Most of us grow up hearing that the universe is huge and mostly empty, but we’re usually told to picture it as a flat expanse stretching out in every direction.
Lately, though, cosmologists have been playing with a far more interesting idea: the universe might loop back on itself in a shape closer to a giant doughnut. That means if you travelled far enough in a straight line, you could, in theory, end up right back where you started. It sounds wild, but it’s based on real questions scientists are wrestling with about geometry, space and what the cosmos is actually doing at the very largest scales.
This idea isn’t just a quirky theory; it would completely change how we think about space, time and what “edge of the universe” even means. For years, researchers have been studying background radiation, cosmic patterns and subtle irregularities that don’t quite fit the usual model. None of this gives a definite answer yet, but the doughnut idea is picking up attention because it helps explain a few puzzling features we see when we look deep into space. The fun part is that something that sounds playful could end up being surprisingly close to the truth.
The universe being flat doesn’t mean it’s shaped like a pancake.
When scientists say the universe is flat, they’re talking about geometry, not shape. Flat just means parallel lines stay parallel. You can have a flat piece of paper, but you can also roll that paper into a cylinder or connect the ends to make a doughnut shape, and it’s still geometrically flat.
Think of it like wrapping paper around a cylinder. The lines on the paper stay parallel even when they’re going around in circles. Mathematicians have discovered 18 possible geometrically flat, 3D topologies that could describe our universe, and most of them haven’t been properly tested yet. Flat doesn’t mean simple.
Scientists are looking for a universe that loops back on itself.
In a universe with complex topology, travelling across space might be like moving across the screen in Pac-Man. Go far enough in one direction and you end up back where you started. Sounds mental, but there’s no definitive proof it’s not happening.
The idea is called a 3-torus, which is basically a cube where exiting one side brings you back through the opposite side. Light could travel around the universe and loop back, meaning we might be seeing the same galaxies multiple times from different angles without realising it.
The cosmic microwave background might hold the answers.
The cosmic microwave background is leftover light from shortly after the Big Bang. It’s spread throughout the universe and contains patterns that could tell us about the shape of space. Scientists are analysing these patterns, looking for signs that the universe loops.
Strange patterns found in echoes of the Big Bang could be explained by a universe with a more complicated shape. If the universe has complex topology, certain features in this ancient light should repeat or show up in specific ways. Finding those patterns would be proof that space isn’t as straightforward as we thought.
Previous searches only looked at the simplest version.
Scientists have searched for signs of a looping universe before, but they only checked for the most basic version of a 3-torus. Earlier research considered only a small subset of the possible topologies the universe could have. They looked for a simple cube that loops back on itself but didn’t account for twisted or rotated variations.
There are loads of other possibilities they haven’t tested. For example, the sides of the cosmic cube might be twisted relative to each other. Exiting the top of the cube would bring you back to the bottom, but rotated by, for example, 180 degrees. You’d come back flipped upside down or facing a different direction. No one’s looked for that yet.
There are 17 possible topologies that haven’t been ruled out.
A recent study considered a total of 17 possible nontrivial topologies for the cosmos, and most of them haven’t been ruled out. That means we’ve barely scratched the surface of what the universe’s shape could actually be.
Each topology would leave different signatures in the cosmic microwave background and in the distribution of galaxies. Scientists are now working on ways to search for these signatures using more sophisticated methods than before. It’s computationally massive work, probably requiring machine learning to speed things up, but it’s possible.
A doughnut universe would mean you could see copies of yourself.
If the universe loops back on itself, light from our galaxy could travel all the way around and come back to us. That means looking in certain directions, we’d see older versions of the Milky Way. If the universe were 1 billion light-years across, astronomers would see a version of the Milky Way as it was 1 billion years ago and, behind that, another copy from 2 billion years ago.
You wouldn’t see yourself directly because we can’t see Earth from that distance, but in theory, there’d be multiple images of our galaxy scattered across the sky at different ages. Scientists have looked for duplicated patterns of galaxies and haven’t found them yet, but that doesn’t mean they’re not there. The universe might just be too big for us to spot the copies.
They’re using a technique inspired by hearing the shape of a drum.
The Compact team plans to analyse sound waves that left their imprint in the cosmic microwave background and other cosmological data. It’s based on an old mathematical idea about whether you can work out the shape of a drum just by listening to it.
The universe has acoustic waves that rippled through space in its early moments, and those waves left patterns we can still detect. Different shapes would produce different patterns, like how different drums make different sounds. By analysing the frequencies and amplitudes in cosmological data, scientists hope to reverse-engineer the shape of the universe.
The Euclid space telescope could help prove it.
Europe’s Euclid space telescope is mapping the distribution of galaxies across the cosmos. Researchers plan to hunt for signs of nontrivial topology in upcoming data from these surveys. If the universe loops, galaxies should show repeating patterns.
Finding those patterns is like looking for cosmic fingerprints. You’d need to spot the same structures appearing in different parts of the sky, which would only happen if space is wrapping back on itself. It’s difficult because you’re looking for matches across enormous distances, but with enough data, it might be possible.
Scientists admit it’s high-risk, high-reward research.
Team member Andrew Jaffe, a cosmologist at Imperial College London, calls it high-risk, high-reward cosmology. There’s a decent chance they won’t find anything, but if they do, it would completely change our understanding of the universe.
Some researchers are cautiously optimistic, but others are sceptical. The prevailing view is still that the universe is infinite and has simple topology, but no one’s definitively proved that. Until they’ve checked all the possibilities properly, the door’s still open for weirder shapes, like the doughnut.
If the universe is too big, we might never know its shape.
Physicist Glenn Starkman said we’ll either find the topology of the universe or determine that it’s so big that its topology cannot be detected, at least with the tools available to us now. If the loops are bigger than the observable universe, we’d never see the repeating patterns.
Even if the universe does loop back on itself, it might be so vast that light hasn’t had time to make the full trip since the Big Bang. We’d be living inside a doughnut, but unable to see far enough to prove it. That would leave the question open indefinitely, which is frustrating but still tells us something important about the scale of everything.