Monday, March 21, 2011

Test of MOND on Gas-Rich Galaxies

This made the news a while back, and it is now finally in print.

The people working on Modified Newtonian Dynamics got a boost recently when an analysis of gas-rich galaxies produced a very good agreement with MOND's prediction, better than the Dark Matter model.

McGaugh collects from the literature a sample of 47 gas-rich galaxies, for which recent 21 cm spectral line observations provide reliable estimates of both their atomic gas masses (which are combined with stellar population model masses to produce Mb) and their asymptotically flat rotation velocities vf. These data show an impressive match with the MOND prediction of Mb∝vf4, and also agree well with the acceleration parameter a0 required to fit the rotation curves of star-dominated galaxies. McGaugh further claims that the data have no scatter about the MOND prediction beyond measurement errors. This statement appears to be premature since statistical incompleteness, large distance uncertainties (many of the galaxy masses rely on estimated distances only), and other observational realities do not seem to have been taken into account. These will introduce biases into the observed scaling and have been shown to reduce the observed scatter (e.g., Ref. [13]). Such biases will most likely not significantly change the observed correlation, but they cast doubt on the exact details, particularly the interpretation of the scatter.

As one can read in that paragraph (and in the article in the link), there are still a lot of skepticism and issues here in turning this into a slam-dunk success. The other part is this:

McGaugh’s result adds a new facet to the argument that MOND is better at explaining galaxies than standard cosmology. However, as McGaugh admits, MOND cannot compete with ΛCDM as a full cosmological theory. Attempts to generalize MOND into a fully relativistic theory of gravity abound, but even the most promising ones (e.g., tensor-vector-scalar, or TeVeS [14]) struggle to interpret the combination of large-scale observations of the Universe that ΛCDM explains so well. The tuning required in MOND, most notably to explain the dynamics of galaxy clusters, is more severe than that faced by ΛCDM to match galaxy rotation curves. We know that standard but poorly understood baryonic physics plays an important role in shaping the properties of galaxies in ΛCDM. Reconciling MOND with galaxy clusters, on the other hand, requires invoking significant amounts of the missing matter, which MOND was conceived to avoid in the first place.

So already, there are problems with this, but it certainly is an encouraging sliver of hope for MOND.


1 comment:

S.D said...

But I think the MOND can hardly stand up to the so-called "Bullet Cluster".