preprint by McCaugh. This paper chooses some "gassy galaxies" which are consistent with MOND - Modified Newtonian Dynamics - while requiring a smaller number of parameters than the standard cosmological model with dark matter.
When I looked at the graphs that are claimed to contain all the support, I was terribly unimpressed. It's just some slope in a noisy graph that MOND is argued to predict pretty well - a linear curve. There are many other things, to be enumerated below, where MOND doesn't work and they contain many more details than a slope of a would-be linear function for a subclass of galaxies.
I think it's obvious that the author has been cherry-picking in order to support a particular viewpoint. And there's no real evidence against the standard model of cosmology in the paper - except for the observation that this model doesn't offer a trivial way to calculate some power law relating the masses of stars and gas in a galaxy.
So I fully subscribe to every sentence written by Sean Carroll. It's not a paper that can revolutionize cosmology and the journalists didn't do a good job in reporting it.
Carroll enumerates a couple of huge reasons why MOND is not a good explanation of the data that are otherwise explained by dark matter:
- MOND predicts a very bad WMAP spectral curve of the microwave background; the agreement of the dark matter model is spectacular, and includes many perfectly reproduced "bumps" which clearly carry much more information than the noisy graphs in McCaugh's paper; with MOND, the agreement becomes very weak
- dark matter and visible matter don't always share the center; the Bullet Cluster shows that these two chunks of mass may get totally separated; at least all the prevailing versions of MOND make such a separation of "body and soul" impossible, so they're really falsified by this and similar observations
- MOND doesn't fit clusters; the scaling laws are OK for most galaxies but at a different distance scales, the relationship just fails; most MOND proponents accept that dark matter is needed to explain the motion of clusters - which makes MOND's importance highly questionable because dark matter may explain the galaxies' behavior, too
- even galaxies may fail; certain small and hard-to-observe galaxies, "dwarf spheroidals", are predicted incorrectly
- MOND realizations are given by unnatural equations; the addition of vectors and scalar fields, and their bizarre non-polynomial interactions, surely looks awkward. From the particle physics perspectives, the interactions are totally unnatural, and the very fact that one needs to add new field content whose only goal is to modify gravity is very contrived or "man-made"
I would add one more point, and that is the criticism of the MOND proponents' main motivation:
- it is not natural to assume that all forms of matter and energy must be visible by our eyes, or by electromagnetic waves that we learned to love
The MOND proponents primarily build on the basic philosophy that there's something wrong with a theory that contains elements that are invisible by electromagnetic waves - the main gadget we got used to when observing the real world. I think that this basic philosophical notion - which is the true driving force behind all their model building - is just deeply flawed.
God or Nature didn't guarantee that our eyes He or She created have to be omni-potent. Quite on the contrary, they're almost certainly not. There is no reason - no theorem and no logical argument - that would imply that everything must be visible by the waves we like to use when watching things on the Earth.
After all, our eyes only see a very small interval of possible frequencies of the electromagnetic waves. But even if we add additional frequencies that may be observed by our telescopes, there's no reason why we should see "everything". We only see the objects that emit the light, and there's absolutely no reason to think that all objects should emit light, or even the same amount of light.
The people who dislike "dark things" seem to think that they're returning us to the era of angels, ghosts, and superstitions. But they don't. Dark matter and dark energy are standard parts of physics because they do have observable consequences. The word "observable" doesn't have to be associated with light or electromagnetic waves.
During its cosmological evolution, Nature just chose what is the right percentage of the things that will be observable 13.7 billion years after the Big Bang. And the answer is that only the gay 4% visible matter is observable. You don't like that the heterosexual 96% majority of the mass in the Universe prefers to hide its intimate places from the lights of your camera? Well, move into another, less puritan Universe. ;-)