Imagine a universe in which you can direct a spaceship in one direction and eventually return to where you started from. If our universe were small in size, then such movements would be possible, and physicists could measure its volume.

"We could say: We now know the size of the universe," astrophysicist Thomas Buchert of the University of Lyon, Center for Astrophysical Research in France, told Live Science.

Studying light from the earliest Universe, Buchert and a group of astrophysicists came to the conclusion that our cosmos can be multiply connected, that is, space is closed on itself in all three dimensions, like a three-dimensional donut.

Such a universe would be finite, and, according to their results, our entire cosmos could only be three to four times larger than the limits of the observable universe, which is about 45 billion light-years away.

Physicists use the language of Einstein's general theory of relativity to explain the universe. This language associates the contents of space-time with the bends and bends of space-time, which then tell those 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 - dark matter, dark energy, ordinary matter, radiation and everything else - with its general geometric shape.

For decades, astronomers have debated the nature of this shape: whether our universe is "flat" (meaning that imaginary parallel lines will always remain parallel), "closed" (parallel lines will eventually intersect), or "open" (these lines will diverge).

This geometry of the universe dictates its fate. Flat and open universes will continue to expand forever, while a closed universe will eventually collapse on its own.

Numerous observations, especially of the cosmic microwave background (a flash of light that occurred when our universe was only 380,000 years old), have firmly established that we live in a flat universe. Parallel lines remain parallel and our universe will continue to expand.

But form is not just geometry. There is also topology, that is, how the shape can change while maintaining the same geometric rules.

For example, let's take a flat piece of paper. It is obviously flat - parallel lines remain parallel. Now take the two edges of this paper and roll it into a cylinder. These parallel lines are still parallel: The cylinders are geometrically flat. Now take the opposite ends of the cylindrical paper and join them together. The result is a donut shape that is also geometrically flat.

While our measurements of the content and shape of the universe tell us about its geometry - it's flat - they don't tell us about topology. They don't tell us if our universe is multiply connected, which means that one or more dimensions of our cosmos are connected to each other.

While a perfectly flat universe would extend to infinity, a flat universe with a multiply connected topology would have a finite size. If we could somehow determine that one or more dimensions are twisting into themselves, then we would know that the Universe is finite in this dimension. We could then use these observations to measure the total volume of the universe.

A group of astrophysicists from the University of Ulm in Germany and the University of Lyon in France have drawn attention to the cosmic microwave background (CMB). When the CMB was received, our universe was a million times smaller than it is today, and therefore, if our universe was truly multi-connected, then the likelihood that it would fold with itself within the observable boundaries of the cosmos was much higher.

Today, due to the expansion of the universe, it is much more likely that the folding occurs on a scale beyond the observed boundaries, and therefore it will be much more difficult to detect the folding. Observing the MGB gives us the best chance of seeing the imprints of a multiply connected universe.

The research team paid particular attention to perturbations - a fancy physical term for bumps and vibrations - in the CMB temperature. If one or several dimensions in our Universe are connected to each other, then the perturbations cannot be greater than the distance around these loops. They just wouldn't fit.

As Buchert explained, "in infinite space, perturbations in the CMB radiation temperature exist at all scales. However, if space is finite, then there are no wavelengths that are greater than the size of space."

In other words: There is a maximum perturbation size that can reveal the topology of the Universe.

Already, an intriguing number of large-scale absent disturbances have already been found on CSBM maps compiled by satellites such as NASA's WMAP and ESA's Planck. Buchert and his colleagues investigated whether these absent perturbations could be caused by a multiply connected universe.

To do this, the team ran many computer simulations of what the MDB would look like if the universe were a tri-torus - the mathematical name for a giant three-dimensional donut in which our cosmos is connected to itself in all three dimensions.

“Therefore, we have to simulate in this topology and compare with what is observed,” explained Buchert. "The properties of the observed CMB fluctuations show 'missing power' at scales larger than the size of the Universe."

The lack of power means that fluctuations in the CMB are not present at these scales. This would mean that our Universe is multiply connected and finite on such scales.

"We find a much better fit for the observed fluctuations compared to the standard cosmological model, which is considered infinite," he added.

“We can vary the size of space and repeat this analysis. As a result, we get the optimal size of the universe that best fits with CMB observations. The answer from our work is unambiguous: a finite universe better matches observations than an infinite model. We can say: Now we know the size Of the Universe ".

The team found that a multiply connected universe about three to four times larger than our observed bubble best fits the CMB data. While this result technically means that you can travel in one direction and end up where you started, in reality you cannot.

We live in an expanding universe, and on a large scale, the universe is expanding faster than the speed of light, so you can never catch up and complete the cycle.