Massive neutrinos and interacting dark matter look alike through the lens of lensing

TL;DR

The paper demonstrates that the lensing power-spectrum suppression attributed to massive neutrinos, quantified by the sum $\\sum m_\\nu$, can be replicated by dark matter–baryon scattering with a velocity-dependent cross section $\\sigma_{\\rm DMb} \\propto v^{-4}$, potentially biasing neutrino mass inferences from next-generation CMB lensing data. Using CLASS with DM–baryon interactions (and HALOFIT for nonlinearities), the authors map the degeneracy between $\\sum m_\\nu$ and the DMb parameters $ (m_{\\rm DM},\\sigma_{\\rm DMb}) $, showing that regions around $ m_{\\rm DM} \\sim 100$ MeV and $ \\sigma_{\\rm DMb} \\sim$ a few $\\times 10^{-42}$ cm$^2$ can produce lensing spectra indistinguishable from higher neutrino masses at projected CMB-S4 precision. They also fix base cosmological parameters to a Planck-like best-fit and find that the degeneracy can persist across a broad parameter space, underscoring the need for joint constraints on DMb interactions and neutrino masses to avoid biased conclusions. Practically, this means that neutrino mass bounds derived without accounting for DM–baryon scattering are conservative, but future analyses must incorporate DMb effects to obtain reliable constraints on the neutrino sector.

Abstract

We demonstrate that the suppression in the lensing power spectrum of the cosmic microwave background (CMB) caused by massive neutrinos can be mimicked by dark matter-baryon interactions at the precision of next-generation CMB experiments. Thus, a determination of neutrino masses from the CMB lensing power spectrum may be compromised. We illustrate the degeneracy for a dark matter-proton cross section $\propto v^{-4}$, which arises in the $t$-channel exchange of an ultralight mediator in the nonrelativistic limit.

Figures (6)

• Figure 1: 95% CL limits on $\sigma_{\rm DMb}$ for $n=-4$, from CMB+BAO spectra, Milky Way subhalos and the Lyman-$\alpha$ forest Buen-Abad:2021mvc.
• Figure 2: Relative differences in the matter power spectra for neutrino masses (left) and DMb interactions with $m_\mathrm{DM}=\qty{0.1}{GeV}$ (right), with respect to $\Lambda$CDM with $\sum m_\nu=0.06\,\mathrm{eV}$ and the parameters in Eq. (\ref{['eq:baselcdm']}) set to $\{0.02242,0.11933,0.6766,0.0561,3.047,0.9665\}$Planck:2018vyg.
• Figure 3: Relative differences in the lensing power spectra for neutrino masses (left) and DMb interactions (right), with the same parameter choices as in Fig. \ref{['fig:mpk']}.
• Figure 4: Values of $\Delta\chi^2$ (left) and $\tilde{\chi}^2_{AB}$ (right) in the ($m_\mathrm{DM},\sigma_\mathrm{DMb}$) plane. The colors represent the entire range of values in the numerical analysis.
• Figure 5: Values of $\tilde{\chi}^2_{AB}$ (blue, left axis) and $\Delta\chi^2$ (green, right axis) for $m_\mathrm{DM}=\qty{100}{MeV}$ and different values of $\sigma_\mathrm{DMb}$.