MOND fits the rotation curves of spiral galaxies well, but it was designed to do that. So do such data provide any test?

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MOND fits the rotation curves of spiral galaxies well, but it was designed to do that. So do such data provide any test?

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Yes. That MOND was “designed” to fit rotation curves and is therefore guaranteed to do so is a common misconception. Both MOND and dark matter were invented to explain the presence of mass discrepancies in astronomical data, especially the flat rotation curves of spiral galaxies. It is often asserted that MOND is ad hoc (and therefore bad), but this is also true of dark matter. Once invoked, dark matter can be distributed in any way necessary to explain just about anything. It is difficult to test, and does not constitute a falsifiable theory. In contrast, once a modified force law is specified, there is no freedom to adjust its predictions. MOND can not fit any arbitrary rotation curve: it is tied down by the observed mass distribution. However, it does fit real data. I find it remarkable that of the infinite variety of things rotation curves might plausibly do were they caused by a Newtonian disk + dark matter halo, they in fact do the one (and only one) thing allowed by MOND.

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