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

2017/04/30 by Lachlan Lancaster, Francis-Yan Cyr-Racine, Lloyd Knox +1 · 1 citation
Physics and Astronomy · #astro-ph.CO #hep-ph

paper · pdf · doi:10.1088/1475-7516/2017/07/033

published as JCAP 1707 (2017) 033 · 22 pages + references, 9 figures, 4 tables, references added

arxiv created 2018/01/19 · arxiv updated 2018/01/22

Abstract

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\rm eff, and resulting in a neutrino opacity τν∝ G\rm eff2 Tν5. Using a conservative prior on the parameter log10(G\rm eff \rm MeV2), we find a bimodal posterior distribution. The first of these modes is consistent with the standard ΛCDM cosmology and corresponds to neutrinos decoupling at redshift z_ν,\rm dec > 1.3×105. 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,\rm dec∼8300. This mode is characterized by a high value of the effective neutrino coupling constant, 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. 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 H0, 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.

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