The freestreaming of cosmological neutrinos prior to recombination of the baryon-photon plasma alters gravitational potentials and therefore the details of the time-dependent gravitational driving of acoustic oscillations.

We report here a first detection of the resulting shifts in the temporal phase of the oscillations, which we infer from their signature in the Cosmic Microwave Background (CMB) temperature power spectrum.

The magnitude of the shift is proportional to the fraction of the total radiation density in neutrinos. Parameterizing the shift via an effective number of neutrino species we find $1.9 < N_\nu^{\delta\phi} < 3.4$ at $68\%$ confidence, consistent with the standard model value of $N_{\nu} = 3.046$, and inconsistent with zero. \\

( http://arxiv.org/abs/1503.07863 , 826kb)