Science

The Universe could have the shape of a “donut”

For many years, cosmologists have questioned the shape of the Universe. Is it a sphere? A dodecahedron? Or something else? But some physicists believe that it could actually have an even more peculiar shape: that of a donut. In such a finite universe, it would be possible to travel in a straight line and come back to our starting point, and thus determine its size.

By examining the light from the very beginning of the Universe, Thomas Buchert and a team of astrophysicists deduced that our cosmos can be multiconnected, which means that space is closed on itself in all three dimensions like a donut. Such a universe would be finite, and depending on their results, our entire cosmos might be only three to four times the size of the boundaries of the observable Universe (about 45 billion light years in radius).

Physicists use Einstein’s language of general relativity to explain the Universe. This language connects the content of space-time to the deformation of space-time, which then tells these contents how to interact. In a cosmological context, this language links the content of the entire universe – dark matter, dark energy, ordinary matter, radiation – to its global geometric form. For decades, astronomers have debated the nature of this shape: is our universe “flat” (meaning that imaginary parallel lines would stay parallel forever), “closed” (parallel lines would eventually end up)? cross) or “open” (these lines would diverge)?

The different possible geometries of the Universe. © Wikimedia

This geometry of the Universe dictates its destiny. Flat, open universes would continue to expand forever, while a closed universe would eventually collapse on itself. Multiple observations, especially from the cosmic diffuse background, have firmly established that we live in a flat universe. The parallel lines remain parallel and our universe will not stop expanding. But form is not limited to geometry.

Topology of the Universe: is it finite and multiconnected?

There is also topology, which is how shapes can change while keeping the same geometric rules. While our measurements of the content and shape of the Universe tell us about its geometry – it is flat – they do not tell us about the topology. They don’t tell us if our universe is multiconnected, which means that one or more dimensions of our cosmos connect to each other.

While a perfectly flat universe would stretch to infinity, a flat universe with a multi-connection topology would have finite size. If we could somehow determine if one or more dimensions are enveloped in themselves, then we would know that the Universe is finite in that dimension. We could then use these observations to measure the total volume of the Universe. But how would a multiconnected universe turn out?

A team of astrophysicists from the University of Ulm in Germany and the University of Lyon in France focused on the study of the cosmic diffuse background (CMB). When the CMB was issued, our universe was a million times smaller than it is today, and so if our universe is indeed multiconnected, then it was much more likely to curl up on itself. within the observable limits of the cosmos at the time.

Today, due to the expansion of the Universe, envelopment is much more likely to occur on a scale beyond observable limits, and therefore envelopment would be much more difficult to detect. The observations of the CMB give us our best chance of seeing the footprints of a multiconnected universe.

The team specifically looked at the temperature disturbances of the CMB. If one or more dimensions of our universe were to reconnect, the disturbances could not be greater than the distance around those loops. ” In an infinite space, temperature disturbances of CMB radiation exist at all scales. If, however, the space is finite, then those wavelengths which are larger than the size of the space are missing. », Explains Buchert. In other words: There would be a maximum size for the perturbations, which could reveal the topology of the Universe.

A CMB card compatible with a 3D torus shape

CMB maps made with satellites like NASA’s WMAP and ESA’s Planck have observed an intriguing amount of large-scale missing disturbance. Buchert and his collaborators examined whether these missing disturbances could be due to a multiconnected universe. To do this, the team performed numerous computer simulations of what the CMB would look like if the Universe were a 3D torus, which is the mathematical name for a giant three-dimensional donut, where our cosmos is connected to. itself in all three dimensions.

simulation cmb form torus universe
CMB simulation for a torus-shaped universe. The results are consistent with the anisotropies detected in the current CMB. © Buchert et al. 2021

It is therefore necessary to make simulations in a given topology and compare with what is observed. The properties of the observed fluctuations of the CMB then show a missing power at scales exceeding the size of the Universe. We find a much better correspondence with the observed fluctuations, compared to the standard cosmological model which is considered to be infinite », Says Buchert. Missing power means that fluctuations in CMB are not present at these scales. This would imply that our universe is multiconnected and finite, at this size scale.

We can vary the size of the space and repeat this analysis. The result is an optimal size of the Universe that best matches the observations of the CMB. The answer of our article is clearly that the finite universe corresponds better to observations than the infinite model. We could say: now we know the size of the Universe “. The team found that a multi-connection universe about three to four times the size of our observable bubble best matched the CMB data. Of course, the authors point out that these results are preliminary and require additional work.

Sources: arXiv

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