The universe is not round, say scientists - it is shaped like a trumpet

At first they thought it was flat. Then that it was shaped like a football. But now, scientists believe the universe could be shaped like a flat-sided trumpet.

At first they thought it was flat. Then that it was shaped like a football. But now, scientists believe the universe could be shaped like a flat-sided trumpet.

That would lead to strange effects in some parts of the universe, where time and light would be so curved that you could see the back of your own head. Also, a long-held theory about the universe - that it looks much the same anywhere - would have to be abandoned. And finally, the universe would be finite, rather than extending in every direction forever.

The new shape, predicted by careful mathematical modelling to fit with known astronomical data, would have the universe stretched out into a long funnel, flaring into a bell-like shape at one end. The thin end would be infinitely long - but so narrow that it would have a finite volume.

The "bell" of the trumpet would also be peculiar: it would flare out, but a spaceship that could somehow navigate towards the open end would eventually find itself flying back along the "outside surface" of the bell.

The research by a team of German physicists led by Frank Steiner at the University of Ulm is reported today in New Scientist magazine.

Their theory uses a complex mathematical model called a "Picard topology", named after a mathematician rather than the Star Trek character. It would mean the universe has a finite volume, although you would not be aware of its "edges"; they would seem to be part of the rest of space.

The concept follows a close examination of the dramatic data produced by Nasa, the US space agency, in 1996, which showed that the very faint microwave radiation left over from the Big Bang 13 billion years ago was not evenly spread throughout the cosmos. This lack of evenness - called "anisotropy" - suggested that large-scale models of the shape of the universe, previously thought to be a flat expanse of space time, were wrong.

Ever since then, cosmologists have puzzled over what the data from the Cosmic Background Explorer (Cobe) spacecraft could tell them about the space-time "shape" of the universe. One rival group proposed last year that the universe was shaped instead like a football. But that would have left a particular pattern of background radiation - which has not been found by further experiments.

In the "flat" space of conventional cosmology, small "blobs" - hot and cold spots - in background radiation should be round. But observations show that instead they are ellipses. The curve of a horn-shaped universe can explain this, says Professor Steiner.

The "space" in any small part of the "horn" is saddle-shaped, like a Pringles potato crisp. Such a "negatively curved" space would act as a warped lens, distorting the image around the blobs so they look elliptical to astronomers.

Another property of a "trumpet" universe would be that large blobs more than about 60 degrees across would be absent. So far, scientists have also found this to be the case.

But if the team at Ulm is right, scientists will have to abandon one of the fundamental tenets of cosmology - that all parts of the cosmos are about the same. Holger Then, a member of Professor Steiner's team, told New Scientist : "If one happens to find oneself a long way up the narrow end of the horn, things indeed look very strange, with two very small dimensions."

At an extreme enough point, you would be able to see the back of your own head. Over the next year or so, astronomers will test whether large blobs really are lacking in cosmic microwave background radiation and small ones really are elliptical.