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[1102.3913] A Novel Test of the Modified Newtonian Dynamics

Posted: March 08 2011
by Maciej Bilicki
This paper presents an analysis of the relation between flat rotation velocities and baryonic masses of 47 gas-rich galaxies. It claims that the observations support the MOND prediction and cannot be agreed with [tex]\Lambda[/tex]CDM. I have however some doubts on that analysis. First, the baryonic mass is not an observable. It is estimated with the use of the distance to the galaxy. However, in this letter it is not provided how the distances were obtained. We however all know that almost all extragalactic distance estimates have a big scatter. This surely affects baryonic mass estimates, as does the uncertainty on the fraction of gas mass w.r.t. stellar mass even in gas-rich galaxies. For these reasons I cannot fully trust the errorbars, hence the goodness of fit (one can however see that the data could be fitted with something else than a power law, too).
Moreover, the line in Fig. 2 that is supposed to present the [tex]\Lambda[/tex]CDM prediction is plotted assuming that the baryon fraction f_b in galaxies is the same as the universal cosmic one. This surely is not true and it varies from one galaxy to another. So at least the normalization of the [tex]\Lambda[/tex]CDM curve presented in the paper is somewhat arbitrary (the author mentions this fact). This generally means that there is not a single prediction of [tex]\Lambda[/tex]CDM in this matter. So this is then not a good test to compare other models with.
There is however a cosmological prediction for which the two models can be tested and the author mentions it. It is the CMB angular power spectrum which MOND cannot reproduce. I am a bit surprised to read that on one hand 47 galaxies may falsify one theory ([tex]\Lambda[/tex]CDM - see additionally the press release ... cleID=2352), but on the other hand, Nobel-prize data from the whole sky gathered by sophisticated instruments cannot falsify the other (MOND). Not to mention other scales at which MOND fails, like galaxy clusters, including the famous Bullet Cluster. Saying in the paper that both theories fare equally badly for that cluster suggests that we shouldn't bother with any of them, should we?
Last but not least, I don't understand why such an analysis is presented in a purely physical journal although the work is purely observational. And the astronomical issues, as the distance estimates that I mentioned, are just skipped.