Simon’s Backup Weblog


Do the sums right and you don’t need a kludge…

Posted in Uncategorized by Simon Bisson on October 10, 2005

…and it looks like there’s now no need for dark matter to account for galactic rotation curves – if you use General Relativity rather than Newtonian gravity.

As the CERN Courier puts it:

This is because even for weak fields and slow speeds, well-known non-linearities change the character of the solution dramatically. The success of Newtonian mechanics in situations like our solar system can be traced to the fact that in this case the planets are basically “test particles”, which do not contribute significantly to the overall field. However, in a galaxy this approximation is not a good one – all the rotating matter is also the source of the gravitational field in which everything rotates.

You’ll find a full text copy of the paper in question here.

A galaxy is modelled as a stationary axially symmetric pressure-free fluid in general relativity. For the weak gravitational fields under consideration, the field equations and the equations of motion ultimately lead to one linear and one non-linear equation relating the angular velocity to the fluid density. It is shown that the rotation curves for the Milky Way, NGC 3031, NGC 3198 and NGC 7331 are consistent with the mass density distributions of the visible matter concentrated in flattened disks. Thus the need for a massive halo of exotic dark matter is removed. For these galaxies we determine the mass density for the luminous threshold as 10^{-21.75} kg.m$^{-3}.

Interesting stuff, and useful for anyone writing space opera…

Original pointer via Slashdot

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One Response to 'Do the sums right and you don’t need a kludge…'

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  1. purplecthulhu said,

    Interesting… I note that the paper is not yet accepted, as far as I can tell, so this might all go away. If it doesn’t then this is a major result. The technique has only been applied so far to a small number of galaxies, so it might not work more generally, and might only work on these because of some hidden (incorrect) assumptions.

    It should be noted that galactic halo dark matter only ever made up a tiny fraction of the dark matter on larger scales (about 30% of closure density). This cannot be produced by normal matter, and so has to be some kind of non-baryonic material. Then there’s the 70% made up by ‘dark energy’ needed to make the universe flat, as observed…


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