Are Light Sterile Neutrinos Preferred or Disfavored by Cosmology?

TL;DR

This work investigates whether a 3+2 sterile-neutrino scenario with masses $m_4=0.68~\mathrm{eV}$ and $m_5=0.94~\mathrm{eV}$ (sum $\sum m_{\nu}=1.62~\mathrm{eV}$, $N_{\mathrm{eff}}=5.046$ if thermally populated) can be reconciled with cosmological data. Using a CosmoMC/CAMB framework, the authors analyze CMB, Hubble constant, galaxy power spectrum, and supernova data from SN Union2 (SALT2) and SDSS (MLCS), exploring both vanilla and extended cosmological parameter spaces with a fixed two-sterile-neutrino prior. They find that the viability of the sterile-neutrino model is highly sensitive to SN treatment: it is strongly disfavored when using SALT2 (large $\Delta\chi^2_{\rm eff}$ and $\Delta{\rm DIC}$), but can be consistent with MLCS SN data, and it remains only marginally constrained when SN data are omitted. Extending the parameter space to include curvature, evolving dark energy, additional relativistic species, running of the spectral index, and helium abundance improves the fit modestly (to $\Delta\chi^2_{\rm eff}\sim3$ without SN data) but incurs Bayesian penalties ($\Delta{\rm DIC}\sim11$), indicating no decisive cosmological preference for two ~eV sterile neutrinos. The study underscores the crucial role of systematic uncertainties in SN analyses and suggests that, if laboratory hints for eV-scale sterile neutrinos persist, cosmology may require more exotic extensions to remain compatible.

Abstract

We find that the viability of a cosmological model that incorporates 2 sterile neutrinos with masses around 1 eV each, as favored by global neutrino oscillation analyses including short baseline results, is significantly dependent on the choice of datasets included in the analysis and the ability to control the systematic uncertainties associated with these datasets. Our analysis includes a variety of cosmological probes including the cosmic microwave background (WMAP7+SPT), Hubble constant (HST), galaxy power spectrum (SDSS-DR7), and supernova distances (SDSS and Union2 compilations). In the joint observational analysis, our sterile neutrino model is equally favored as a LCDM model when using the MLCS light curve fitter for the supernova measurements, and strongly disfavored by the data at Δχ^2 ~ 18 when using the SALT2 fitter. When excluding the supernova measurements, the sterile neutrino model is disfavored by the other datasets at Δχ^2 ~ 12, and at best becomes mildly disfavored at Δχ^2 ~ 3 when allowing for curvature, evolving dark energy, additional relativistic species, running of the spectral index, and freedom in the primordial helium abundance. No single additional parameter accounts for most of this effect. Therefore, if laboratory experiments continue to favor a scenario with roughly eV mass sterile neutrinos, and if this becomes decisively disfavored by cosmology, then a more exotic cosmological model than explored here may become necessary.

Figures (3)

• Figure 1: Left: CMB temperature power spectrum measurements with WMAP7 (orange) and SPT (blue). The $\Lambda$CDM model without sterile neutrinos is shown with the solid (black) line, and the $\Lambda$CDM model with 2 sterile neutrinos is shown in dashed (red). Right: Assuming the $\Lambda$CDM model is centered on the DR7 data, with error bars given by the shaded band (in blue), we show the absolute difference with our sterile neutrino model in solid (red).
• Figure 2: Joint two-dimensional marginalized constraints on $\sigma_8 (\Omega_m/0.25)^{0.47}$ against $\Omega_m$ from combining the measurements of WMAP+SPT+$P(k)$+HST+SNe. The purple and pink shaded confidence regions (inner 68%, outer 95%) are obtained using SNe from the Union2 compilation (SALT2), while the solid and dashed transparent ellipses are obtained using SNe from the SDSS compilation (MLCS). The overlapping ellipses preferring a lower matter density ( left) are for the $\Lambda$CDM model without sterile neutrinos, while the overlapping ellipses preferring a larger matter density ( right) are for the $\Lambda$CDM model with sterile neutrinos. The horizontal dashed lines (in brown) denote the 68% confidence interval about the mean from the local ($0.025 < z < 0.25$) galaxy cluster abundance measurement of Vikhlinin et al. (2009) Vikhlinin:2008ym.
• Figure 3: Joint two-dimensional marginalized constraints on the spectral index $n_s$ and matter density $\Omega_m$ (inner 68%, outer 95%). The green shaded ellipses are for WMAP+HST, blue shaded ellipses are for WMAP+SPT+HST, the solid transparent ellipses (in red) are for WMAP+$P(k)$+HST, and the dashed transparent ellipses (in black) are for WMAP+HST+SNe, where the SNe are from the SDSS compilation (MLCS). The panel to the left assumes a $\Lambda$CDM model without sterile neutrinos, while the panel to the right includes two sterile neutrinos.