# Radiation in equilibrium with plasma and plasma effects on cosmic microwave background

## TL;DR

This paper derives the spectrum of equilibrium radiation in plasma, showing how plasma effects alter the cosmic microwave background and discussing the detectability of these effects with current technology.

## Contribution

It provides three methods to derive the equilibrium radiation spectrum in plasma and analyzes plasma's influence on the cosmic microwave background.

## Key findings

- Plasma modifies the equilibrium radiation spectrum from the Planck law.
- Current technology cannot realistically detect plasma effects on the CMB.
- Quantifiable impacts of plasma on the CMB are identified.

## Abstract

The spectrum of the radiation of a body in equilibrium is given by Planck's law. In plasma, however, waves below the plasma frequency cannot propagate; consequently, the equilibrium radiation inside plasma is necessarily different from the Planck spectrum. We derive, using three different approaches, the spectrum for the equilibrium radiation inside plasma. We show that, while plasma effects cannot be realistically detected with technology available in the near future, there are a number of quantifiable ways in which plasma affects cosmic microwave background (CMB) radiation.