The failure of testing for cosmic opacity via the distance-duality relation

TL;DR

This paper critically examines the use of the distance-duality relation for testing cosmic opacity, highlighting methodological limitations and the lack of convincing evidence for a transparent universe.

Contribution

It reveals the challenges and limitations in applying the DDR for cosmic opacity tests, emphasizing the need for more accurate data and cautious interpretation.

Findings

Applying DDR for opacity testing is limited by data quality and interpretation issues.

Current methods often assume frequency independence, which may bias results.

No convincing evidence of universe transparency has been established using DDR.

Abstract

The distance-duality relation (DDR) between the luminosity distance $D_L$ and the angular diameter distance $D_A$ is viewed as a powerful tool for testing for the opacity of the Universe, being independent of any cosmological model. It was applied by many authors, who mostly confirm its validity and report a negligible opacity of the Universe. Nevertheless, a thorough analysis reveals that applying the DDR in cosmic opacity tests is tricky. Its applicability is strongly limited because of a non-unique interpretation of the $D_L$ data in terms of cosmic opacity and a rather low accuracy and deficient extent of currently available $D_A$ data. Moreover, authors usually assume that cosmic opacity is frequency independent and parametrize it in their tests by a prescribed phenomenological function. In this way, they only prove that cosmic opacity does not follow their assumptions. As a consequence, no convincing evidence of transparency of the universe using the DDR has so far been presented.