A New Theorem Predicts that Stationary Black Holes Must Have at Least One Light Ring
martyb writes:
A new theorem predicts that stationary black holes must have at least one light ring:
Researchers at the Max Planck Institute for Gravitational Physics in Germany and Universidade de Aveiro in Portugal have recently introduced a theorem that makes predictions about the light rings around stationary black holes. Their theorem, presented in a paper published in Physical Review Letters, suggests that equilibrium black holes must, as a general rule, have at least one light ring in each of their sense of rotation.
"Remarkably, the properties of light rings can encode much relevant black hole information," Pedro Cunha and Carlos Herdeiro, the two researchers who carried out the study, told Phys.org via email. "Measuring these properties grants a direct window into the elusive and yet fairly uncharted regime of very strong gravity close to a black hole. At this moment it is still unclear whether Einstein's theory of general relativity remains a good description of the laws of gravity under such extreme conditions. Therefore, a key question is: does any black hole model, in any theory of gravity, need to have a light ring?"
[...] "In our paper, we introduce a generic and mathematically innovative argument that establishes that an equilibrium black hole must indeed have, as a rule, at least one standard light ring in each rotational sense," Cunha and Herdeiro said. "To analyze light rings, typically, one considers families of solutions of a given theory of gravity, like general relativity, or some particular model of modified gravity. Here, however, the argument is of a topological nature."
[...] "The prediction that black holes always have light rings and they are always outside the horizon has important consequences," Cunha and Herdeiro say. "For instance, it implies that the silhouette of a black hole, known as the black hole shadow, is generically different and usually larger than what one would expect the size of the black hole itself to be. So the shadow should always be a magnification of the black hole."
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