Kavli Affiliate: John E. Carlstrom
| First 5 Authors: Dimitrios Psaltis, Lia Medeiros, Pierre Christian, Feryal Ozel, Kazunori Akiyama
| Summary:
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.
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