30 September 2009 by Stuart Clark
Dark matter is either weirder than we thought or does not exist at all, a new study suggests.
A galaxy is supposed to sit at the heart of a giant cloud of dark matter and interact with it through gravity alone. The dark matter originally provided enough attraction for the galaxy to form and now keeps it rotating. But observations are not bearing out this simple picture.
Since dark matter does not radiate light, astronomers infer its distribution by looking at how a galaxy’s gas and stars are moving. Previous studies have suggested that dark matter must be uniformly distributed within a galaxy’s central region – a confounding result since the dark matter’s gravity should make it progressively denser towards a galaxy’s centre.
Now, the tale has taken a deeper turn into the unknown, thanks to an analysis of the normal matter at the centres of 28 galaxies of all shapes and sizes. The study shows that there is always five times more dark matter than normal matter where the dark matter density has dropped to one-quarter of its central value.
The finding goes against expectations because the ratio of dark to normal matter should depend on the galaxy’s history – for example, whether it has merged with another galaxy or remained isolated during its entire existence. Mergers should skew the ratio of dark to normal matter on an individual basis.
“There is absolutely no rule in physics that explains these results,” says study co-author Hong Sheng Zhao of the University of St Andrews in the UK.
The authors suggest there may be an undiscovered force of nature working between the dark matter and the normal matter, since gravity alone cannot maintain this constant ratio.
Alternatively, they say our understanding of gravity may need modification to eliminate the need for dark matter entirely. Some existing theories, such as MOND (modified Newtonian dynamics) attempt to do just that, suggesting that gravity does not fade away as quickly as current theories predict.
But Mark Wilkinson of the University of Leicester, who was not involved in the study, urges caution in interpreting the results.
“Although this clearly shows much more interplay between normal and dark matter than expected, it is too early to say exactly what this means,” he told New Scientist.
Wilkinson speculates that normal matter may wield more of an influence on dark matter than expected. He says supernova explosions might somehow be able to blow dark matter out of the galaxies’ central regions.
Journal reference: Nature (vol 461, p 627)
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