Dark Radiation after Planck

TL;DR

Planck data are used to test whether there are extra relativistic species and a nonstandard CMB lensing amplitude. The study jointly varies the neutrino effective number $N_{\rm eff}$ and the lensing parameter $A_{\rm L}$ while analyzing Planck+WP data and combining BAO and HST measurements, comparing with ACT and SPT results. The Planck constraints favor a higher $N_{\rm eff}$ and $A_{\rm L}$ than the standard values, though the strength of the hints depends on the dataset, with BAO reducing the significance and HST maintaining it. The authors emphasize correlations between $N_{\rm eff}$ and $A_{\rm L}$ and call for the full Planck mission polarization data to confirm whether these hints indicate new physics.

Abstract

We present new constraints on the relativistic neutrino effective number N_eff and on the Cosmic Microwave Background power spectrum lensing amplitude A_L from the recent Planck 2013 data release. Including observations of the CMB large angular scale polarization from the WMAP satellite, we obtain the bounds N_eff = 3.71 +/- 0.40 and A_L = 1.25 +/- 0.13 at 68% c.l.. The Planck dataset alone is therefore suggesting the presence of a dark radiation component at 91.1% c.l. and hinting for a higher power spectrum lensing amplitude at 94.3% c.l.. We discuss the agreement of these results with the previous constraints obtained from the Atacama Cosmology Telescope (ACT) and the South Pole Telescope (SPT). Considering the constraints on the cosmological parameters, we found a very good agreement with the previous WMAP+SPT analysis but a tension with the WMAP+ACT results, with the only exception of the lensing amplitude.

Figures (4)

• Figure 1: Comparison of the results for Planck+WP, WMAP9+SPT and WMAP9+ACT datasets in terms of the 1-D posterior distribution functions for the parameters $N_{\rm eff}$ (left) and $A_{\rm L}$ (right).
• Figure 2: Comparison of the 2-D posterior distribution function from the Planck+WP, WMAP9+ACT and WMAP9+SPT datasets in the $N_{\rm eff}-A_{\rm L}$ plane.
• Figure 3: Comparison of the 1-D posterior distribution functions from the CMB-only (Planck+WP), CMB+HST, CMB+BAO and CMB+BAO+HST datasets for $N_{\rm eff}$ (left) and $A_{\rm L}$ (right).
• Figure 4: Comparison of the 2-D posterior distribution functions from the CMB-only, CMB+HST, CMB+BAO and CMB+BAO+HST datasets in the $N_{\rm eff}-A_{\rm L}$ parameters plane.