Manahil Ali

Light passing by a massive body bends around it, like a stream of cars swerving around a deep pothole in the road. The steeper the pothole's slopes, the more the trajectories curve, and the fewer straight paths remain to the observer. That's how a black hole's shadow is born: a region from which light can no longer escape.

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.

The first three minutes after the birth of the world are a cosmic kitchen where nature cooked the lightest atomic nuclei. The recipe is set by gravity: it controls the cooling rate of the hot 'broth' of particles. If gravity worked differently than we are used to, the dish would have turned out different — with different amounts of helium and deuterium.