Friday, October 10, 2014
More Problems With Dark Matter
In a new preprint by Wu and Kroupa of October 8, 2014, accepted for publication in MNRAS and entitled "Galactic rotation curves, the baryon-to-dark-halo-mass relation and space-time scale invariance" these advocates for MOND and related theories in lieu of dark matter raise multiple new serious concerns about both cold dark matter and warm dark matter theories. They also use statistical methods applied to large sets of galactic rotation curve fits to determine which of the MOND interpolation formulas is more accurate.
* Empirically, the fit of galactic rotation curves to baryonic mass distributions is much more tight than in the galaxies produced by CDM and WDM simulations, despite the fact that the observational evidence has measurement errors which are absent in the simulation data.
The "standard" interpolation function in MOND models outperforms the "simplified" version, although both are very good fits. The WDM model has about twice as much scatter as the "simplified" MOND theoretical prediction line. The "tamed" CDM models that make unphysical assumptions to replicate reality are twice as bad as WDM simulations. And, the fully realistic CDM models have massive scatter despite the fact that the real world galaxies behave almost precisely as the standard interpolation function from MOND predicts.
A non-parameteric statistical test disfavors all of the CDM and WDM models and the simple MOND model when compared to the observational evidence by 99.99%, while finding it 80%-90% likely that "standard" MOND and another sophisticated MOND variant called SID are a fit to the data. This is particularly stunning given the amount of measurement error involved.
The link between baryonic matter distributions and apparent dark matter halo distributions is so tight with such a simple formula that it is very hard to see how any DM theory could achieve this result. The "scatter" of different possible dark mater halos for given baryonic matter distributions in DM theories are fundamental, built in, parts of the theory.
* All DM model simulations systemically over predict the amount of dark matter in galaxies of a given baryonic mass, while the amount of dark matter that would be necessary to reproduce the MOND results is right on the money.
* Kroupa acknowledges to important facts that one has to correct for when using MOND to predict galaxy dynamics: (1) the non-luminous interstellar gas to star mass ratio, and (2) the impact of fields from nearby objects (e.g. the impact of gravity from a central galaxy on the dynamics of a satellite galaxy) aka the "external field effect".
In general, more non-luminous interstellar gas simply requires an adjustment in the estimated amount of ordinary baryonic matter in a system.
Outside gravitational fields, in general, reduce the apparent amount of dark matter in the system in the eyes of a Newtonian observer.
Bonus trick: If you know the amount of luminous matter and interstellar gas in a system, and how fast it is rotating, you can use this slightly more sophisticated version of MOND to determine how far the influenced gravitational system is from the outside object created gravitational system.
* Dark matter halos inferred by Newtonian observers when MOND is used to govern dynamics have the correct isothermal shape the observational evidence implies, rather than the NFW distribution that CDM models naturally produce.
* About 70% of disk galaxies are "bulgeless" and MOND can easily reproduce these dynamics in the face of various galaxy formation scenarios. But, WDM and CDM models create far, far too few bulgeless galaxies, because those models tend to create serious bulges whenever DM halos or galaxies merge, which happens a lot. Some WDM/CDM simulations cheat to prevent relatively equal galaxy collisions like the Bullet cluster and get results more like real life, but there is no physical mechanism to cause that to happen and we know that big collisions are far more common than the "tame" simulations permit.
* These very accurate results are achieved with the usual one MOND parameter (plus external field effects and gas-star mass ratios in particular galaxies), and very easy math compared to many competing theories.
* It sounds like some progress has been made in physically motivating the equations and in reducing the amount by which apparent DM is underestimated at cluster scales from the "old days" although these continue to be problems.
I'll admit that I've definitely flirted with WDM models, which are definitely closer to observational evidence than CDM models that perform just dismally. But, this analysis makes it extremely hard to believe that there are really any dark matter particles out there at all.