Original URL: http://www.theregister.co.uk/2007/11/10/dwarf_stars/

Dwarfs threaten Kepler and Newton

Dark matters

By Anton Wylie

Posted in Science, 10th November 2007 07:02 GMT

It seems you don't need a disk to thumb your nose at celestial mechanics - if you're big enough to be a galaxy.

Recent investigations of the velocities of stars in dwarf galaxies have reinforced the need for explanatory dark matter to account for their motion, according to Professor Mario Mateo and and Dr Matthew Walker of the Universities of Michigan and Cambridge respectively, who collaborated on research announced recently.

For some time, astronomers used dark matter to explain how the disks of spiral galaxies rotate. But Mateo and Walker's investigation of dwarf galaxies in the immediate vicinity of the Milky Way, which has more then doubled the data available on the motions of stars in dwarf galaxies, indicates that what goes for big spirals also goes for little dwarfs. Neither seem to want to respect the dynamical laws of Kepler and Newton.

"These galaxies show a problem right from the centre," Mateo is reported as saying. "The velocity doesn't get smaller. It just stays the same, which is eerie."

Kepler, in the early days of modern science, first formulated a mathematical relationship between the time taken for a planet to orbit our sun and the remoteness of the planet from the sun. It was therefore assumed that stars in the disks of spiral galaxies should behave in a comparable way, stars further from the galactic core taking longer to revolve than stars close to the core. Or so it was thought, until astronomers started measuring.

It was then discovered that stars in spirals' disks have a more or less constant angular velocity throughout the disk, contradicting their expectation. The behaviour is more what one would expect of a solid disk, rather than of billions of stars whose separations vastly exceed their size.

The significance of Mateo and Walker's work is in apparently demonstrating that the non-Keplerian movement of stars in galaxies is not necessarily connected with the morphology of the galaxy.

The difficult with accommodating the facts of stellar kinetics with classical mechanics is that the forces expected of the visible parts of the galaxies do not by themselves seem to fully influence the motions of the bodies. Hence the postulate of an invisible "substance" which corrects the universe to our expectations.

"Our research shows that dwarf galaxies are utterly dominated by dark matter, so long as Newtonian gravity adequately describes these systems," Walker is quoted as saying.

Mateo, on the other hand, prefers a model of dark matter that modifies the force of gravity itself according to the accelerations within the system. So while both researchers subscribe to their respective dark materials, they find themselves in disagreement about how these operate clandestinely.

So Matteo finds himself in the camp of currently less favoured accounts of dark matter. But he might secretly enjoy the predicament into which his findings dump those espousing a cosmology dominated by the activity of electrically energised plasmas at all distance scales.

The outre model of spiral galaxies as cosmic Faraday motors, espoused by some theorists, is in danger of burning. For the prima facie appeal of that model, the visual and dynamical analogies between a spiral galaxy and an electricity meter becomes that much harder to maintain when the rotor component of the system is missing, as looks to be the case for the dwarf galaxies studied.

The advantage of Mateo's Modified Newtonian Dynamics is that it adjusts the concept of what is evidently already there to account for the observed facts. It requires merely a clarification of the nature (i.e. the properties) of gravity itself.

The alternative speculation, that the presently obscure dark matter will in due course come to light (in a manner of speaking) in the guise of undetected or currently unknown elementary particles, faces what appears to be a much sterner program of describing the distribution and density of these substances of the future to account for the perceived motions of organised stellar populations.

Or, to put it differently, the former needs to venture into the field of metaphysics for salvation, while the latter must take its chances juggling multitudes of vectors within computer model hyperspace. It is, to say the very least, an invidious choice.

While this explanatory face-off has yet to be resolved, Mateo it appears is not above drawing on tactics more commonly associated with prize fighters outside the ring, and more recently with football managers away from the touchlines, in the psychological battle for advantage.

Matteo alleges that Neptune was once "dark matter". But no, this is not apparently a claim entailing alchemical transformations.

Space Daily notes:

"Before the term was even coined, astronomers predicted its [Neptune's] existence based on an anomaly in the orbit of Neptune's neighbor Uranus. They knew just where to look for Neptune."

Say that last sentence again, Prof? ®