The y-sky: diffuse spectral distortions of the cosmic microwave background

TL;DR

This paper predicts a diffuse y-sky caused by non-linear evolution of cosmic microwave background radiation, providing a new way to trace the Universe's reionization history through spectral distortions.

Contribution

It introduces the first detailed theoretical prediction of the diffuse y-sky from non-linear cosmological perturbations and derives its power spectrum from the second order Boltzmann equation.

Findings

The diffuse y-sky is a significant spectral distortion component.

The power spectrum of the y-sky is computed numerically.

The y-sky effectively traces the reionization history.

Abstract

The non-linear evolution of the energy density of the radiation is shown to induce spectral distortions of the cosmic microwave background both at recombination and during the reionization era. This distortion has the same spectral signature as the one produced by the re-scattering of photons by non-relativistic hot electrons, the thermal Sunyaev-Zeldovich effect, whose amplitude is quantified by a Compton y parameter. A diffuse y-sky is then expected to emerge from mode couplings in the non-linear evolution of the cosmological perturbations and to superimpose to the point source contributions of galaxy clusters. The equations describing the evolution of the y field and a hierarchy governing its angular multipoles are derived from the second order Boltzmann equation. These equations are then integrated numerically to obtain the first predicted power spectrum of the diffuse y-sky. It is found to be a remarkable tracer of the reionization history of the Universe.