Optical Depth of the Cosmic Microwave Background due to Scattering and Absorption

TL;DR

This paper investigates how absorption processes, alongside scattering, in a partially ionized cosmic plasma affect the optical depth of the CMB, especially at low frequencies, potentially altering our understanding of the universe's ionization history.

Contribution

It introduces the significance of absorption effects in the optical depth calculation of the CMB, which were previously overlooked, highlighting their dependence on plasma temperature and frequency.

Findings

Absorption contributes significantly to CMB optical depth at low frequencies.

Absorption effects depend on plasma temperature and radiation frequency.

Including absorption may revise models of the universe's ionization history.

Abstract

It is shown that, in addition to the Thomson scattering, the absorption due to the electron-electron, electron-ion and the electron -atom collisions in a partially ionized cosmic plasma would also contribute to the optical depth of the cosmic microwave background (CMB). The absorption depth depends on the plasma temperature and frequency of the CMB radiation. The absorption effects are prominent at the low frequency part of the CMB spectrum. These effects when included in the interpretation of the CMB spectrum may necessitate a revised view of the ioniziation of the universe.