Galactic rings: the smoking gun for modified gravity

For a while now, them star gazers have been banging heads over the nature o’ gravity.

Here’s the problem: when you look at the way galaxies are a-spinnin and a-spiralin, there just ain’t enough gravity to hold em together. So either there is some hidden mass putting its gravitational shoulder to the wheel: the so-called dark matter. Or there’s something wrong with the 1/r^2 rule at large distances and gravity is stronger than we think at these scales.

In 1981, Mordehai “Mouse” Milgrom modified gravity to cope with this problem in a theory called MOdified Newtonian Dynamics or MOND. It says that gravity is a little stronger over galactic distances and this is what stops spinning galaxies from tearin’ themselves apart.

But the theory makes some strange predictions too. For example, it says that some galaxies should look as if they are surrounded by a ring of dark matter.

Earlier this year, the Hubble Space telescope found exactly that — a ring of dark matter around the galaxy cluster Cl 0024+17, which lies 5 billion light years from Earth. What astrogawpers actually saw were distorted images of galaxies behind Cl 0024+17 caused by the mass of Cl 0024+17 bendin’ light as it passed by, a phenomenon called gravitational lensing. The astrobods used these distortions to infer the distribution of mass around Cl 0024+17 and concluded it must have a ring of dark matter even though they ain’t actually seen a ring of any kind.

So Mouse Milgrom is a-shoutin and a-ravin from the roof tops: this is exactly what his theory predicts. Could it be enough to prove MOND once and for all?

Probably not. Gravitational lensing data is notoriously difficult to interpret so the work is by no means done and dusted. But even if it’s right, one swallow, don’t a summer make. He’s gonna need a lot more evidence before his ideas become mainstream.

Ref: arxiv.org/abs/0709.2561 : Rings and Shells of “Dark Matter” as MOND Artifacts

One Response to “Galactic rings: the smoking gun for modified gravity”

  1. Zephir says:

    The same phenomena (space-ring surrounding massive bodies) was published recently even at the case of probes, which are revolving Earth in equatorial plane:

    http://arxiv.org/abs/astro-ph/0608087
    http://space.newscientist.com/article/dn13411-earths-rotation-may-account-for-wayward-spacecraft.html

    We can consider it as an analogy of rotating black holes at large scales.