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

Understand

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.