Global Deep-MOND Parameter as a Theory Discriminant
  Different formulations of MOND predict somewhat different rotation curves for
the same mass distribution. Here I consider a global attribute of the rotation
curve that might provide a convenient discriminant between theories when
applied to isolated, pure-disk galaxies that are everywhere deep in the MOND
regime. This parameter is Q=<V^2>/V0^2, where <V^2> is the mean squared
rotational speed of the galaxy, and V0 is the asymptotic (constant) rotational
speed. The comparison between the observed and predicted values of Q is
oblivious to the distance, the inclination, the mass, and the size of the disk,
and to the form of the interpolating function. For the known modified-gravity
theories Q is predicted to be a universal constant (independent of the mass
distribution in the disk): Q=2/3. The predicted Q value for modified-inertia
theories does depend on the mass distribution. However, surprisingly, I find
here that it varies only little among a very wide range of mass distributions,
Q=0.73+-0.01. While the difference between the theories amounts to only about 5
percent in the predicted RMS velocity, a good enough sample of galaxies may
provide the first discerning test between the two classes of theories.
