu$DM cross section using a dimensionless coupling $u_{\nu\mathrm{DM}}$, evolves perturbations via modified Boltzmann equations, and captures nonlinear effects with an emulator trained on $N$-body simulations. By combining Planck, BAO, ACT, and DES Y3 cosmic shear data, the authors find a nearly $3\sigma$ preference for non-zero $u_{\nu\mathrm{DM}}$ around $u_{\nu\mathrm{DM}}\sim 10^{-4}$, which simultaneously reduces the $S_8$ tension and yields consistent fits to both CMB and weak-lensing observations. Forecasts for future weak-lensing surveys (CSST and LSST) indicate substantially improved constraints, offering a clear path to decisively test this dark-sector interaction and potentially reveal new physics beyond $\Lambda$CDM.

Abstract

Neutrinos and dark matter (DM) are two of the least understood components of the Universe, yet both play crucial roles in cosmic evolution. Clues about their fundamental properties may emerge from discrepancies in cosmological measurements across different epochs of cosmic history. Possible interactions between them could leave distinctive imprints on cosmological observables, offering a rare window into dark sector physics beyond the standard $Λ$CDM framework. We present compelling evidence that DM-neutrino interactions can resolve the persistent structure growth parameter discrepancy, $S_8 = σ_8\,\sqrt{Ω_m/0.3}$, between early and late universe observations. By incorporating cosmic shear measurements from current Weak Lensing surveys, we demonstrate that an interaction strength of $u \sim 10^{-4}$ not only provides a coherent explanation for the high-multipole observations from the Atacama Cosmology Telescope (\texttt{ACT}), but also alleviates the $S_8$ discrepancy. Combining early universe constraints with \texttt{DES Y3 cosmic shear} data yields a nearly $3σ$ preference for non-zero DM neutrino interactions. This strengthens previous observational claims and provides a clear path toward a significant breakthrough in cosmological research. Our findings challenge the standard $Λ$CDM paradigm and highlight the potential of future large-scale structure surveys, which can rigorously test this interaction and unveil the fundamental properties of DM.

Figures (7)

• Figure 1: Posterior distributions of the DM–neutrino interaction parameter $\mathbf{u_{\nu\mathrm{\textbf{DM}}}}$. The red dash-dotted(blue dotted) line represents the results obtained using the Planck+BAO+ACT(DES Y3 cosmic shear) likelihood. The combined results for the likelihood Planck+BAO+ACT+DES Y3 cosmic shear are presented as a green solid line.
• Figure 2: Profile likelihood distribution and marginalized 2D posterior distribution in the ($\mathbf{S_8}$, $\mathbf{u_{\nu\textrm{DM}}}$) plane. Left: Marginalized 2D posterior distribution in the ($S_8$, $u_{\nu\textrm{DM}}$) plane. The red and blue contours show the results for the Planck+BAO+ACT and DES Y3 cosmic shear datasets, respectively. The green region represents the results for the combined Planck+BAO+ACT+DES Y3 cosmic shear dataset. Right: The $\Delta\chi^2$ with the parameter $u_{\nu\rm{DM}}$ obtained using the profile likelihood method from the analysis of Planck+BAO+ACT, DES Y3 cosmic shear and combined datasets.
• Figure 3: Forecasted constraints on the DM–neutrino interaction strength from future weak-lensing surveys. Variation of $\Delta \chi^2$ with the parameter $u_{\nu\textrm{DM}}$ obtained using the profile likelihood method Herold:2024enb. The brown line represents the Planck+BAO likelihood, while the light green and gray lines additionally include mock cosmic shear data from CSST, and LSST, respectively. The dashed black line indicates $\Delta \chi^2 = 2.71$, corresponding to the $2\sigma$ upper limit. The orange-shaded region indicates the $95\%$ CL preferred range of the $u_{\nu\rm{DM}}$ parameter found in the Planck+BAO+ACT+DES Y3 cosmic shear data.
• Figure 4: Impact of DM–neutrino interactions on the linear matter power spectrum. The ratio of the linear matter power spectrum in the interacting $\nu$DM scenario to that in $\Lambda$CDM at $z=0$. Cosmological parameters are set to $h=0.68$, $\Omega_{\rm{b}} h^2=0.0223$, $\Omega_{\textrm{DM}} h^2=0.120$, $\rm{A}_s=2.215\times 10^{-9}$, $n_s=0.97$, and $\tau_{\rm reio}=0.053$. The topmost solid blue line corresponds to the standard $\Lambda$CDM model. The left panel shows results for different fractions of interacting DM, $\hat{r}$. The red solid line corresponds to $\hat{r}=1$ and $\rm{log_{10}}u_{\nu\rm{DM}} = -4.6$. The green dashed and black dash-dotted lines correspond to scenarios with $\hat{r} = 0.5$ and $0.1$, respectively, with the same value of $u_{\nu\rm{DM}}$. For comparison, the brown dotted line shows the case where $\rm{log_{10}}u_{\nu\rm{DM}} = -5$ and $\hat{r} = 1$. The right panel shows results for scenarios where DM interacts with neutrinos over certain redshift ranges. The red solid line is the same as in the left panel, with the interaction described by a fixed parameter, $\rm{log_{10}}u_{\nu\rm{DM}} = -4.6$, constant across all redshifts. The green line depicts the case where the same value of $u_{\nu\rm{DM}}$ applies, but only for high redshifts $z > 10^3$. The black dotted, dash-dotted and orange long-dashed lines correspond to cases with a non-zero value of $u_{\nu\rm{DM}}$ only for the limited redshift ranges $2\times 10^5 > z > 10^3$, $10^5 > z > 10^3$ and $10^6 > z > 10^3$, respectively. The gray-shaded region indicates scales not used in our weak lensing analysis.
• Figure 5: Validation of the emulator against $\mathbf{N}$-body simulations. Left: Comparison of the nonlinear matter power spectrum, $P(k)$, obtained from an emulator (blue line) with that from a full $N$-body simulation (red) at z=0. Cosmological parameters are set to $h=0.68$, $\Omega_{\rm{b}} h^2=0.0223$, $\Omega_{\rm{DM}} h^2=0.120$, $\rm{A_s}=2.215\times 10^{-9}$, $n_s=0.97$, $\tau_{\rm reio}=0.053$, and $\rm{log}_{10}u_{\nu\rm{DM}}=-4.6$. The lower panel shows the percentage difference between the emulator and $N$-body results. The shaded band indicates the statistical uncertainty of the simulations, arising from finite box size (200 $h^{-1}$Mpc) and limited realization sampling. As a result, the power spectrum measurements at small wavenumbers are affected by sample variance. Right: The quantity $\Delta P(k)/P_{em}(k)$, which corresponds to the relative difference between the matter power spectra in $N$-body simulation and the result obtained with the emulator at z=0, as a function of wavenumber $k \ [h/\mathrm{Mpc}]$, shown with the statistical uncertainty of the simulations. The blue band corresponds to the best-fit point in our analysis, while the red band represents the same cosmological parameters with the exception that $u_{\nu\rm{DM}}=10^{-5}$.