Continental Hotspot

One may wonder why scientists consider rotation curves evidence for MOND. After all wasn’t MOND designed to fit rotation curves? Well no, not quite. I’ll let the major review by Famaey & McGaugh on MOND do the talking for me:

“It is often wrongly stated that Milgrom’s formula was constructed in an ad hoc way in order to reproduce galaxy rotation curves, while this statement is only true of these two observations: (i) the asymptotic flatness of the rotation curves, and (ii) the slope of the baryonic Tully-Fisher relation (but note that, at the time, it was not clear at all that this slope would hold, nor that the Tully-Fisher relation would correlate with baryonic mass rather than luminosity, and even less clear that it would hold over orders of magnitude in mass). All the other successes of Milgrom’s formula related to the phenomenology of galaxy rotation curves were pure predictions of the formula made before the observational evidence. The predictions that are encapsulated in this simple formula can be thought of as sort of “Kepler-like laws” of galactic dynamics. These various laws only make sense once they are unified within their parent formula, exactly as Kepler’s laws only make sense once they are unified under Newton’s law.”

To put a picture to it:

The same can be found in this paper by Sanders & Verheijen:

It is sometimes argued that MOND is “designed” to fit rotation curves, so that it is no surprise that it works so well on this scale. It is true that MOND, in some sense, is designed to reproduce asymptotically flat rotation curves and a TF relation of the form L ∝ V 4. But it by no means evident that the variety of detailed shapes of rotation curves exhibited by the total sample of 80 galaxies so far considered could be so precisely reproduced by using MOND to calculate the radial force from the observed distribution of detectable matter. MOND works well throughout the entire galaxy, not just where the rotation attains its asymptotic constant value, and it was in no sense designed to do this. Using MOND Milgrom predicted that the discrepancy between the observed curve and the
Newtonian rotation curve should be small in regions of high surface-brightness, and that the discrepancy should be large in galaxies of low surface-brightness– even before such galaxies were discovered.