Against dark matter: globular clusters

One of the best 'sign-post' papers I ever read, which convinced me which way to go, was by Scarpa et al. (2006) (see reference below). There have been more conclusive ones published since, but this was the one I happened to read first.

To make dark matter fit general relativity to the oberved galaxy rotations you have to assume that it stays spread out in a halo around the galaxy, and therefore does not have structure on small scales. Scarpa et al. looked at globular clusters which are small areas within the Milky Way, where the stars are arranged slightly more densely than in surrounding areas. They found that the globular clusters behaved like little galaxies: whenever their internal accelerations dropped below a critical acceleration, a0, their dynamics became non-Newtonian. Their crucial point was that you can't use dark matter to explain the anomalous dynamics of tiny globular clusters since to fit it to galaxies you've already specified it must spread out: you can't have it both ways.

Scarpa et al. also pointed out that the external acceleration on the globular clusters due to the galaxy was larger than a0, but the anomalous behaviour still occured when the internal accelerations dropped below a0. I'm happy to say that this points away from MoND, and towards MiHsC which relies on the mutual accelerations of nearby matter.

I still have this glorious paper and I wrote on my copy: “Brilliant stuff! Tells me which way to go :)”. It seems to have been ignored by most of the astrophysical community, but it shows that the dark matter idea is unworkable.

Scarpa, R., G. Marconi and R. Gilmozzi, 2006. Globular clusters as a test for gravity in the weak acceleration regime. Arxiv: 0601581v1.

McCulloch, M.E., 2012. Testing quantised inertia (MiHsC) on galactic scales. A&SS, 342, 575. Arxiv: 1207.7007.

Globular clusters: crucial experiments?

I think that the way to approach physics is not to aim to invent beautiful theories, but to look for the data that shows the way. One of the sign-post papers that happened to influence me in this way was: Scarpa, Marconi and Gilmozzi, 2006, although I've read similarly clear ones by, eg: M. Milgrom, S. McGaugh & X. Hernandez.

Scarpa et al. make the point in their introduction that Newton's laws have never been tested at the tiny accelerations that exist at the edge of galaxies and that "deviations from Newtonian dynamics are always observed when, and only when, the gravitational acceleration falls below ~10^-10 m/s^2 as computed considering only baryons". They go on to state that it is agreed that dark matter cannot affect things on the small scale of globular clusters (dark matter haloes are large and smooth), so they looked at three globular clusters and showed that, indeed, below the critical acceleration (from the mass in the cluster) they deviate from Newton, just like larger galaxies. This suggests the presence of new physics rather than dark matter.

They also make the point that the external gravitational field, from the larger galaxy, acting on the globular clusters is above the critical acceleration, but the non-Newtonian behaviour is still seen. This points away from MoND where dynamics depend on the total acceleration, and points towards MiHsC where it is the local mutual accelerations that matter.

Scarpa et al. isn't perfect, only three globular clusters were analysed, but I'd like to express my appreciation to all those, like them, who risk unpopularity to base their conclusions on direct observations of nature, eg: M. Milgrom, S. McGaugh, X. Hernandez, J. Anderson, M. Tajmar, CERN (especially the OPERA team).... Observing new regimes is hugely risky, but is the only way to get to new physics.