28 Feb. 2011

 

More evidence in favour of MOND?

 

Modified Newtonian dynamics (MOND) is the name given to a theory proposed in 1983 by the physicist Mordehai Milgrom, to get over some of the apparent shortcomings shown by Newtonian gravity when applied to very large systems, such as galaxies.
The best known problem is the fact that the rotation curves of spiral galaxies often remain very flat, even at very large radii, well beyond the stellar disk. The problem is that if material at larger and larger radii continues to orbit with a similar velocity, then the mass must continue to increase well beyond the visible disk.
To explain this, and other, anomalies, the concept of dark matter was introduced; a kind of invisible material able to gravitationally influence a galaxy's dynamics by virtue of its mass. The presence of dark matter in galaxies can therefore explain flat rotation curves, without making any modifications to Newton's law of gravity. Whether dark matter or a modification to Newtonian gravity is the correct description of nature is an ongoing debate.
The latest voice to be heard is that of Stacy McGaugh (University of Maryland), whose results are to be published in the March issue of Physical Review Letters. McGaugh studied a relation between the rotation velocity and mass of galaxies called the Tully-Fisher relation. The problem is that estimating the mass of a galaxy, dominated by stars, from the star light emitted is very difficult; it depends on the exact stellar population of the object.
McGaugh tried to avoid this problem by studying galaxies that are dominated by gas, not stars (see photo), to measure the total gas mass. Comparing the total masses and rotational velocities of 47 gas-rich galaxies, McGaugh found that they all followed the Tully-Fisher relation predicted by MOND more closely than that predicted by Newtonian gravity and dark matter.
This isn't the first time that MOND appears to beat dark matter, but if the mass determinations of McGaugh are reliable then it is a result that should provoke a long look at the role, and even existence, of dark matter. Nonetheless, we mustn't forget that on larger scales, like in galaxy clusters, dark matter appears to work rather better than MOND in explaining the dynamical behaviour. The debate certainly won't end here...

 

by Michele Ferrara & Marcel Clemens

credit: University of Maryland-Department of Astronomy, NASA