SCIENCE: Is there really nothing out there?

Theories about the formation of galaxies may have to be revised now that new evidence suggests there are large holes in space devoid of matter, says Charles Arthur

Space really does consist of large amounts of nothing, according to a new set of results from the European Southern Observatory (ESO). An international team using telescopes in Chile, Europe, Australia and the US have discovered that there are voids up to 100 million light years across that contain no matter at all.

The finding - which suggests (as so many other astronomical calculations do) that there is less matter around than predicted - could mean that theories about the formation of galaxies will have to be revised.

Astronomers have known for years that there are areas of the sky where optical telescopes cannot find any visible matter such as galaxies, gas or stars. But there was always the suspicion that these voids might contain something which did not show up.

Proving that there's nothing out there is not a straightforward task: if the matter was present but too cool to emit radiation, it would not show up by any telescopic examination.

So the ESO team tried a different tack: they examined the movement of more than 2,000 well-known galaxies through space, using their international resources of optical and radio telescopes. The intention now was to use the inescapable property that any matter in the voids would have: gravity.

If there was something there, then it would exert a gravitational pull on those galaxies - just as the supercluster of galaxies known as the "Great Attractor", 150 million light years away, is doing for everything in our local Universe, including our own home galaxy, the Milky Way. By measuring the deflection of the movements of the galaxies, and correlating that with their positions and velocities, it would be possible to build a three-dimensional map of the distribution of matter in the "nearby Universe".

The conclusion that the team reached, after months of computer analysis and checking, was blunt: apart from the galaxies, there really is nothing there.

The picture here is part of the computer-generated view of the local Universe, covering an area 600 million light years across. At the centre is the Milky Way, though on the scale used in this image, its size - tens of thousands of light years - means it is only a speck.

The voids have diameters of about 100 to 200 million light years, and have minimal matter density. But this raises a number of questions about the formation of galaxies that have troubled some astronomers.

For example, it has been known for some time that the distribution of energy after the Big Bang was not even: the COBE (Cosmic Background Radiation) experiment, showing "ripples" in the background temperature of the Universe, indicated that. But there were no "holes" in the energy/matter distribution. For that reason, the astronomers have concluded, the holes that we can observe today must have formed later in the Universe's development. The conventional theory of galaxy formation is that stars began to form, and that their gravity pulled them together into galaxies. This would leave voids - as observed; but the average distribution of matter should be equal. There would be regions of the Universe that make up for the voids' low density by having larger numbers of galaxies.

Certainly, there are plenty of clusters of galaxies that do contain enormous numbers of stars - the Great Attractor (discovered in 1986) being one of them. But, say the ESO team, there is still not enough observed matter in those "superclusters" to compensate for the the emptiness of the newly discovered holes. The result: a big question mark over how galaxies are formed. Though the results are not being formally released until September - in the Journal of Astrophysical Letters - it seems astronomers have a new problem to grapple with in keeping track of the matter that should be present in the Universe - but which we can't find.