A universe that ends by beginning.

Look far enough ahead and the universe runs down. The last stars burn out. Even black holes, by the slow glow named after Stephen Hawking, are expected to give off all their energy and disappear. What is left is thin: faint radiation and stray particles, spread across a space that keeps on growing.

In 2005 the mathematician and physicist Roger Penrose put forward a startling idea about that ending. A universe left with nothing but light has lost every way to measure time or size. Then its vast, cold end can be read as the small, hot beginning of another. He calls each universe an aeon, and the idea conformal cyclic cosmology.

It is a bold idea, and Penrose names its costs himself. Every particle that has mass must one day lose it, and every black hole must be gone before the end. Many cosmologists are sceptical, and no measurement has settled it. In 2018 Penrose and two colleagues reported hot spots in the oldest light that they read as traces of an earlier universe; other cosmologists were unconvinced, one arguing that chance could explain the spots far more easily than the paper allowed. To us it is still a beautiful idea about how everything ends, and how it might begin again.

Why would losing mass matter? Because a clock needs it. Light can tell which events are able to reach which: the shape of time. Penrose counts that as nine of the ten numbers that describe space and time at each point. The tenth is the scale, and only something with mass can keep it. With nothing but light, he writes, there is no way to build a clock.

So if the last mass goes, the universe loses its sense of size. Nothing in it could measure a second or a metre. A vast, cold end and a small, hot beginning would then be the same shape, and only the scale, which nothing is left to measure, would tell them apart.

Our index makes a move like Penrose’s, and it makes it every time a mind hands back what it found. Walk round a circle once or a thousand times and you arrive at the same place, and how many times you went round is gone for good. A look crosses into the index like that: its place is kept, and it carries no count.

And what nobody has looked at yet has the same gift. Scale up whatever is still dark after any number of looks, and it is just like the dark on the very first day. The end of one stretch of looking reads as the start of the next. Penrose’s handover would wait at the end of a universe. Ours happens at every look.

Penrose’s picture asks every black hole to be gone before the end. We asked the question that turns it round: could a black hole be a crossing of its own, where the scale is shed and only the shape passes through?

We read what physics says. At the centre of the simplest black hole, as Einstein’s theory describes it, anything falling in is stretched one way and squeezed the other two, more and more without limit. Physicists call the stretching spaghettification. A change of scale stretches every way alike, so it cannot undo a stretch in one direction: at the centre, the shape itself is torn. Penrose’s own hypothesis about beginnings says as much: a universe starts smooth, while an end inside a black hole is left free.

Some physicists rewrite gravity so that a falling body passes through, into a new universe or out of a white hole. Each of those needs new physics of its own, and none is settled.

No physics we have read has a law that would let only the shape survive. What our question gives is a description of the law it would take: an end as smooth as Penrose’s beginning. It is a question, not a result, and we are holding it open.