Simultaneous constraints on the number and mass of relativistic species

TL;DR

This paper addresses whether there are additional relativistic species beyond the three active neutrinos by jointly constraining the effective number of neutrino species $N_\\mathrm{eff}$ and the sum of neutrino masses $\\sum m_\\nu$ using a suite of cosmological observations. It demonstrates that $N_\\mathrm{eff}$ and $\\sum m_\\nu$ are correlated with standard $\\Lambda$CDM parameters and should be fitted simultaneously. The joint analysis of CMB data (WMAP7, SPT), BAO, SNLS, $H(z)$, and WiggleZ yields $N_\\mathrm{eff} = 3.58^{+0.15}_{-0.16}$ (68% CL) and $\\sum m_\\nu < 0.60$ eV (95% CL), with a mild 2$\\sigma$ preference for $N_\\mathrm{eff} > 3$. The results constitute the strongest cosmological constraints to date on these parameters and highlight the importance of considering neutrino properties in tandem with the LCDM framework for accurate inference of early-Universe physics.

Abstract

Recent indications from both particle physics and cosmology suggest the existence of more than three neutrino species. In cosmological analyses the effects of neutrino mass and number of species can in principle be disentangled for fixed cosmological parameters. However, since we do not have perfect measurements of the standard Lambda Cold Dark Matter model parameters some correlation remains between the neutrino mass and number of species, and both parameters should be included in the analysis. Combining the newest observations of several cosmological probes (cosmic microwave background, large scale structure, expansion rate) we obtain N_eff=3.58(+0.15/-0.16 at 68% CL) (+0.55/-0.53 at 95% CL) and a sum of neutrino masses of less than 0.60 eV (95 CL), which are currently the strongest constraints on N_eff and M_nu from an analysis including both parameters. The preference for N_eff >3 is now at a 2sigma level.

Figures (5)

• Figure 1: Illustration of how the CMB and matter power spectra change for varying neutrino mass (solid lines) and effective number of neutrinos (dashed lines) fixing all other parameters (to WMAP 7-yr values for $\Lambda$CDM). $\sum m_\nu$ does not affect the CMB power spectrum much, but changes the matter power spectrum significantly. The effect of $N_\mathrm{eff}$ is clearly visible for small scales (high values of $l$) in the CMB power spectrum, and the two parameters are clearly degenerate in particular for the matter power spectrum. The shaded regions indicate the normalised uncertainties of current experiments.
• Figure 2: The 68% and 95% CL contours in the $N_\mathrm{eff}$-$\sum m_\nu$ parameter space of fitting a $\Lambda$CDM+$\sum m_\nu$+$N_\mathrm{eff}$ model to a selection of data combinations.
• Figure 3: The one dimensional probability distributions for $N_\mathrm{eff}$ and $\sum m_\nu$ of fitting a $\Lambda$CDM+$\sum m_\nu$+$N_\mathrm{eff}$ model to a selection of data combinations (same colours as in Fig. \ref{['fig:mnuNeff']}). The grey shaded area indicates the probability distribution when correlations are neglected by fixing $\sum m_\nu$=0 (left) and $N_\mathrm{eff}$=3.046 (right).
• Figure 4: The 68% and 95% CL contours for fitting $\Lambda$CDM+$\sum m_\nu$+$N_\mathrm{eff}$ (solid black), $\Lambda$CDM+$\sum m_\nu$ (dotted blue), $\Lambda$CDM+$N_\mathrm{eff}$ (dot-dashed green), and $\Lambda$CDM (dashed red) models to CMB$_\mathrm{WMAP}$ + CMB$_\mathrm{SPT}$ + WiggleZ + $H(z)$ + BAO + SN data. The resulting contours are consistent with each other for all parameters.
• Figure 5: Results of fitting a $\Lambda$CDM+$\sum m_\nu$+$N_\mathrm{eff}$ model to a selection of data combinations (same colours as in Fig. \ref{['fig:mnuNeff']}).