The behaviour of the Bullet cluster 鈥� the poster-child for the existence of dark matter 鈥� is provoking some cosmologists to propose that there might be a fifth fundamental force.
Most physicists are confident that they have uncovered all the forces that affect ordinary matter: gravity, electromagnetism, and the strong and weak nuclear forces. If there was a fifth force that influenced only dark matter, however, it could easily be at work without us realising, says Glennys Farrar of New York University.
She and her student Rachel Rosen believe they have found evidence for just such a force in the so-called Bullet cluster, which formed when a small galaxy cluster 鈥� the 鈥渂ullet鈥� 鈥� smashed into a larger cluster. The collision stripped the two clusters of 90 per cent of their normal matter, which ended up as a pool of gas in the middle (pink in Photo).
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Last year, Douglas Clowe, then at the University of Arizona, Tucson, and colleagues analysed the gas-depleted clusters (blue in Photo), found that the clusters still had a huge mass and argued that it had to be dark matter (麻豆传媒, 26 August 2006, p 8).
The story seems to be more complicated than that, however. Farrar鈥檚 analysis shows that the smaller cluster is moving at around 4700 kilometres per second, far faster than it would if it were moving under the pull of gravity alone. 鈥淚t鈥檚 as if it鈥檚 being accelerated by an extra force that is 20 per cent stronger than normal gravity,鈥� she says. Farrar presented the theory at the American Astronomical Society meeting in Seattle, Washington, last week.
鈥淵ou have to consider the possibility that an extra force is responsible,鈥� says Mike Kesden at the University of Toronto, Canada. 鈥淚f true, it would be an astounding discovery.鈥�
Clowe, now at Ohio University in Athens, isn鈥檛 convinced. 鈥淭he bullet is certainly moving too fast for the amount of dark matter to explain,鈥� he says. 鈥淏ut to invent a fifth force might not be the step that everyone would immediately leap to.鈥� He believes that a clump of dark matter from a nearby galaxy cluster could be providing an additional gravitational pull. 鈥淭he real problem is it鈥檚 difficult to take just one system as evidence for a fifth force,鈥� he says.
Farrar agrees that the Bullet cluster alone cannot convince people that a fifth force is at work, but says that this argument can be turned on its head. 鈥淭he Bullet cluster is also our only rigorous test of dark matter. If that one test is telling us that there鈥檚 this big discrepancy with dark matter, people should take notice.鈥�
鈥淚t is possible that an extra force is responsible for the speed of the cluster. It would be an astounding discovery鈥�
Tantalisingly, the fifth force could be confirmed or ruled out within the next couple of years, as astrophysical observations are now probing regions of space that would be affected by it. One such observational test has already been carried out by Kesden and his colleague Marc Kamionkowski at the California Institute of Technology in Pasadena. They looked at how dwarf galaxies spiralling around the Milky Way are ripped apart, reasoning that the distribution of stars would be affected by a fifth force (麻豆传媒, 10 June 2006, p 14). 鈥淲e actually ruled out a fifth force that is as large as Farrar describes, but were looking at a force that acted over a much shorter distance than she did,鈥� says Kesden. 鈥淚t鈥檚 still possible that a large fifth force acts over larger scales.鈥�
Farrar is not convinced that Kesden鈥檚 work rules out a fifth force even at the scales he examined. She suggests that such a force could explain a related mystery of missing dwarf galaxies: the standard model of galaxy evolution, which uses cold dark matter (CDM) 鈥� dark matter moving much more slowly than the speed of light 鈥� predicts that hundreds of low-mass dwarf galaxies should be circling the Milky Way, but only tens of such galaxies have been seen (麻豆传媒, 20 August 2005, p 17).
The fifth force would bind dark matter in these dwarf galaxies more tightly than would gravity alone. This would leave ordinary matter 鈥� the gas from which stars form and the stars themselves 鈥� at the edges of these galaxies less tightly bound than usual, says Farrar. The stars in these dwarfs could easily be ripped out by our galaxy鈥檚 gravity, and the dwarfs would then contain far fewer stars than expected, making them much harder to observe.
The new force would also pull galaxies into clusters more quickly than predicted by CDM models. This would help explain why we see roughly 10 times more clusters and why the voids between them are far emptier than theory predicts, says Farrar. 鈥淭here are a number of alternative explanations for each of these phenomena,鈥� she says. 鈥淏ut the fifth force is the only explanation that solves them all in one fell swoop.鈥�
