The unprecedented image of the M87* supermassive black hole has sparked some controversy over its usefulness as a test of the general relativistic Kerr metric. The criticism is mainly related to the black hole’s quasi-circular shadow and advocates that its radius depends not only on the black hole’s true spacetime properties but also on the poorly known physics of the illuminating accretion flow. In this paper we take a sober view of the problem and argue that our ability to probe gravity with a black hole shadow is only partially impaired by the matter degrees of freedom and the number of non-Kerr parameters used in the model. As we show here, a more intriguing situation arises from the mass scaling of the dimensional coupling constants that typically appear in non-GR theories of gravity. Existing limits from gravitational wave observations imply that supermassive systems like the M87* black hole would suffer a suppression of all non-GR deviation parameters in their metric, making the spacetime and the produced shadow virtually Kerr. Therefore, a supermassive black hole shadow is likely to probe only those extensions of General Relativity which are endowed with dimensionless coupling constants or other special cases with a screening mechanism for black holes or certain types of spontaneous scalarization.

中文翻译：

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超大质量黑洞阴影可以测试克尔度量吗？

M87* 超大质量黑洞的史无前例的图像引发了一些争议，即它作为一般相对论克尔度量的测试的有用性。批评主要与黑洞的准圆形阴影有关，并主张其半径不仅取决于黑洞的真实时空特性，还取决于鲜为人知的发光吸积流物理学。在本文中，我们对这个问题持冷静的看法，并认为我们用黑洞阴影探测引力的能力只是部分地受到物质自由度和模型中使用的非克尔参数数量的影响。正如我们在这里展示的，一个更有趣的情况来自于通常出现在非 GR 引力理论中的维度耦合常数的质量缩放。引力波观测的现有限制意味着像 M87* 黑洞这样的超大质量系统会受到度量中所有非 GR 偏差参数的抑制，使得时空和产生的阴影实际上是克尔。因此，超大质量黑洞阴影可能只探测广义相对论的那些具有无量纲耦合常数的扩展，或其他具有黑洞筛选机制或某些类型的自发标化机制的特殊情况。