First Constraint on the Neutrino-Induced Phase Shift in the BAO Spectrum

TL;DR

This paper presents the first constraint on the neutrino-induced phase shift in the BAO spectrum, providing new evidence for the cosmic neutrino background and confirming aspects of standard cosmology.

Contribution

It introduces the first measurement of the neutrino-induced phase shift in BAO data, linking early universe neutrino physics with large-scale structure observations.

Findings

Detected a non-zero phase shift at >95% confidence

Validated the robustness of the phase shift detection through simulations

Demonstrated the application of BAO to probe early universe physics

Abstract

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.