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arXiv · 2505.21431

Non-minimal light-curvature couplings and black-hole imaging

Raúl Carballo-Rubio·Héloïse Delaporte·Astrid Eichhorn·Pedro G. S. Fernandes
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Abstract

Non-minimal couplings between the electromagnetic field strength and the spacetime curvature are part of the effective field theory of gravity and matter. They alter the local propagation of light in a significant way if the ratio of spacetime curvature to the non-minimal coupling is of order one. Spacetime curvature can become appreciable around black holes, and yet the effect of non-minimal couplings on electromagnetic observations of black holes remains underexplored. A particular feature of the non-minimal coupling between the electromagnetic field-strength and the Riemann tensor is that it generates two distinct photon rings for different polarizations. Working within the paradigm of lensing bands and focusing on the $n = 1$ lensing band, we illustrate by which diagnostics a modified light propagation may be distinguished from a modified spacetime geometry and how constraints on the value of the non-minimal coupling can be obtained

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Raúl Carballo-Rubio, Héloïse Delaporte, Astrid Eichhorn, Pedro G. S. Fernandes. 2025-11-06. Non-minimal light-curvature couplings and black-hole imaging. https://arxiv.org/abs/2505.21431

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