Are light sterile neutrinos preferred or disfavored by cosmology?

We find that the viability of a cosmological model that incorporates two 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 data sets included in the analysis and the ability to control the systematic uncertainties associated with these data sets. Our analysis includes a variety of cosmological probes including the cosmic microwave background ($\mathrm{WMAP}7+\mathrm{SPT}$), 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 $\Lambda \mathrm{CDM}$ model when using the MLCS light curve fitter for the supernova measurements, and strongly disfavored by the data at $\Delta {\chi }_{\mathrm{eff}}^2\approx 18$ when using the SALT2 fitter. When excluding the supernova measurements, the sterile neutrino model is disfavored by the other data sets at $\Delta {\chi }_{\mathrm{eff}}^2\approx 12$, and at best becomes mildly disfavored at $\Delta {\chi }_{\mathrm{eff}}^2\approx 3$ 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.

Figure 1

Left: CMB temperature power spectrum measurements with WMAP7 on large angular scales (orange) and SPT on small scales (blue). The $\Lambda \mathrm{CDM}$ model without sterile neutrinos is shown with the solid (black) line, and the $\Lambda \mathrm{CDM}$ model with two sterile neutrinos is shown in dashed (red). Right: Assuming the $\Lambda \mathrm{CDM}$ 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 $\mathrm{WMAP}+\mathrm{SPT}+P(k)+\mathrm{HST}+\mathrm{SNe}$. The left (purple) and right (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 $\Lambda \mathrm{CDM}$ model without sterile neutrinos, while the overlapping ellipses preferring a larger matter density (right) are for the $\Lambda \mathrm{CDM}$ 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. [71].

Figure 3

Joint two-dimensional marginalized constraints on the spectral index $n_s$ and matter density ${\Omega }_m$ (inner 68%, outer 95%). The background (green) shaded ellipses are for $\mathrm{WMAP}+\mathrm{HST}$, foreground (blue) shaded ellipses are for $\mathrm{WMAP}+\mathrm{SPT}+\mathrm{HST}$, the solid transparent ellipses (in red) are for $\mathrm{WMAP}+P(k)+\mathrm{HST}$, and the dashed transparent ellipses (in black) are for $\mathrm{WMAP}+\mathrm{HST}+\mathrm{SNe}$, where the SNe are from the SDSS compilation (MLCS). The panel to the left assumes a $\Lambda \mathrm{CDM}$ model without sterile neutrinos, while the panel to the right includes two sterile neutrinos.