#### The universe would be finite. A team of astrophysicists has deduced that the cosmos could be closed on itself in three dimensions, as if it were a donut.

Our universe could be finite and have the shape of a kind of giant three-dimensional donut. The discovery is by a team of astrophysicists of which Thomas Buchert, from the University of Lyon, is a member. Examining the light of the early universe scientists have deduced that our cosmos can be closed on itself in all three dimensions as if it were a donut. It would then be finite and, according to the results of the analysis, could only be about three to four times larger than the limits of the observable universe, about 45 billion light years away.

#### The donut shape of the universe

To explain the universe, physicists use Einstein's general relativity, which, in a cosmological context, links the contents of the entire cosmos - dark matter, dark energy, regular matter, radiation and everything else - to its overall geometric shape. For decades, astronomers have debated the nature of our universe by asking whether it was flat, closed, or open. Flat, open universes would continue to expand forever, while a closed universe would eventually collapse in on itself.

Numerous observations, particularly of the cosmic microwave background, or the flash of light released when our universe was only 380,000 years old, have established that we live in a flat universe. Parallel lines remain parallel and our universe will continue to expand. But geometry must be joined by topology which is how shapes can change while maintaining the same geometric rules. For example if we take a piece of paper lying down, it has a flat shape, but we can roll it up into a cylinder shape. Cylinders are also geometrically flat and can be connected, by joining the opposite ends, to form a donut.

Measurements of the universe, so far, have focused on geometry while leaving out tolopogy and so do not tell us if our universe is multiconnected. In case it is, we may know that it is finite and we may even get to measure its total volume. Research by astrophysicists at the University of Ulm in Germany and the University of Lyon in France examined the cosmic microwave background and, specifically, perturbations in its temperature. If one or more dimensions in our universe were to reconnect with themselves, the perturbations could not be larger than the distance around those circuits.

From maps made with satellites such as NASA's WMAP and ESA's Planck, an interesting amount of large-scale missing perturbations were observed. Buchart and his team then examined whether those missing perturbations could be due to a multiconnected universe and found that a multiconnected universe, about three to four times larger than our observable space, best matched the cosmic microwave background data. Although the results are still preliminary, instrumental effects could explain the missing fluctuations on a large scale and would suggest that our universe may be shaped like a giant doughnut.

In the meantime, an older-than-universe spiral galaxy has been photographed in Chile, which could explain how star systems, such as the Milky Way, formed.

Stefania Bernardini