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Recently, Tsupko et al. have put forward the very interesting proposal to usethe shadows of high-redshift supermassive black holes (SMBHs) as standardrulers. This would in principle allow us to probe the expansion history withina redshift range which would otherwise be challenging to access. In this note,we critically examine this proposal, and identify a number of important issueswhich had been previously overlooked. These include difficulties in obtainingreliable SMBH mass estimates and reaching the required angular resolution, andan insufficient knowledge of the accretion dynamics of high-redshift SMBHs.While these issues currently appear to prevent high-redshift SMBH shadows frombeing used as robust standard rulers, we hope that our flagging them early willhelp in making this probe theoretically mature by the time it will beexperimentally feasible.

Concerns regarding the use of black hole shadows as standard rulers