The blackness of the cosmic microwave background spectrum as a probe of the distance-duality relation

TL;DR

This paper investigates how potential violations of the reciprocity relation could cause spectral distortions in the cosmic microwave background, using observational constraints to limit such violations to very small levels.

Contribution

It demonstrates that current observational data tightly constrain violations of the reciprocity relation, ensuring the cosmic microwave spectrum remains nearly black.

Findings

Violations of the reciprocity relation are limited to less than 0.01% since decoupling.

Spectral distortions from other sources are compared to those from reciprocity violations.

Current constraints strongly support the validity of the distance duality relation.

Abstract

A violation of the reciprocity relation, which induces a violation of the distance duality relation, reflects itself in a change in the normalisation of the cosmic microwave spectrum in such a way that its spectrum is grey. We show that existing observational constraints imply that the reciprocity relation cannot be violated by more than 0.01% between decoupling and today. We compare this effect to other sources of violation of the distance duality relations which induce spectral distortion of the cosmic microwave background spectrum.