Time is a fickle thing. Inside a massive star, it flows calmly, like sand in an old clock on a mantelpiece. Outside, on the scale of the entire universe, it stretches along with space, as if the same clock were in an elevator that speeds up and slows down. For a long time, physicists puzzled over whether these two worlds could be glued together without visible seams.

The authors of the new work took two pieces of spacetime. The first is the interior of a heavy, static sphere, like that of Schwarzschild. The second is an expanding cosmos with a peculiar flow of time, where the rate is set not just by distance but also by how fast the world inflates. They joined them according to rules devised by Israeli physicist Werner Israel — known as junction conditions. It turns out that gluing without wrinkles, that is, without a thin shell of matter at the boundary, is possible only under one condition: the expansion of the background must follow the same law as the geometry inside.

The main conclusion is reassuring. Local physics, the kind that works in a lab on Earth, does not have to break down even if time flows differently in the rest of the universe. It is enough that a simple compatibility rule holds at the interface. It is like your wristwatch and the time zone on another continent not conflicting, as long as you correctly adjust the date at the International Date Line.