A tale of two modes: neutrino freestreaming in the early universe
Abstract
We present updated constraints on the freestreaming nature of cosmological neutrinos from cosmic microwave background (CMB) temperature and polarization power spectra, baryonic acoustic oscillation data, and distance ladder measurements of the Hubble constant. Specifically, we consider a Fermilike fourfermion interaction between massless neutrinos, characterized by an effective coupling constant G_{eff}, and resulting in a neutrino opacity dot tau_{νpropto} G_{eff}^{2} T_{ν}^{5}. Using a conservative flat prior on the parameter log_{10}( G_{eff} MeV^{2}), we find a bimodal posterior distribution with two clearly separated regions of high probability. The first of these modes is consistent with the standard ΛCDM cosmology and corresponds to neutrinos decoupling at redshift z_{ν,dec} > 1.3×10^{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_{ν,dec}~8300. This mode is characterized by a high value of the effective neutrino coupling constant, log_{10}( G_{eff} MeV^{2}) = 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 ΛCDM cosmology, and determine this result to be largely driven by the lowl 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 matterradiation equality in order to fit current cosmological data.
 Publication:

Journal of Cosmology and Astroparticle Physics
 Pub Date:
 July 2017
 DOI:
 10.1088/14757516/2017/07/033
 arXiv:
 arXiv:1704.06657
 Bibcode:
 2017JCAP...07..033L
 Keywords:

 Astrophysics  Cosmology and Nongalactic Astrophysics;
 High Energy Physics  Phenomenology
 EPrint:
 22 pages + references, 9 figures, 4 tables, references added