GOODBYE dark matter, hello dark energy. Two physicists in Britain say the
motion of stars in spiral galaxies such as the Milky Way can be explained by
energy floating in empty space. This puts paid to the idea that galaxies are
stuffed full of invisible dark matter.
Stars in the outer regions of spiral galaxies orbit so fast that, by rights,
they ought to fly off into intergalactic space. Until now, astronomers have
explained their motion by claiming that the stars are gripped by the gravity of
dark matter. Theories suggest that the dark matter, be it in the form of black
holes or planets or even undiscovered subatomic particles, forms a galactic
鈥渉alo鈥� 10 to 100 times as massive as all the stars put together.
Now Steven Whitehouse and George Kraniotis of London University鈥檚 Royal
Holloway College are challenging that view. They selected five nearby spiral
galaxies with well-known 鈥渞otation curves鈥� measurements of how the orbital speed
of stars varies with distance from the centre of the galaxy. They subtracted the
rotation curve that would have been expected if the gravity of the visible
material was the only force keeping the whirling stars in check.
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What Whitehouse and Kraniotis were left with was a rotation curve in which
the velocity increased linearly, in direct proportion to radial distance. 鈥淚t
was exactly the effect you would expect from dark energy,鈥� says Kraniotis. Two
years ago, a team led by Saul Perlmutter of Lawrence Berkeley Laboratory in
California discovered that empty space appeared to contain energy鈥攄ark
energy鈥攚hich was pushing galaxies apart and speeding up the expansion of the Universe
(New 杏吧原创, 3 April, p 28).
This energy, identified with the 鈥渃osmological constant鈥� in big bang models
of the Universe, is tiny and its effect is only apparent over very large scales.
鈥淥ddly enough, nobody considered whether it might have an effect on the scale of
individual galaxies,鈥� says Kraniotis.
Whitehouse and Kraniotis claim that the linear contribution to the rotation
curves of their five galaxies can only be explained by Perlmutter鈥檚 dark energy.
Since the dark energy in galaxies has an equivalent mass to dark matter, it
exerts a gravitational pull on the stars. 鈥淚t is this that prevents them from
flying off,鈥� says Kraniotis. 鈥淵ou don鈥檛 need to postulate large amounts of dark
尘补迟迟别谤.鈥�
While other researchers are cautious about the claim, they nevertheless
believe it is worth taking seriously. 鈥淚 think it is very important to seek
alternatives to the dark matter hypothesis in explanations of galactic rotation
curves,鈥� says Miloslav Svec of McGill University in Montreal. 鈥淯nobservable dark
matter in a completely transparent Universe is a very unsatisfying idea.鈥�
Kraniotis and Whitehouse hope to persuade astronomers to examine more spiral
galaxies and spur theorists to find an explanation for the dark energy. They
have submitted a paper on their work to the journal Monthly Notices of the
Royal Astronomical Society.