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.

Zz.

Einstein’s Theory of Gravity and the Problem of Missing Mass

There is a very good review of the current progress in the theory of Modified Newtonian Dynamics (MOND) in the current issue of Science[1].

Abstract: The observed matter in the universe accounts for just 5% of the observed gravity. A possible explanation is that Newton’s and Einstein’s theories of gravity fail where gravity is either weak or enhanced. The modified theory of Newtonian dynamics (MOND) reproduces, without dark matter, spiral-galaxy orbital motions and the relation between luminosity and rotation in galaxies, although not in clusters. Recent extensions of Einstein’s theory are theoretically more complete. They inevitably include dark fields that seed structure growth, and they may explain recent weak lensing data. However, the presence of dark fields reduces calculability and comes at the expense of the original MOND premise, that the matter we see is the sole source of gravity. Observational tests of the relic radiation, weak lensing, and the growth of structure may distinguish modified gravity from dark matter.

Whether you buy into it or not, this will at least get you up to speed on the strengths and weaknesses of this theory.

Zz.

[1] P.G. Ferreira1 and G.D. Starkman, Science v.326, p.812 (2009).

More Problems With MOND

There are more challenges to the Modified Newtonian Dynamics (MOND) theory.

[Note: a few of the links may require free registration to gain full access to the article]

First of course was the observation from the collision in the Bullet cluster from a year ago. The result from this strongly favors the presence of Dark Matter. In fact, many astrophysicists even proclaimed that MOND is dead after this observation. Of course, things don't die off that easily (and that fast) in physics. The MOND advocates came back with a scenario that could be consistent with the Bullet cluster observation without any need for Dark Matter. This was met with major skepticism by others in the field, as can be read at the end of that article in the link.

Now comes another publication that might seriously challenge MOND's analysis of the Bullet cluster.

I. Ferreras et al., "Necessity of Dark Matter in Modified Newtonian Dynamics within Galactic Scales", Phys. Rev. Lett. v.100, p.031302 (2008).

Abstract: To test modified Newtonian dynamics (MOND) on galactic scales, we study six strong gravitational lensing early-type galaxies from the CASTLES sample. Comparing the total mass (from lensing) with the stellar mass content (from a comparison of photometry and stellar population synthesis), we conclude that strong gravitational lensing on galactic scales requires a significant amount of dark matter, even within MOND. On such scales a 2 eV neutrino cannot explain the excess of matter in contrast with recent claims to explain the lensing data of the bullet cluster. The presence of dark matter is detected in regions with a higher acceleration than the characteristic MOND scale of ~10^-10 m/s^2. This is a serious challenge to MOND unless lensing is qualitatively different [possibly to be developed within a covariant, such as Tensor-Vector-Scalar (TeVeS), theory].

This lensing issue was brought up in one of the article that I linked above, so obviously, this is a serious issue with MOND. Let's see if they can dig out of this one.

Zz.