A black hole is not just a dark spot in the sky. Its shadow falls on the glowing backdrop of infalling matter and carries the imprint of spacetime geometry itself. If gravity works even slightly differently than we think, the shadow will give it away.
In a new study, physicists traced light rays around a rotating black hole in so-called Rastall gravity. This theory adds two new numbers to Einstein's equations: the structure parameter γ and the Rastall parameter μ. Using computer simulations, they launched rays from a distant observer and watched what silhouette emerged against the accretion disk — the superheated matter swirling around the hole.
The result was clear: at a fixed spin rate, as γ increases, the shadow expands noticeably. The photon capture region grows, as if the black hole were 'swelling' in the eyes of a distant viewer. At the same time, the shadow becomes rounder — rotation deforms it less. The parameter μ also contributes, and the combined influence of both numbers leaves a distinct pattern on the shadow.
Thus, the shadow becomes a tool: by measuring its size and deviation from a perfect circle, we can probe the fundamental structure of gravity. Perhaps future observations with telescopes like the Event Horizon Telescope will help choose between the Einsteinian picture and its extensions.
as written by the authors arXiv:2606.15727
We investigate the observational appearance of a rotating black hole (BH) in Rastall gravity by analyzing its shadow and accretion signatures under different illumination environments. The spacetime geometry is characterized by the Rastall parameter $$\mu$$, the structure parameter $$\gamma$$, and the rotation parameter $$a$$. To visualize the BH environment, we employ a ray-tracing algorithm that follows photon trajectories from the observer's screen to the emission region. We analyze how the shadow radius, distortion, and photon ring morphology respond to changes in the spacetime parameters. For a fixed value of $$a$$, the shadow observables exhibit a pronounced dependence on the Rastall gravity parameters. In particular, increasing the structure parameter $$\gamma$$ leads to a gradual enlargement of the shadow radius, indicating an expansion of the photon capture region surrounding the BH. At the same time, the distortion parameter decreases, implying that the shadow boundary becomes progressively more circular and less deformed. These results suggest that larger values of $$\gamma$$ tend to suppress the asymmetry induced by rotation and enhance the apparent size of the shadow. Similar modifications are observed for different values of the Rastall parameter $$\mu$$, demonstrating that the combined effects of $$\mu$$ and $$\gamma$$ leave distinct signatures on the shadow morphology. Consequently, shadow observations may provide an effective tool for constraining the parameter space of rotating BHs in Rastall gravity.
A black hole's shadow is about 2.5 times larger than its event horizon, the boundary from which not even light can return.