Scientific Method / Science & Exploration

Dark matter is unnecessary. Oh wait, just kidding.

Flip-flopping on the subject of dark matter.

In the three months that I've been writing about science, I've noticed a few things. Perhaps the biggest one is that everything is a constant state of flux. This is what makes science simultaneously great, yet maddening to write about. If you check Eurekalert for any appreciable amount of time, you'll notice that many of the headlines are contradictory, because studies contradict one another regularly. The trick is finding which ones are really worth reporting, and which are largely fluff. It's a bit easier for me when it comes to topics in my own field, and quite a bit harder when it comes to things like astrophysics.

On October 7, it seemed as though dark matter was here to stay. Then yesterday we find out that general relativity seems to solve the problem of galactic rotation without resorting to exotic dark matter. By using general relativity in place of Newtonian gravity, F I Cooperstock and S Tieu of the University of Victoria have found no reason to assume the existence of an exotic halo of dark matter (PDF fulltext).

We will demonstrate that via general relativity, the generating potentials producing the observed flattened galactic rotation curves are necessarily linked to the mass density distributions of the flattened disks, obviating any necessity for dark matter halos in the total galactic composition.

However this claim is not without its detractors. Korzynksi's response claims that the Cooperstock and Tieu model is unphysical. Namely, that it includes a "fudge factor" in a shell of matter which isn't observed and seems to be little more than a mathematical artifact... one which skews the results of Cooperstock and Tieu.

Indeed, merely resolving galactic rotation alone is insufficient to dispense with the need for dark matter. Motions of galaxies themselves within clusters, as well as diffuse hot intergalactic gas, also require far more mass than is observed. The most direct observations we have made of dark matter, using gravitational lensing of more distant galaxies, also confirm that there is far more mass than we can see there.

Most clusters have their component galaxies at such high velocities that their observed masses would have the cluster flying apart in just a few hundred million years and hence there would be no clusters remaining today. Intergalactic gas is so hot that it, too, would rapidly escape the cluster if not for the presence of much more mass than we can see. The Cooperstock/Tieu model addresses neither of these problems.

So it looks like dark matter is here to stay for the time being.

Hat Monster contributed to this writeup.

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