Gravitational Test beyond the First Post-Newtonian Order with the Shadow of the M87 Black Hole

Psaltis, D.; Medeiros, L.; Christian, P.; Ozel, F.; Akiyama, K.; Alberdi, A.; Alef, W.; Asada, K.; Azulay, R.; Ball, D.; Balokovic, M.; Barrett, J.; Bintley, D.; Blackburn, L.; Boland, W.; Bower, G. C.; Bremer, M.; Brinkerink, C. D.; Brissenden, R.; Britzen, S.; Broguiere, D.; Bronzwaer, T.; Byun, D. -Y.; Carlstrom, J. E.; Chael, A.; Chan, C. -K.; Chatterjee, S.; Chatterjee, K.; Chen, M. -T.; Chen, Y.; Cho, I.; Conway, J. E.; Cordes, J. M.; Crew, G. B.; Cui, Y.; Davelaar, J.; De Laurentis, M.; Deane, R.; Dempsey, J.; Desvignes, G.; Dexter, J.; Eatough, R. P.; Falcke, H.; Fish, V. L.; Fomalont, E.; Fraga-Encinas, R.; Friberg, P.; Fromm, C. M.; Gammie, C. F.; Garcia, R.; Gentaz, O.; Goddi, C.; Gomez, J. L.; Gu, M.; Gurwell, M.; Hada, K.; Hesper, R.; L. C., Ho; Ho, P.; Honma, M.; Huang, C. -W. L.; Huang, L.; Hughes, D. H.; Inoue, M.; Issaoun, S.; James, D. J.; Jannuzi, B. T.; Janssen, M.; Jiang, W.; Jimenez-Rosales, A.; Johnson, M. D.; Jorstad, S.; Jung, T.; Karami, M.; Karuppusamy, R.; Kawashima, T.; Keating, G. K.; Kettenis, M.; Kim, J. -Y.; Kim, J.; Kim, J.; Kino, M.; Koay, J. Y.; Koch, P. M.; Koyama, S.; Kramer, M.; Kramer, C.; Krichbaum, T. P.; Kuo, C. -Y.; Lauer, T. R.; Lee, S. -S.; Y. -R., Li; Li, Z.; Lindqvist, M.; Lico, R.; Liu, J.; Liu, K.; Liuzzo, E.; W. -P., Lo; Lobanov, A. P.; Lonsdale, C.; R. -S., Lu; Mao, J.; Markoff, S.; Marrone, D. P.; Marscher, A. P.; Marti-Vidal, I.; Matsushita, S.; Mizuno, Y.; Mizuno, I.; Moran, J. M.; Moriyama, K.; Moscibrodzka, M.; Muller, C.; Musoke, G.; Mus Mejias, A.; Nagai, H.; Nagar, N. M.; Narayan, R.; Narayanan, G.; Natarajan, I.; Neri, R.; Noutsos, A.; Okino, H.; Olivares, H.; Oyama, T.; Palumbo, D. C. M.; Park, J.; Patel, N.; Pen, U. -L.; Pietu, V.; Plambeck, R.; Popstefanija, A.; Prather, B.; Preciado-Lopez, J. A.; Ramakrishnan, V.; Rao, R.; Rawlings, M. G.; Raymond, A. W.; Ripperda, B.; Roelofs, F.; Rogers, A.; Ros, E.; Rose, M.; Roshanineshat, A.; Rottmann, H.; Roy, A. L.; Ruszczyk, C.; Ryan, B. R.; Rygl, K. L. J.; Sanchez, S.; Sanchez-Arguelles, D.; Sasada, M.; Savolainen, T.; Schloerb, F. P.; Schuster, K. -F.; Shao, L.; Shen, Z.; Small, D.; Sohn, B. W.; Soohoo, J.; Tazaki, F.; Tilanus, R. P. J.; Titus, M.; Torne, P.; Trent, T.; Traianou, E.; Trippe, S.; Van Bemmel, I.; Van Langevelde, H. J.; Van Rossum, D. R.; Wagner, J.; Wardle, J.; Ward-Thompson, D.; Weintroub, J.; Wex, N.; Wharton, R.; Wielgus, M.; Wong, G. N.; Wu, Q.; Yoon, D.; Young, A.; Young, K.; Younsi, Z.; Yuan, F.; Yuan, Y. -F.; Zhao, S. -S.

doi:10.1103/PhysRevLett.125.141104

The 2017 Event Horizon Telescope (EHT) observations of the central source in M87 have led to the first measurement of the size of a black-hole shadow. This observation offers a new and clean gravitational test of the black-hole metric in the strong-field regime. We show analytically that spacetimes that deviate from the Kerr metric but satisfy weak-field tests can lead to large deviations in the predicted black-hole shadows that are inconsistent with even the current EHT measurements. We use numerical calculations of regular, parametric, non-Kerr metrics to identify the common characteristic among these different parametrizations that control the predicted shadow size. We show that the shadow-size measurements place significant constraints on deviation parameters that control the second post-Newtonian and higher orders of each metric and are, therefore, inaccessible to weak-field tests. The new constraints are complementary to those imposed by observations of gravitational waves from stellar-mass sources.