Sterile neutrinos help reconcile the observational results of primordial gravitational waves from Planck and BICEP2

Original · EN

We show that involving a sterile neutrino species in the ΛCDM+r model can help relieve the tension about the tensor-to-scalar ratio r between the Planck temperature data and the BICEP2 B-mode polarization data. Such a model is called the ΛCDM+r+νₛ model in this paper. Compared to the ΛCDM+r model, there are two extra parameters, N eff and mν, sterile eff, in the ΛCDM+r+νₛ model. We show that in this model the tension between Planck and BICEP2 can be greatly relieved at the cost of the increase of nₛ. However, comparing with the ΛCDM+r+dnₛ/d k model that can significantly reduce the tension between Planck and BICEP2 but also makes trouble to inflation due to the large running of the spectral index of order 10⁻² produced, the ΛCDM+r+νₛ model is much better for inflation. By including a sterile neutrino species in the standard cosmology, besides the tension with BICEP2, the other tensions of Planck with other astrophysical data, such as the H₀ direct measurement, the Sunyaev-Zeldovich cluster counts, and the galaxy shear data, can all be significantly relieved. So, this model seems to be an economical choice. Combining the Planck temperature data, the WMAP-9 polarization data, and the baryon acoustic oscillation data with all these astrophysical data (including BICEP2), we find that in the ΛCDM+r+νₛ model nₛ=0.999± 0.011, r=0.21⁺⁰.⁰⁴₋₀.₀₅, N eff=3.95± 0.33 and mν, sterile eff=0.51⁺⁰.¹²₋₀.₁₃ eV. Thus, our results prefer ΔN eff>0 at the 2.7σ level and a nonzero mass of sterile neutrino at the 3.9σ level.