First Constraint on the Neutrino-Induced Phase Shift in the BAO Spectrum
  The existence of the cosmic neutrino background is a fascinating prediction
of the hot big bang model. These neutrinos were a dominant component of the
energy density in the early universe and, therefore, played an important role
in the evolution of cosmological perturbations. The energy density of the
cosmic neutrino background has been measured using the abundances of light
elements and the anisotropies of the cosmic microwave background (CMB). A
complementary and more robust probe is a distinct shift in the temporal phase
of sound waves in the primordial plasma which is produced by fluctuations in
the neutrino density and has recently been detected in the CMB. In this paper,
we report on the first constraint on this neutrino-induced phase shift in the
spectrum of baryon acoustic oscillations (BAO) of the BOSS DR12 data.
Constraining the acoustic scale using Planck data while marginalizing over the
effects of neutrinos in the CMB, we find a non-zero phase shift at greater than
95\% confidence. We also demonstrate the robustness of this result in
simulations and forecasts. Besides providing a new test of the cosmic neutrino
background, our work is the first application of the BAO signal to early
universe physics and a non-trivial confirmation of the standard cosmological
history.
