The shadow radius lower bound is more robust than the photon sphere radius lower bound in n-dimensional Einstein gravity
Abstract
The photon sphere and the black hole shadow are two closely related geometric features of strong-field spacetimes: the photon sphere is a hypersurface foliated by circular null geodesics, while the shadow boundary is determined by the critical impact parameter for null-geodesic capture. For static, spherically symmetric, asymptotically flat black holes in n-dimensional (n≥4) Einstein gravity with anisotropic matter, we derive lower bounds on the shadow radius and the photon sphere radius and compare their matter assumptions. Assuming that an auxiliary function Ξ(r) satisfies −ρ≤Ξ≤ pr and (r2Ξ)'≥0 between the horizon and the photon sphere under consideration, we establish rγ≥[(n−1)/2]1/(n−3)rH, where rH is the event horizon radius. Writing the shadow radius as rsh=[maxr>rHU(r)]−1/2, where U(r) is the null effective potential, we obtain rsh/rH≥[(n−1)/2]1/(n−3)√((n−1)/(n−3)) using only ρ≥0 and ρ+pr≥0, both implied by the weak energy condition (WEC). This shadow bound holds irrespective of the number of photon spheres and does not require the additional matter conditions used in the photon sphere lower-bound proofs. In this sense, the lower bound on the shadow radius is more robust than that on the photon sphere radius. A four-dimensional configuration satisfying the WEC has a single exterior photon sphere and obeys the shadow bound while violating the photon sphere lower-bound inequality, demonstrating this distinction even when both radii are associated with the same photon sphere. Both lower bounds are saturated by the Schwarzschild--Tangherlini solution. In Appendix~A, we also derive the mass-dependent upper bounds under the WEC, the strong energy condition, and suitable asymptotic decay assumptions.
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BibTeX
@misc{fu2026shadow,
title = {{The shadow radius lower bound is more robust than the photon sphere radius lower bound in $n$-dimensional Einstein gravity}},
author = {Jiaqi Fu and Yong Song},
howpublished = {arXiv},
year = {2026},
month = oct,
doi = {10.48550/arxiv.2606.27795},
eprint = {2606.27795},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2606.27795},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - The shadow radius lower bound is more robust than the photon sphere radius lower bound in n-dimensional Einstein gravity
AU - Jiaqi Fu
AU - Yong Song
PY - 2026
DA - 2026/10/08
PB - arXiv
DO - 10.48550/arxiv.2606.27795
UR - https://arxiv.org/abs/2606.27795
AB - The photon sphere and the black hole shadow are two closely related geometric features of strong-field spacetimes: the photon sphere is a hypersurface foliated by circular null geodesics, while the shadow boundary is determined by the critical impact parameter for null-geodesic capture. For static, spherically symmetric, asymptotically flat black holes in n-dimensional (n≥4) Einstein gravity with anisotropic matter, we derive lower bounds on the shadow radius and the photon sphere radius and compare their matter assumptions. Assuming that an auxiliary function Ξ(r) satisfies −ρ≤Ξ≤ p_r and (r^2Ξ)'≥0 between the horizon and the photon sphere under consideration, we establish r_γ≥[(n−1)/2]^1/(n−3)r_H, where r_H is the event horizon radius. Writing the shadow radius as r_sh=[max_r>r_HU(r)]^−1/2, where U(r) is the null effective potential, we obtain r_sh/r_H≥[(n−1)/2]^1/(n−3)√((n−1)/(n−3)) using only ρ≥0 and ρ+p_r≥0, both implied by the weak energy condition (WEC). This shadow bound holds irrespective of the number of photon spheres and does not require the additional matter conditions used in the photon sphere lower-bound proofs. In this sense, the lower bound on the shadow radius is more robust than that on the photon sphere radius. A four-dimensional configuration satisfying the WEC has a single exterior photon sphere and obeys the shadow bound while violating the photon sphere lower-bound inequality, demonstrating this distinction even when both radii are associated with the same photon sphere. Both lower bounds are saturated by the Schwarzschild--Tangherlini solution. In Appendix~A, we also derive the mass-dependent upper bounds under the WEC, the strong energy condition, and suitable asymptotic decay assumptions.
ER -