Episode 712 - Cosmic Conundrums
from Kevin McFarlane's podcast Hosted by Kevin McFarlane
The historic Event Horizon Telescope (EHT) observations of the supermassive black hole at the center of the Messier 87 galaxy (M87*) have resolved a bright, ring-like emission structure approximately 40\ \mu\text{as} in diameter. This is comparable to the Sagittarius A* (Sgr A*) shadow diameter of 48.7 \pm 7\ \mu\text{as}. General Relativity (GR) interprets this bright ring as the product of strong gravitational lensing, where an unstable critical photon shell surrounds the Kerr black hole. Within this classical geometric paradigm, lensed photons orbiting near-circular trajectories produce an infinite sequence of self-similar subrings that asymptotically converge toward a theoretical boundary known as the critical curve. However, while the macroscopic features of the M87* image strongly align with the predictions of the Kerr metric, the reconstructed images exhibit subtle asymmetries, localized variations in ring thickness, and temporal dynamics that stretch the limits of purely geometric spacetime descriptions.
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