First Sagittarius A* Event Horizon Telescope Results. VI. Testing the Black Hole Metric
Authors: Event Horizon Telescope Collaboration  , Akiyama Kazunori , Alberdi Antxon , Alef Walter , Algaba Juan Carlos , Anantua Richard , Asada Keiichi , Azulay Rebecca , Bach Uwe , Baczko Anne-Kathrin , Ball David , Baloković Mislav , Barrett John , Bauböck Michi , Benson Bradford A. , Bintley Dan , Blackburn Lindy , Blundell Raymond , Bouman Katherine L. , Bower Geoffrey C. , Boyce Hope , Bremer Michael , Brinkerink Christiaan D. , Brissenden Roger , Britzen Silke , Broderick Avery E. , Broguiere Dominique , Bronzwaer Thomas , Bustamante Sandra , Byun Do-Young , Carlstrom John E. , Ceccobello Chiara , Chael Andrew , Chan Chi-kwan , Chatterjee Koushik , Chatterjee Shami , Chen Ming-Tang , Chen 陈 Yongjun 永军 , Cheng Xiaopeng , Cho Ilje , Christian Pierre , Conroy Nicholas S. , Conway John E. , Cordes James M. , Crawford Thomas M. , Crew Geoffrey B. , Cruz-Osorio Alejandro , Cui 崔 Yuzhu 玉竹 , Davelaar Jordy , De Laurentis Mariafelicia , Deane Roger , Dempsey Jessica , Desvignes Gregory , Dexter Jason , Dhruv Vedant , Doeleman Sheperd S. , Dougal Sean , Dzib Sergio A. , Eatough Ralph P. , Emami Razieh , Falcke Heino , Farah Joseph , Fish Vincent L. , Fomalont Ed , Ford H. Alyson , Fraga-Encinas Raquel , Freeman William T. , Friberg Per , Fromm Christian M. , Fuentes Antonio , Galison Peter , Gammie Charles F. , García Roberto , Gentaz Olivier , Georgiev Boris , Goddi Ciriaco , Gold Roman , Gómez-Ruiz Arturo I. , Gómez José L. , Gu 顾 Minfeng 敏峰 , Gurwell Mark , Hada Kazuhiro , Haggard Daryl , Haworth Kari , Hecht Michael H. , Hesper Ronald , Heumann Dirk , Ho 何 Luis C. 子山 , Ho Paul , Honma Mareki , Huang Chih-Wei L. , Huang 黄 Lei 磊 , Hughes David H. , Ikeda Shiro , Impellizzeri C. M. Violette , Inoue Makoto , Issaoun Sara , James David J. , Jannuzi Buell T. , Janssen Michael , Jeter Britton , Jiang 江 Wu 悟 , Jiménez-Rosales Alejandra , Johnson Michael D. , Jorstad Svetlana , Joshi Abhishek V. , Jung Taehyun , Karami Mansour , Karuppusamy Ramesh , Kawashima Tomohisa , Keating Garrett K. , Kettenis Mark , Kim Dong-Jin , Kim Jae-Young , Kim Jongsoo , Kim Junhan , Kino Motoki , Koay Jun Yi , Kocherlakota Prashant , Kofuji Yutaro , Koch Patrick M. , Koyama Shoko , Kramer Carsten , Kramer Michael , Krichbaum Thomas P. , Kuo Cheng-Yu , Bella Noemi La , Lauer Tod R. , Lee Daeyoung , Lee Sang-Sung , Leung Po Kin , Levis Aviad , Li 李 Zhiyuan 志远 , Lico Rocco , Lindahl Greg , Lindqvist Michael , Lisakov Mikhail , Liu 刘 Jun 俊 , Liu Kuo , Liuzzo Elisabetta , Lo Wen-Ping , Lobanov Andrei P. , Loinard Laurent , Lonsdale Colin J. , Lu 路 Ru-Sen 如森 , Mao 毛 Jirong 基荣 , Marchili Nicola , Markoff Sera , Marrone Daniel P. , Marscher Alan P. , Martí-Vidal Iván , Matsushita Satoki , Matthews Lynn D. , Medeiros Lia , Menten Karl M. , Michalik Daniel , Mizuno Izumi , Mizuno Yosuke , Moran James M. , Moriyama Kotaro , Moscibrodzka Monika , Müller Cornelia , Mus Alejandro , Musoke Gibwa , Myserlis Ioannis , Nadolski Andrew , Nagai Hiroshi , Nagar Neil M. , Nakamura Masanori , Narayan Ramesh , Narayanan Gopal , Natarajan Iniyan , Nathanail Antonios , Fuentes Santiago Navarro , Neilsen Joey , Neri Roberto , Ni Chunchong , Noutsos Aristeidis , Nowak Michael A. , Oh Junghwan , Okino Hiroki , Olivares Héctor , Ortiz-León Gisela N. , Oyama Tomoaki , Özel Feryal , Palumbo Daniel C. M. , Paraschos Georgios Filippos , Park Jongho , Parsons Harriet , Patel Nimesh , Pen Ue-Li , Pesce Dominic W. , Piétu Vincent , Plambeck Richard , PopStefanija Aleksandar , Porth Oliver , Pötzl Felix M. , Prather Ben , Preciado-López Jorge A. , Psaltis Dimitrios , Pu Hung-Yi , Ramakrishnan Venkatessh , Rao Ramprasad , Rawlings Mark G. , Raymond Alexander W. , Rezzolla Luciano , Ricarte Angelo , Ripperda Bart , Roelofs Freek , Rogers Alan , Ros Eduardo , Romero-Cañizales Cristina , Roshanineshat Arash , Rottmann Helge , Roy Alan L. , Ruiz Ignacio , Ruszczyk Chet , Rygl Kazi L. J. , Sánchez Salvador , Sánchez-Argüelles David , Sánchez-Portal Miguel , Sasada Mahito , Satapathy Kaushik , Savolainen Tuomas , Schloerb F. Peter , Schonfeld Jonathan , Schuster Karl-Friedrich , Shao Lijing , Shen 沈 Zhiqiang 志强 , Small Des , Sohn Bong Won , SooHoo Jason , Souccar Kamal , Sun 孙 He 赫 , Tazaki Fumie , Tetarenko Alexandra J. , Tiede Paul , Tilanus Remo P. J. , Titus Michael , Torne Pablo , Traianou Efthalia , Trent Tyler , Trippe Sascha , Turk Matthew , van Bemmel Ilse , van Langevelde Huib Jan , van Rossum Daniel R. , Vos Jesse , Wagner Jan , Ward-Thompson Derek , Wardle John , Weintroub Jonathan , Wex Norbert , Wharton Robert , Wielgus Maciek , Wiik Kaj , Witzel Gunther , Wondrak Michael F. , Wong George N. , Wu 吴 Qingwen 庆文 , Yamaguchi Paul , Yoon Doosoo , Young André , Young Ken , Younsi Ziri , Yuan 袁 Feng 峰 , Yuan 袁 Ye-Fei 业飞 , Zensus J. Anton , Zhang Shuo , Zhao Guang-Yao , Zhao 赵 Shan-Shan 杉杉 
Published: May 2022

Abstract
Astrophysical black holes are expected to be described by the Kerr metric. This is the only stationary, vacuum, axisymmetric metric, without electromagnetic charge, that satisfies Einstein’s equations and does not have pathologies outside of the event horizon. We present new constraints on potential deviations from the Kerr prediction based on 2017 EHT observations of Sagittarius A* (Sgr A*). We calibrate the relationship between the geometrically defined black hole shadow and the observed size of the ring-like images using a library that includes both Kerr and non-Kerr simulations. We use the exquisite prior constraints on the mass-to-distance ratio for Sgr A* to show that the observed image size is within ∼10% of the Kerr predictions. We use these bounds to constrain metrics that are parametrically different from Kerr, as well as the charges of several known spacetimes. To consider alternatives to the presence of an event horizon, we explore the possibility that Sgr A* is a compact object with a surface that either absorbs and thermally reemits incident radiation or partially reflects it. Using the observed image size and the broadband spectrum of Sgr A*, we conclude that a thermal surface can be ruled out and a fully reflective one is unlikely. We compare our results to the broader landscape of gravitational tests. Together with the bounds found for stellar-mass black holes and the M87 black hole, our observations provide further support that the external spacetimes of all black holes are described by the Kerr metric, independent of their mass.