WoU-MMA: Can Black Hole Images Constrain Modified Gravity Theories?

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The recent imaging of the black hole at the center of the galaxy M87 by the Event Horizon Telescope holds the promise of revealing wonders about the astrophysics of material accreting onto a supermassive black hole. The information contained in these images has promise to also allow for tests of general relativity in the region of strong gravitational fields outside the event horizon of a supermassive black hole. A research group at the University of Illinois at Urbana-Champaign will study whether these images can indeed be used to constrain deviations from Einstein's theory. This research will be integrated with activities aimed at increasing the interest of middle-school students and the general public in physics and science in general. The research plan is aimed at studying whether the images obtained from the observation of the black holes at the center of the galaxy M87 and at the center of the Milky Way can be used to constrain deviations from Einstein's theory of general relativity. First, the investigators will study how the shape of the so-called shadow in these images changes in modified gravity, developing a fit for the boundary and calculating the Lyapunov exponents of unstable geodesics near the light-ring, which produce certain features in the image. Then, they will develop simple, phenomenological models of accretion flow in modified gravity, which will allow them to generate simple synthetic images and study the effect of deviations from general relativity. This study will be followed by an analysis of the Blandford-Znajek process in modified gravity to test how sensitive the jet power and magnetic field structure are to departures from general relativity. The project will conclude with a thorough treatment of general relativistic magnetohydrodynamics and accretion in modified gravity, and the generation of synthetic polarized images from realistic accretion models to determine whether deviations from general relativity can be disentangled from other astrophysical processes. This project advances the goals of the NSF Windows on the Universe Big Idea.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.