A spotlight beam fades into the darkness, but if you trace it backward, you hit the lamp. The history of the universe might be the same: tracing its expansion backward, we run into a beginning.

Physicists took the simplest model — a universe that stretches equally in all directions, like a cube with all sides growing. They didn't look into the very first moments but simply asked: could such expansion have lasted infinitely into the past? It turns out, no. If today the cosmos is expanding in all directions (and observations confirm this), then in the past its lines must converge to a single point. It's like inflating a balloon: no matter how far back you look, there's always a moment when it was a point.

But there's a loophole: if the universe had always been contracting in at least one direction, the past could be infinite. The authors give an example, such a world is like a sheet of rubber stretched in length and width but squeezed in thickness. It has no "beginning" in the usual sense, because contraction in one dimension compensates for expansion in others. However, our cosmos seems not to be like that: it swells in all directions at once, so its history is finite.

So the spotlight beam hits the lamp, and the expansion of the universe hits a moment before which there was nothing. Or maybe something entirely different, which our physics does not yet describe.