We could be living in a small Universe where space is curved in on itself, rather like a football, say researchers in this week's Nature journal.
By Dr David Whitehouse
BBC News Online science editor
More precisely, we may inhabit a dodecahedral cosmos. It is, according to the scientists, the best way to account for the latest satellite observations.
Leonardo da Vinci had the right idea
Dodecahedrons, and similar shapes, have long fascinated mankind. Plato believed that the Universe was made up of them.
Leonardo da Vinci also studied them, as did the great astronomer Kepler, who thought the structure of the Solar System was based on geometrical shapes.
Further observations, especially from space probes yet to be launched, may settle the matter, and may at last reveal the hidden geometry of the Universe.
Ripples in the sky
The Cosmic Microwave Background (CMB) radiation, the "echo" of the Big Bang, contains a wealth of data about the early history of the Universe, as well as its large-scale structure.
If only we had precise enough observations of it to discriminate between competing ideas of what the Universe is like.
Will it expand forever? Is space infinite? Such profound questions may have their answers in the CMB.
The scientists were writing in Nature magazine
Specifically, the answers may be found in the ripples in the CMB - miniscule, regular, fluctuations in its strength over the sky.
Data from the US space agency's Wilkinson Microwave Anisotropy Probe (WMAP), which maps the CMB, suggests that at the very largest scales its temperature fluctuations seen across the sky are smaller than would be produced by an infinite Universe.
It seems the WMAP data shows the Universe is too small for large fluctuations to be seen in the microwave background radiation.
Positively curved space sections
Astronomers from the US and France suggest that space itself is not big enough to support such waves.
A small, cosmologically speaking, finite Universe, however, made of curved pentagons joined together into a sphere, would fit the observations.
Writing a commentary in Nature, George Ellis of the University of Cape Town, says we live in a Universe "with positively curved space sections and a non-standard topology".
The answer could be in the CMB
Indeed, a dodecahedral Universe, were you able to traverse it, would have some interesting properties.
If you went out to the edge of the dodecahedron, you would come back in through the opposite face.
More precise observations made by WMAP and by its successor, the Planck satellite, to be launched in 2007, will tell scientists if the cosmos does have such a shape, or if it is even stranger.