The arXiv brought an early Xmas gift in the form of a measurement of the velocity dispersion of Crater 2. Crater 2 is an extremely diffuse dwarf satellite of the Milky Way. Upon its discovery, I realized there was an opportunity to predict its velocity dispersion based on the reported photometry. The fact that it is very large (half light radius a bit over 1 kpc) and relatively far from the Milky Way (120 kpc) make it a unique and critical case. I will expand on that in another post, or you could read the paper. But for now:

The predicted velocity dispersion is σ = 2.1 +0.9/-0.6 km/s.

This prediction appeared in press in advance of the measurement (ApJ, 832, L8). The uncertainty reflects the uncertainty in the mass-to-light ratio.

The measured velocity dispersion is σ = 2.7 ± 0.3 km/s

as reported by Caldwell et al.

Isn’t that how science is suppose to work? Make the prediction first? Not just scramble to explain it after the fact?


11 thoughts on “Crater 2: prediction verified.

  1. Could you please explain a bit more, for those of us unfamiliar with the details, how good/bad a match your prediction is with the measurement?


    1. Well, yes, of course.
      What I meant was that “2.1 +0.9/-0.6” seems a very large range for such a small number. Superficially it looks like a +43%/-29%
      If the possible range of measured values goes from, say, 0.5 to 30.0, then your prediction is impressive. However if the possible range of measured values is 1.0 to 3.0, then your prediction is not as impressive because it includes a very large subset of the possible values.
      I’m not familiar enough with Dwarf Galaxies to know what the possible range of results would be.


  2. The predicted +/- allows for a factor of two uncertainty in the stellar mass-to-light ratio, so this is the full plausible range in MOND. Five is right out. For dark matter, the allowed range is 0.5 to infinity, and beyond. See the next post for a more detailed discussion of what we plausibly expect in LCDM.


    1. Yes, I saw in the next post that the expected range, based on previous observations, was roughly from 10 km/s to 25 km/s. A prediction of ~2 km/s would seem outragious and foolish to lCDM advocates. This triumph of your use of MoND, while not really a surprise to me, should be a wake up call to most astronomers. Of course it won’t be….


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