91av

THE mysterious dark matter that holds galaxies together could be just another
facet of the equally bizarre dark energy that is pushing the Universe apart. So
says a team of physicists who’ve included several extra, invisible dimensions in
their model of the Universe.

Scientists have known for decades that galaxies must contain huge amounts of
invisible matter, because without the extra gravity it supplies, the stars would
fly apart into space. Then, three years ago, astronomers discovered signs that
the expansion of the Universe is speeding up. Some researchers think this may be
an illusion
(91av, 15 December, p 15)
but if it’s real, then some form of “dark energy” must be pushing space apart.

No one knows what dark matter and dark energy are. But astrophysicist Aharon
Davidson and his colleagues from Ben-Gurion University in Beersheba, Israel, are
suggesting that dark matter may just be another manifestation of dark energy.
“It’s two for the price of one,” Davidson says.

The team modelled the Universe as an entity with four dimensions of space and
time called a “brane”, embedded in a background space with five or more
dimensions. We perceive only the four dimensions, just as people in earlier
times thought the Earth was flat instead of a 3D sphere.

When the researchers tried to reconstruct Einstein’s theory of gravity on the
brane, they found that if the brane contains dark energy, then the equations
that describe it automatically contain a term that acts like dark
matter—even though there is no extra matter in the brane. That would mean
dark matter isn’t matter at all, just a kind of geometrical distortion.

But other physicists see serious flaws in the idea. If Davidson’s theory is
correct, then dark matter should appear to be spread evenly through the
Universe, says Joel Primack, an astrophysicist at the University of California,
Santa Cruz—and in that case it couldn’t serve as the glue that holds
galaxies together.

Yet the only reason dark matter was ever dreamed up was that it clumps
galaxies. Davidson realises this, and believes that eventually he will be able
to account for the clumpiness.