Relic Neutrinos, thermal axions and cosmology in early 2014

TL;DR

This work analyzes cosmological constraints on the sum of active neutrino masses $\sum m_\nu$ and extensions with hot thermal relics (thermal axions, sterile neutrinos) using data available up to early 2014, including DR11 BAO. Employing Planck CMB data, lensing, BAO, SN Ia, $H_0$, and large-scale structure measurements (including WiggleZ full shape and CFHTLenS/Planck SZ priors), the authors explore minimal and extended models, highlighting how degeneracies with $N_{\textrm{eff}}$ and $\sigma_8$ affect bounds. In the baseline three-neutrino case they find $\sum m_\nu<0.22$ eV (95% CL), but adding Planck SZ cluster information can favor $\sum m_\nu\sim0.3$ eV; introducing thermal axions or massive sterile neutrinos mitigates this preference. When extra dark radiation is allowed, $N_{\textrm{eff}}$ tends to rise (with modest evidence for $N_{\textrm{eff}}>3$ in some setups), and the allowed neutrino mass bounds weaken due to $\sum m_\nu$–$N_{\textrm{eff}}$ degeneracies; the BICEP2 tensor results further elevate $N_{\textrm{eff}}$ in $\Lambda$CDM+$r$ analyses. Overall, the paper underscores how upcoming or complementary measurements of light-element abundances and cluster counts critically influence constraints on hot relics and relativistic degrees of freedom.

Abstract

We present up to date cosmological bounds on the sum of active neutrino masses as well as on extended cosmological scenarios with additional thermal relics, as thermal axions or sterile neutrino species. Our analyses consider all the current available cosmological data in the beginning of year 2014, including the very recent and most precise Baryon Acoustic Oscillation (BAO) measurements from the Baryon Oscillation Spectroscopic Survey. In the minimal three active neutrino scenario, we find Sum m_nu < 0.22 eV at 95% CL from the combination of CMB, BAO and Hubble Space Telescope measurements of the Hubble constant. A non zero value for the sum of the three active neutrino masses of about 0.3 eV is significantly favoured at more than 3 standard deviations when adding the constraints on sigma_8 and Omega_m from the Planck Cluster catalog on galaxy number counts. This preference for non zero thermal relic masses disappears almost completely in both the thermal axion and massive sterile neutrino schemes. Extra light species contribute to the effective number of relativistic degrees of freedom, parameterised via Neff. We found that when the recent detection of B mode polarization from the BICEP2 experiment is considered, an analysis of the combined CMB data in the framework of LCDM+r models gives Neff=4.00pm0.41, suggesting the presence of an extra relativistic relic at more than 95 % c.l. from CMB-only data.

Figures (5)

• Figure 1: Left panel: the blue contours show the $68\%$ and $95\%$ CL allowed regions from the combination of CMB data, BOSS DR11 BAO measurements, additional BAO measurements and a prior on the Hubble constant from HST in the ($\sum m_\nu$ (eV), $H_0$) plane. The red (green) contours depict the results when the $\sigma_8-\Omega_m$ weak lensing (galaxy number counts) constraint is added in the analysis. Right panel: as in the left panel but in the ($\sum m_\nu$ (eV), $\sigma_8$) plane.
• Figure 2: Left panel: the blue contours show the $68\%$ and $95\%$ CL allowed regions from the combination of CMB data, BOSS DR11 BAO measurements, additional BAO measurements and a prior on the Hubble constant from HST (depicted by the blue contours) in the ($\sum m_\nu$ (eV), $m_a$ (eV)) plane. The red (green) contours depict the results when the $\sigma_8-\Omega_m$ weak lensing (galaxy number counts) constraint is added in the analysis. Right panel: as in the left panel but replacing the WiggleZ BAO geometrical information by the WiggleZ full-shape matter power spectrum measurements.
• Figure 3: Left panel: the red contours show the $68\%$ and $95\%$ CL allowed regions from the combination of CMB data, BOSS DR11 BAO measurements and additional BAO measurements in the ($\sum m_\nu$ (eV), $N_{\textrm{eff}}$) plane. The blue contours depict the constraints after a prior on the Hubble constant from HST is added in the analysis. Right panel: as in the left panel but in the ($N_{\textrm{eff}}$, $H_0$) plane.
• Figure 4: Left panel: the red contours show the $68\%$ and $95\%$ CL allowed regions from the combination of CMB data, BOSS DR11 BAO measurements and WiggleZ full shape power spectrum measurements in the ($\sum m_\nu$ (eV), $N_{\textrm{eff}}$) plane. The blue contours depict the constraints after a prior on the Hubble constant from HST and the remaining BAO data are added in the analysis. Right panel: as in the left panel but in the ($\sum m_\nu$ (eV), $m^\textrm{eff}_s$ (eV)) plane.
• Figure 5: Left panel: Constraints in the $N_{eff}$ vs $r$ plane from Planck+WP and Planck+WP+BICEP2 data. Notice how the inclusion of the BICEP2 constraint shifts the contours towards $N_{\textrm{eff}}>3$. Right panel: constraints on the $\Sigma m_{\nu}$ vs $r$ plane from Planck+WP and Planck+WP+BICEP2 data. In this case there is no indication for neutrino masses from the combination of CMB data.