A cosmic object spins, but its shadow isn't round. Astrophysicists examined a black hole in an unusual version of gravity — the so-called brane world (where our universe is a three-dimensional surface in a space of higher dimensions). In such a model, the black hole gains an extra parameter — the tidal charge (a measure of how extra dimensions distort gravity). The authors traced how light rays bend around such a hole and constructed its image.
A light ray here is like a messenger running through the twisted streets of an unfamiliar city. When the observer looks almost straight at the hole's pole, the secondary ring, a faint ghostly rim, hides inside the main bright circle. Tilt your gaze, and the picture changes dramatically: the rings spread apart, and a sharp asymmetry appears. Three factors are responsible for this skew: the hole's own rotation, the viewing angle, and that tidal charge.
The authors didn't stop at the shadow's shape. They added an infalling matter flow to the model — gas that rushes toward the hole from the last stable orbit down to the event horizon. This allowed them to compute frequency shifts of radiation across the entire disk. The main conductor of these shifts turned out to be the observer's tilt. Comparing two radio bands—86 GHz and 230 GHz — the scientists saw that brightness and total radiation power are higher at the lower frequency. Also, a disk rotating in the same direction as the hole shines differently than a counter-rotating one.
Such a detailed portrait of an invisible object is not just a theoretical exercise. When next-generation telescopes capture images of black hole shadows, it will be the asymmetry and frequency coloring that tell us whether gravity hides traces of extra dimensions.
The tidal charge in the brane-world model can be negative — unlike the electric charge of an ordinary black hole, which always enters formulas squared.
In the movie 'Interstellar', the black hole Gargantua is shown with rotation taken into account, but without the effect of extra dimensions: its shadow would be even more bizarre if the authors had added the tidal charge.