Searching for Light Relics with the CMB

Abstract

Fluctuations in the cosmic neutrino background are known to produce a phase shift in the acoustic peaks of the cosmic microwave background (CMB). It is through the sensitivity to this effect that the recent CMB data has provided a robust detection of free-streaming neutrinos. In this chapter, we revisit the phase shift of the CMB anisotropy spectrum as a probe of new physics. The phase shift is particularly interesting because its physical origin is strongly constrained by the analytic properties of the Green’s function of the gravitational potential. For adiabatic fluctuations, a phase shift requires modes that propagate faster than the speed of fluctuations in the photon-baryon plasma. This possibility is realized by free-streaming relativistic particles, such as neutrinos or other forms of dark radiation. Alternatively, a phase shift can arise from isocurvature fluctuations. We present simple models to illustrate each of these effects and provide observational constraints from the Planck temperature and polarization data on additional forms of radiation. We also estimate the capabilities of future CMB Stage-4 experiments. Whenever possible, we give analytic interpretations and consider possible implications of our results.