## The universe could be shaped like a donut, not a pancake, new research suggests

The universe could, in fact, be one giant donut, despite all the evidence to suggest it’s as flat as a pancake, new research suggests.

The strange patterns found in the Big Bang echoes could be explained by a universe with a more complicated shape, and astronomers have not fully proven the universe’s flatness, the study finds.

Flat surfaces
All observations so far suggest the universe is flat. In geometry, “flatness” refers to the behavior of parallel lines as they go to infinity. Think of a table: lines that start out parallel will stay that way as they extend across the length of the table.

Instead, look at Earth. The lines of longitude start out perfectly parallel to each other at the equator but eventually converge at the poles. The fact that the parallel lines initially intersect reveals that the Earth is not flat.

The same logic applies to the 3D universe. For example, the cosmic microwave background (CMB), the light released when the cosmos was only 380,000 years old, is now more than 42 billion light-years away with small temperature fluctuations across the sky. Astronomers have calculated the predicted size of those fluctuations compared to observations. If their measured size differs from the predictions, that means those light rays, which started out parallel, changed direction in spacetime, indicating that the geometry of the universe is curved.

But those same measurements have revealed that, ignoring the small-scale deviations of galaxies and black holes, the overall geometry of the universe is flat.

Different types of floor
But there is more than one type of floor. For example, draw parallel lines on a piece of paper. Then wrap one end of the paper to connect to the other, forming a cylinder. The lines remain parallel as they go around the cylinder. In the language of mathematics, any cylinder is geometrically flat but is said to have a different topology. Close both sides of the paper and make a torus or donut shape.

For another example of an oddly flat shape, wrap a thin strip of paper around a circle, but do a 180 degree turn at one end. The end result is a Möbius strip, which remains geometrically flat, because the parallel lines stay parallel, even when flipped.

Mathematicians have discovered 18 possible geometrically flat 3D topologies. In each, at least one dimension wraps around itself, sometimes flipping like a Möbius strip or doing partial rotations. In such a twisted universe, if we were to look far away, we would see a copy (perhaps backwards) of ourselves from a much younger age. For example, 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 it another version from 2 billion years ago, and so on.

If the universe were a giant donut, astronomers could look in two directions to see such copies.

the shape of the universe
Astronomers have measured the topology of the universe in multiple ways, from looking for duplicate galaxy patterns to matching circles in the CMB. All the evidence suggests that the universe is geometrically flat and has a simple unenveloped topology.

But a paper published on February 23 in the arXiv preprint database (opens in a new tab) suggests that previous measurements have been limited. In particular, observations have assumed that the universe envelopes itself in a single dimension and does not have a more complicated topology. In addition, CMB observations have revealed some strange and inexplicable anomalies, such as large patterns appearing where they shouldn’t.

In fact, a universe with a complicated topology could explain at least some of the anomalies in the CMB. While this is not a foolproof case for complicated topologies, the researchers offered insights for more sophisticated forward searches, such as CMB tracking studies.

In that case, there may be a mirror image of us somewhere in our twisted universe.