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. These include difficulties in obtaining reliable SMBH mass estimates and reaching the required angular resolution, and an insufficient knowledge of the accretion dynamics of high-redshift SMBHs. While these issues currently appear to prevent high-redshift SMBH shadows from being used as robust standard rulers, we hope that our flagging them early will help in making this probe theoretically mature by the time it will be experimentally feasible.

中文翻译：

---

关于使用黑洞阴影作为标准尺子的担忧

最近，Tsupko 等人。提出了一个非常有趣的建议，即使用高红移超大质量黑洞 (SMBH) 的阴影作为标准统治者。原则上，这将允许我们在红移范围内探测扩张历史，否则很难访问。在本说明中，我们批判性地审查了该提案，并确定了一些以前被忽视的重要问题。这些包括难以获得可靠的 SMBH 质量估计和达到所需的角分辨率，以及对高红移 SMBH 的吸积动力学了解不足。虽然这些问题目前似乎阻止了高红移 SMBH 阴影被用作强大的标准标尺，