In 1983, Milgrom made a series of predictions about the kinematics of low surface brightness disk galaxies. To even discuss this at that time was fairly remarkable since it was widely held that such objects were extremely rare if they existed at all. However, acclereation was the crucial parameter for testing the force law (MOND) he had proposed to explain the flat rotation curves of spiral galaxies without dark matter. The lower the acceleration the better, and his force law indicated that for disk galaxies this meant low surface density. So he made the predictions tabulated below. Should low surface brightness galaxies ever be found and studied, they would provide strong tests of his predictions.

As time progressed, it became obvious that low surface brightness galaxies were actually rather common. The data necessary for testing MOND began to accumulate in a piecemeal fashion in the late 80s and early 90s due to the efforts of Skillman & Lake, Carignan, Casertano & van Gorkom, Sanders, Begeman & Broeils and others. Larger systematic studies directed specifically at low surface brightness galaxies include work by van der Hulst et al. and de Blok et al.

We did not collect these data because we were interested in testing MOND. Rather, we were interested in low surface brightness galaxies. We expected the data would tell us something about the mass distribution of these galaxies, which we hoped would tell us something about how they formed. I developed my own theory of galaxy formation within the paradigm of dark matter halos. Other scientists developed other such theories. We identified how these theories differed and which of their predictions were testable and different. Enough rotation curve data for low surface brightness galaxies should be able to distinguish between them.

The problem is, all of the dark matter theories fail, including my own. In my 1992 thesis, I predicted a shift in the correlation function and slowly rising rotation curves for low surface brightness galaxies. Both of these predictions came true, a big boost for my ego. I also predicted that low surface brightness galaxies should not fall on the same luminosity-circular velocity (Tully-Fisher) relation as high surface brightness galaxies. Much to my dismay, they do. Other theories have been careful to correctly postdict this fact, but inevitably get the shapes of the rotation curves wrong.

The problem is fundamental. Explaining the data with dark matter always leads to fine-tuning problems. Disk and halo parameters which should be unrelated are strongly correlated with zero intrinsic scatter. The ratio of dark to luminous matter varies randomly from place to place.

This bothers me. A lot. It was driving me crazy when I happened to hear Milgrom give a talk. In a few lines, he derived predictions for low surface brightness galaxies which succinctly described all the properties which made so little sense in the context of dark matter. At the time we had never met. I had only heard MOND discussed dismissively on rare occassions in terms more commonly encountered in toilet stalls.

So, I decided to look into it for myself. I went back and read his 1983 papers, discovering for the first time (in late 1994) his long list of detailed predictions for low surface brightness galaxies. This was great: something I could test with the data that had been accumulating for over a decade, and which I had worked very hard on myself from 1991. Still, MOND seemed so unlikely that I didn't actually bother to perform most of the tests for nearly a year.

Every prediction of MOND is born out by the data.
This can not be said of any other theory, whether it is based on cold dark matter, hot dark matter, baryonic dark matter, mixed dark matter, conformal Weyl gravity, or nonsymmetric gravity.

Here is a comparison of how Dark Matter and MOND fare in various observational tests

Observational TestDark MatterMOND
LSBG Tully-Fisher Relation??+
M/L-Surface Brightness Relation X +
Stellar Mass to Light Ratios NP +
Mass Surface Densities X +
Local M/L NP +
Transition Radii NP +
Characteristic Accelerations NP +
Disk-Halo Conspiracy X +
Rotation Curve Shapes X +
Rotation Curve Rate of Rise X +
Rotation Curve Fits X - NFW +
Thin LSB Disks X +
Disk Stability ?? ?
Dwarf Spheroidal Galaxies NP +
Giant Elliptical Galaxies X NT
Galaxy Clusters ? ??
Gravitational Lensing ? ??
Large Scale Structure ? ?
CMB Geometry + NT
CMB Peak Ratios +? +
Omega(matter) = 1 BBN
+ = prediction confirmed
X = prediction falsified*
? = uncertain but promising
?? = uncertain but not promising
NP = no prediction
NT = no test

* - Dark matter theories make few testable predictions, so the entries in this table often reflect rational prior expectations. Any one of these can be adjusted as needed to fit observations. It is however not possible to adjust all of them simultaneously in the way needed without causing contradictions and fine-tuning problems.

MOND does not [yet] explain everything.
Yet every prediction it has made has come true.
What more can we ask?

© 1997,2002 Stacy McGaugh
Table modified 10/2000 to include CMB results.
Table modified 09/2002 to include cluster results.
New table updated 11/2011.