Searching for violations of the no-hair theorem (NHT) is a powerful way to test gravity, and more generally fundamental physics, particularly with regards to the existence of additional scalar fields. The first observation of a black hole (BH) shadow by the Event Horizon Telescope (EHT) has opened a new direct window onto tests of gravity in the strong-field regime, including probes of violations of the NHT. We consider two scenarios described by the Einstein-Maxwell equations of General Relativity and electromagnetism, to which we add a scalar field. In the first case we consider a minimally-coupled scalar field with a potential, whereas in the second case the field is conformally-coupled to curvature. In both scenarios we construct charged BH solutions, which are found to carry primary scalar hair. We then compute the shadows cast by these two BHs as a function of their electric charge and scalar hair parameter. Comparing these shadows to the shadow of M87* recently imaged by the EHT collaboration, we set constraints on the amount of scalar hair carried by these two BHs. The conformally-coupled case admits a regime for the hair parameter, compatible with EHT constraints, describing a so-called mutated Reissner-Nordstrom BH: this solution was recently found to effectively mimic a wormhole. Our work provides novel constraints on fundamental physics, and in particular on violations of the no-hair theorem and the existence of additional scalar fields, from the shadow of M87*.

中文翻译：

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视界望远镜观测到的带有标量头发的黑洞

寻找违反无毛定理 (NHT) 的方法是测试重力和更普遍的基础物理学的有效方法，尤其是关于额外标量场的存在。事件视界望远镜 (EHT) 对黑洞 (BH) 阴影的首次观测为强场区重力测试打开了一个新的直接窗口，包括对违反 NHT 的探测。我们考虑广义相对论和电磁学的爱因斯坦-麦克斯韦方程所描述的两种情况，我们向其中添加了一个标量场。在第一种情况下，我们考虑具有势能的最小耦合标量场，而在第二种情况下，场与曲率共形耦合。在这两种情况下，我们都构建了带电 BH 解决方案，发现它们带有主要的标量头发。然后，我们将这两个 BH 投射的阴影计算为它们的电荷和标量头发参数的函数。将这些阴影与最近由 EHT 合作拍摄的 M87* 的阴影进行比较，我们对这两个 BH 携带的标量头发数量设置了限制。共形耦合的情况允许头发参数的制度与 EHT 约束兼容，描述了所谓的突变 Reissner-Nordstrom BH：最近发现该解决方案可以有效地模拟虫洞。我们的工作为基础物理学提供了新的约束，特别是对无毛定理的违反和来自 M87* 阴影的额外标量场的存在。我们对这两个 BH 携带的标量头发数量设置了限制。共形耦合的情况允许头发参数的制度与 EHT 约束兼容，描述了所谓的突变 Reissner-Nordstrom BH：最近发现该解决方案可以有效地模拟虫洞。我们的工作为基础物理学提供了新的约束，特别是对无毛定理的违反和来自 M87* 阴影的额外标量场的存在。我们对这两个 BH 携带的标量头发数量设置了限制。共形耦合的情况允许头发参数的制度与 EHT 约束兼容，描述了所谓的突变 Reissner-Nordstrom BH：最近发现该解决方案可以有效地模拟虫洞。我们的工作为基础物理学提供了新的约束，特别是对无毛定理的违反和来自 M87* 阴影的额外标量场的存在。