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Matter environments around black holes: Geodesics, light rings, and ultracompact configurations

Dylan S. Fonseca1,*, Caio F. B. Macedo2,1,†, Mateus Malato Corrêa1,‡, and Diego Rubiera-Garcia3,§

  • *Contact author: dylan.fonseca@icen.ufpa.br
  • †Contact author: caiomacedo@ufpa.br
  • ‡Contact author: malato.mateus@gmail.com
  • §Contact author: drubiera@ucm.es
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Phys. Rev. D 113, 124039 – Published 12 June, 2026

DOI: https://doi.org/10.1103/x95d-ry14

Abstract

Astrophysical black holes are invariably embedded in matter environments whose gravitational influence can alter key strong-field features of the spacetime. In this work, we investigate the impact of spherically symmetric dark-matter distributions on black hole geometry, geodesic structure, and ringdown phenomenology. Modeling the surrounding matter through Einstein clusters, we construct self-consistent spacetimes for three widely used density profiles—the Hernquist, Navarro-Frenk-White, and Jaffe models—and examine how their near-horizon behavior modifies the location and stability of circular timelike and null geodesics, including the innermost stable circular orbit (ISCO) and light rings. In the low-compactness regime, we derive analytical expressions showing that environmental effects generically shift the ISCO inward and the principal light ring outward, leading to parametric deviations in their associated orbital frequencies and Lyapunov exponents. At higher compactness, we explore the emergence of additional light rings, marginally stable orbits, and secondary horizons, identifying the regions of parameter space in which these additional geodesic structures arise. We introduce a perturbative approach to find approximate analytical solutions to the spacetime metric that are valid in astrophysical scenarios and compare them with the numerical solutions. Using time-domain evolutions of scalar perturbations, we demonstrate how such structures can imprint characteristic signatures on the ringdown signal, including long-lived trapped modes and echolike modulations associated with multiple potential barriers. Our results provide a unified framework for assessing environmental effects around black holes and highlight the importance of matter-induced corrections for interpreting upcoming electromagnetic and gravitational-wave observations.

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Phys. Rev. D 113, 124039– Published 12 June, 2026
  • Received 13 January 2026
  • Accepted 15 May 2026
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