Comment on "21-cm Radiation: A New Probe of Variation in the Fine-Structure Constant"

TL;DR

This paper critiques previous methods for analyzing 21-cm radiation as a probe of alpha variation, highlighting issues with dimensionful parameters and proposing a more consistent approach.

Contribution

It introduces a method that avoids problems with dimensionful parameters to reliably estimate the effect of alpha variation on 21-cm brightness temperature.

Findings

Previous estimates are sensitive to units used.

Proposed method yields consistent estimates of alpha variation effects.

Highlights importance of unit-invariant analysis in cosmological probes.

Abstract

Khatri and Wandelt reported that change in the value of alpha by 1% changes the mean brightness temperature $T_b$ decrement of the CMB due to 21 cm absorption by 5% over the redshift range z $<$ 50. A drawback of their approach is that the dimensionful parameters are used. Changing of units leads to the change of the magnitude and even sign of the effect. Similar problems may be identified in a large number of other publications which consider limits on the variation of alpha using dimentionful parameters. We propose a method to obtain consistent results and provide an estimate of the effect.