This is an interesting case. Though I worry about the temporal evolution and current dynamical state of this system, this, together with the work of Aguirre et al., persuades me that the residual mass discrepancy in rich clusters is most probably real. This is a serious problem for MOND, and it is not obvious to me whether the most obvious candidate for extra mass in clusters, heavy neutrinos, will do the trick here.

Our colleagues in experimental physics can really help us out here. Detection of WIMPs would of course be most helpful. So would a mass measurement of a particle we actually know to exist - the neutrino. If mnu is near its lower limit (~0.02 eV), neutrinos would be inadequate to explain the residual mass discrepancy in rich clusters. While there could still be baryons unaccounted for there, such a situation would favor CDM and we would be stuck trying to understand how MONDian behavior arises from galaxy formation. On the other hand, a neutrino mass closer to the heavy end of the currently allowed range (~ 1 or 2 eV) would be about right for MOND in clusters and would cause insurmountable problems for structure formation in CDM.

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