Physics & Math

It's one of the biggest questions of science: what shape is the universe? From Ptolemy onwards, physicists thought of the cosmos as a sphere, but this may not be correct.

Many alternative shapes have been proposed. And they have something in common: all can be described with reference to popular snack foods. So prise open a bag of your favourite munchie, and join us on a journey through food-based theoretical physics.

Ring doughnut

The idea that the universe might be shaped like a torus – or ring doughnut – has been around for many decades in various forms.

One variation proposes that the doughnut has a single twist – like a Möbius strip that has been inflated.

Our universe may even be floating around inside a torus-shaped space. In one interpretation of string theory, our universe is a three-dimensional region of space called a "brane" floating in higher-dimensional space.

One model suggests that the cosmos once included various branes with up to eight dimensions, floating about in a nine-dimensional space with each dimension circling back on itself like a doughnut. The high-dimension branes may have smashed together and evaporated, whereas our universe survived.

Pringle

Is space flat? If it is, then light beams that set off parallel to each other should keep travelling in parallel forever. If it is curved, then the beams will either drift apart or come together.

One possibility is that space is curved like a Pringle. At the centre of a Pringle, the surface curves up and down at the same time (see diagram). In theory, every point in our universe could be like that – in mathematical language, space could be negatively curved.

If this is true, it could explain why time only moves forwards, and possibly why the universe is expanding so fast.

So far, the evidence suggests that the universe really is flat, and not Pringle-shaped, but the question is not settled.

Peanut

Around 14 billion years ago, the universe formed in the big bang. Starting from an infinitely tiny speck, it exploded in all directions, gradually cooling as it did so.

Yet it may not have spread out evenly. Magnetic fields spanning the early universe could have caused the universe to expand more in one direction than others.

If so, this would mean that our universe is an ellipsoid like a peanut or olive (Physical Review Letters, vol 97, p 131302).

Bugle

Apologies to our non-American readers: Bugles are a popular corn snack in the US, and are cone-shaped like the horns of trumpets or bugles.

A universe shaped like a Bugle may sound odd, but it could help explain some puzzling observations of the cosmic microwave background radiation: the relic radiation left over from the big bang.

The CMB radiation has many hot and cold spots in it, but none above a certain size. On the face of it, there is no reason why this should be so. But a Bugle-shaped universe offers a simple explanation: at the time the spots formed 380,000 years after the big bang, there would not have been enough room for large spots to grow within the Bugle shape.

Apple

Although apples are not a shape that could describe our universe as we see it, this healthier snack could help us understand our universe's hidden dimensions.

Several of the big theories in physics, like string theory, predict that our universe has more dimensions than humans are aware of. We live in a four-dimensional universe, moving in length, breadth, height and time. But there may be many other dimensions – each of them curled up so tightly they are too small for us to perceive them.

Some physicists have tentatively suggested that they should be apple-shaped, because that shape helps explain why the universe's fundamental particles come in threes.

For example, we observe three types of neutrino. But it is possible that there is only one type of neutrino, and the three varieties we see are the result of neutrinos taking different routes through the hidden dimensions. Because apple shapes are both concave and convex, there are three types of route the particles could take, potentially explaining the three observed types.

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