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arXiv · 2311.17462

Lower bound on the radii of light rings in traceless black-hole spacetimes

Shahar Hod
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Abstract

Photonspheres, curved hypersurfaces on which massless particles can perform closed geodesic motions around highly compact objects, are an integral part of generic black-hole spacetimes. In the present compact paper we prove, using analytical techniques, that the innermost light rings of spherically symmetric hairy black-hole spacetimes whose external matter fields are characterized by a traceless energy-momentum tensor cannot be located arbitrarily close to the central black hole. In particular, we reveal the physically interesting fact that the non-linearly coupled Einstein-matter field equations set the lower bound $r_γ\geq {6\over5}r_{\text{H}}$ on the radii of traceless black-hole photonspheres, where $r_{\text{H}}$ is the radius of the outermost black-hole horizon.

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Shahar Hod. 2023-12-28. Lower bound on the radii of light rings in traceless black-hole spacetimes. https://arxiv.org/abs/2311.17462

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