Concerns regarding the use of black hole shadows as standard rulers

Understand

Recently, Tsupko et al.

• have put forward the very interesting proposal to use the shadows of high-redshift supermassive black holes (SMBHs) as standard rulers.
• This would in principle allow us to probe the expansion history within a redshift range which would otherwise be challenging to access.
• In this note, we critically examine this proposal, and identify a number of important issues which had been previously overlooked.