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Imagine a universe where you could steer a spaceship in one direction and eventually come back to where you started. If our universe were a finite donut, then such movements would be possible and physicists could potentially measure its size.
“We could say: now we know the size of the universe,” astrophysicist Thomas Buchert, of the University of Lyon, France’s Astrophysics Research Center, told Live Science in an email.
Related: 10 crazy theories about the universe
By examining the light of the very first universe, Buchert and a team of astrophysicists deduced that our cosmos can be multiconnected, which means that space is closed on itself in three dimensions like a three-dimensional donut. Such a universe would be finite, and depending on their results, our entire cosmos might be only about three to four times the size of the limits of the observable universe, about 45 billion light years away.
A tasty problem
Physicists use the language of Einstein’s general relativity to explain the universe. This language connects the content of space-time to the bending and deformation of space-time, which then tells these contents how to interact. This is how we feel the force of gravity. In a cosmological context, this language connects the contents of the entire universe – black matter, dark energy, regular matter, radiation and everything in between – to its overall geometric form. For decades, astronomers have debated the nature of this shape: whether our universe is “flat” (meaning that imaginary parallel lines would stay parallel forever), “closed” (parallel lines would eventually intersect) or “Open” (these lines would differ).
Related: 8 ways to see Einstein’s theory of relativity in real life
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 (the flash of light emitted when our universe was only 380,000 years old), 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. There is also topology, this is how shapes can change while keeping the same geometric rules.
For example, take a flat sheet of paper. It’s obviously flat – the parallel lines stay parallel. Now take two edges of this paper and roll it into a cylinder. These parallel lines are always parallel: the cylinders are geometrically flat. Now take the opposite ends of the cylindrical paper and connect them. This gives the shape of a donut, which is also geometrically flat.
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 multi-connected, which means that one or more dimensions of our cosmos connect to each other.
Look towards the light
While a perfectly flat universe would extend to infinite, a flat universe with a multi-connection topology would have a 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 studied the cosmic diffuse background (CMB). At the end of the CMB, 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 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. CMB observations give us our best chance of seeing the footprints of a multiconnected universe.
Related: 5 reasons why we can live in a multiverse
The team specifically looked at perturbations – the sophisticated physics term for bumps and tremors – in the temperature of the CMB. If one or more dimensions of our universe reconnected with themselves, the disturbances could not be greater than the distance around those loops. They just wouldn’t do.
As Buchert explained to Live Science in an email, “In infinite space, temperature disturbances from CMB radiation exist at all scales. If, however, space is finite, then these lengths are missing. waves that are larger than the size of space. “
In other words: There would be a maximum size for the perturbations, which could reveal the topology of the universe.
Make the connection
CMB maps made with satellites such as NASA’s WMAP and ESA’s Planck have already seen an intriguing amount of large-scale missing disturbance. Buchert and his collaborators examined whether these missing disturbances could be due to a multi-connected universe. To do this, the team performed numerous computer simulations of what the CMB would look like if the universe were a three-dimensional torus, which is the mathematical name for a giant three-dimensional donut, where our cosmos is connected to. itself in all three dimensions. .
“So you have to do simulations in a given topology and compare with what is observed”, explains Buchert. “The properties of the observed fluctuations of the CMB then show ‘missing power’ at scales exceeding the size of the universe.” Missing power means that fluctuations in CMB are not present at these scales. This would imply that our universe is multi-connected and finite, at this size scale.
“We find a much better match with the observed fluctuations, compared to the standard cosmological model which is considered to be infinite,” he added.
“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 to our paper is clearly that the finite universe matches the observations as 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 was the best match for the CMB data. While this result technically means that you could travel in one direction and come back to where you started, you wouldn’t be able to accomplish this in reality. We live in an expanding universe, and on a large scale the universe is expanding at a rate faster than the speed of light, so you can never catch up and come full circle.
Buchert stressed that the results are still preliminary. Instrument effects could also explain the missing large-scale fluctuations.
Still, it’s fun to imagine living on the surface of a giant donut.
Originally posted on Live Science.
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