Is there a radio excess from the decoupling of pre-recombination bremsstrahlung?

TL;DR

This paper argues that the proposed radio excess from pre-recombination bremsstrahlung decoupling cannot occur if matter and radiation stay kinetically coupled, challenging previous suggestions of an additional non-thermal component.

Contribution

It clarifies that no non-thermal radio excess can originate from bremsstrahlung decoupling under kinetic coupling conditions.

Findings

No excess from bremsstrahlung decoupling when matter and radiation are kinetically coupled

Reinforces the standard thermal spectrum of the cosmic microwave background

Challenges previous claims of a radio excess from early universe processes

Abstract

Recently it has been suggested that thermal bremsstrahlung emission, when it decouples prior to recombination, creates an excess over the Planck cosmic microwave background spectrum at sub-GHz frequencies. Remarkable by itself, this would also explain a long-standing unexplained deficit in the predictions of the extragalactic radio background. In this brief note we reiterate that no such non-thermal component can arise by itself when matter and radiation remain kinetically coupled.