https://doi.org/10.1140/epjc/s10052-025-14721-w
Regular Article - Theoretical Physics
Shadows of naked singularity in Brans–Dicke gravity
1
PPGCosmo, Núcleo de Astrofísica e Cosmologia (Cosmo-Ufes), Universidade Federal do Espírito Santo, 29075-910, Vitória, ES, Brazil
2
Institute of Theoretical Astrophysics, University of Oslo, Sem Sælands vei 13, 0371, Oslo, Norway
3
Departamento de Física, Núcleo de Astrofísica e Cosmologia (Cosmo-Ufes), PPGCosmo, Universidade Federal do Espírito Santo, 29075-910, Vitória, ES, Brazil
4
Centro Brasileiro de Pesquisas Físicas (CBPF), R. Xavier Sigaud 150, 22290-180, Rio de Janeiro, RJ, Brazil
a
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Received:
16
June
2025
Accepted:
30
August
2025
Published online:
12
September
2025
Abstract
We investigate exact vacuum solutions in Brans–Dicke (BD) gravity, focusing on their implications for black hole shadow imaging. Motivated by the Event Horizon Telescope (EHT) observations, we revisit a class of BD solutions that exhibit a naked singularity. These solutions, despite lacking a conventional event horizon, exhibit photon spheres and produce shadow-like features. We analyze null geodesics and perform ray-tracing simulations under an optically thin accretion disk model to generate synthetic images. Our results show that BD naked singularities can cast shadows smaller than those of Schwarzschild black holes of equivalent mass. This requires, however, negative values for the BD parameter 
(in the range 
). The lower bound on is compatible with the absence of ghosts. Although such range is outside the Solar System bounds, our results are relevant for a larger class of modified gravity models whose local behaviour can be approximated by Brans–Dicke gravity. These findings suggest that BD naked singularities are possible candidates for compact astrophysical objects.
© The Author(s) 2025
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