Phases of New Physics in the CMB

TL;DR

This paper explores how the phase shift in the CMB spectrum, caused by free-streaming particles or isocurvature fluctuations, can serve as a probe for new physics beyond the standard model, with constraints from Planck data and future forecasts.

Contribution

It provides a detailed analysis of the phase shift as a diagnostic tool for new relativistic particles and presents observational constraints and forecasts for future experiments.

Findings

Planck data constrains additional dark radiation.

Phase shift analysis distinguishes free-streaming particles from isocurvature modes.

Future CMB experiments will improve sensitivity to new physics.

Abstract

Fluctuations in the cosmic neutrino background are known to produce a phase shift in the acoustic peaks of the cosmic microwave background. It is through the sensitivity to this effect that the recent CMB data has provided a robust detection of free-streaming neutrinos. In this paper, 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. We then provide observational constraints from the Planck temperature and polarization data on additional forms of radiation. We also forecast the capabilities of future CMB Stage IV experiments. Whenever possible, we give analytic interpretations of our results.