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.

Against Dark Matter - If the shoe fits..

Well done to the people behind the Alpha Magnetic Spectrometer (AMS) on the Space Station that has just reported some results. It is always good to collect data in new regimes. Whatever they find, they will find something new. They do not seem to have anything significant to say about where their extra positrons are coming from though.

The AMS was designed to search for dark matter: a terrible theory in my view, because it is not falsifiable and it looks to me like an attempt to support a theory (general relativity), that was devised before we knew about galaxy rotations, by adding invisible matter ad hoc to galaxies to force the galactic mass and rotation data to fit general relativity (dark matter is so flexible that you can fit many models to the data). There was a similar argument in Galileo's time. Aristotle had said heavier objects fall faster, but Galileo noticed in a hailstorm that big and small hailstones fell together and later on he showed, using balls and inclined planes, why this must be. His contemporaries, to play safe and support Aristotle, said "Aha! The big ones must have fallen from a greater height". This could have been right, but the clue was in the ad hoc way they had to set up the initial height of the various sizes of hail: there was no reason for it. There is also no reason why dark matter should be in a diffuse halo around a galaxy.

If dark matter is too vague to be falsified, all one can do is suggest less arbitrary alternatives.

Milgrom's empirical MoND (Modified Newtonian Dynamics) is a little less arbitrary. MoNDians suggest that we tweak either gravity or inertia at low accelerations (so there are two varieties). They do not have a specific mechanism in mind, but at least they have less wriggle room than dark matter. MoND has a formula (well, a few choices), but it does still have one fitting parameter: a0. By varying a0 MoND can be made to fit galaxy rotation curves very well, but is a bit off with clusters.

Then there is MiHsC, which has a very specific physical model of inertia. MiHsC predicts galaxy rotations and galaxy cluster dynamics within the uncertainty in the observations, with comparable accuracy to MoND, and without any adjustable parameters at all, see: http://arxiv.org/abs/1207.7007 (Journal: Astro. & Space Sci., 342, 2, 575-578). See also the figure in my blog entry here.

An analogy: You go into a shoe shop. The owner proudly produces his revered Einstein Shoe (nice curves, no sock needed). He measures your foot, concludes with regret that your foot is too big for the shoe and suggests chopping most of your foot off. That's the dark matter way. A MoND shoe would fit with some adjustment at the cobblers. A MiHsC shoe would fit with no adjustments, but it's an unknown brand..

Quantised inertia on galactic scales

I'm pleased that my paper (Testing quantised inertia on galactic scales) has just been accepted by the journal Astrophysics and Space Science. I used the data published by McGaugh, Schombert, de Blok and Zagursky (2006) listing the baryonic mass and circular velocity of dwarf galaxies, spiral galaxies and galaxy clusters. In the paper I show that MiHsC (quantised inertia) predicts the observed circular velocity of all these structures (within the uncertainties) from their baryonic mass only (ie: without dark matter).

In more detail: MiHsC overestimates the circular velocity of galaxies (but still agrees within the error bars) whereas MOND agrees better with galaxies, if you set its adustable parameter (a0) correctly. On the other hand MiHsC agrees better with large galaxy clusters, where MoND has always had trouble, and a very important advantage of MiHsC is that it doesn't have any adjustable parameters. The paper is on the arxiv here.