A tale of two modes: neutrino free-streaming in the early universe

作者：Lancaster Lachlan^*; Cyr Racine Francis Yan; Knox Lloyd; Pan Zhen

来源：Journal of Cosmology and Astroparticle Physics, 2017, 2017(7): 033.

DOI：10.1088/1475-7516/2017/07/33

• cosmology of theories beyond the SM
• cosmological neutrinos
• cosmological parameters from CMBR
• neutrino properties

We present updated constraints on the free-streaming nature of cosmological neutrinos from cosmic microwave background (CMB) power spectra, baryonic acoustic oscillation data, and local measurements of the Hubble constant. Specifically, we consider a Fermi-like four-fermion interaction between massless neutrinos, characterized by an effective coupling constant G(eff), and resulting in a neutrino opacity tau(nu) proportional to G(eff)(2)T(nu)(5). Using a conservative prior on the parameter log(10) (G(eff) MeV2), we find a bimodal posterior distribution. The first of these modes is consistent with the standard Lambda CDM cosmology and corresponds to neutrinos decoupling at redshift z(nu),(dec) > 1.3 x 10(5), that is before the Fourier modes probed by the CMB damping tail enter the causal horizon. The other mode of the posterior, dubbed the "interacting neutrino mode", corresponds to neutrino decoupling occurring within a narrow redshift window centered around z(nu),(dec) similar to 8300. This mode is characterized by a high value of the effective neutrino coupling constant, log(10) (G(eff) MeV2) = -1.72 +/- 0.10 (68% C.L.). together with a lower value of the scalar spectral index and amplitude of fluctuations, and a higher value of the Hubble parameter. Using both a maximum likelihood analysis and the ratio of the two mode's Bayesian evidence, we find the interacting neutrino mode to be statistically disfavored compared to the standard Lambda CDM cosmology, and determine this result to be largely driven by the low-l CMB temperature data. Interestingly, the addition of CMB polarization and direct Hubble constant measurements significantly raises the statistical significance of this secondary mode, indicating that new physics in the neutrino sector could help explain the difference between local measurements of H-0, and those inferred from CMB data. A robust consequence of our results is that neutrinos must be free streaming long before the epoch of matter-radiation equality in order to fit current cosmological data.

• 出版日期2017-7