Friday, May 18, 2007

Dark matter 2

A continuation of Dark matter: what do we know.

So dark matter is mysterious. How do we learn more?

Good question. The trouble is it doesn't appear to interact much with laboratory equipment. If it came in small, dense chunks, laboratory evidence would have turned up by now. We've done quite a few experiments with enough sensitivity to detect tonne-scale chunks—ultracold calorimeters and acoustic resonators, pendulums, optical interferometers, and so forth. The excellent stability of atomic clocks rules out higher-order quantum effects caused by dark matter.

We likewise see very little evidence of it in solar space. The planets and assorted rubble appear to move to ancient Newton's laws, with an ocassional tiny correction for that upstart Einstein. Deep space radars don't show spacecraft bobbing around in the wake of tonne+ masses. (Although maybe nobody has looked closely enough—I'll ask my personal space radar expert.) The Pioneer 10 and Pioneer 11 probes have subtle anomalies in their deceleration curves, but I expect that is just a chemical or mechanical effect.

Even within our galaxy, dark matter's effects have not been clearly observed. We are getting lots of good data from distant galaxies. However I doubt this will illuminate the quantum physics of dark matter, assuming it has any. We just don't have the resolving power to see fine enough details, and are not likely to get it soon.

So where do we look? I favor looking for dynamical friction near our galaxy's gigantic central black hole (Sagittarius A*). It has both immense gravity and close-orbiting stars. The stars orbit so fast that we could just about afford a telescope to watch them move from week to week. If dark matter comes in chunks less than a light-year wide, that's where we will see it.

Nearby globular clusters are also a good place to look for dynamical effects. If anything smaller than a galaxy is big enough to have a dark matter density gradient, they will. (Globular clusters can reach hundreds of light years in size.) Depending on how dark matter works, they might even have captured some of it. At the same time they are close and bright enough that we can distinguish individual stars, which lets us do detailed measurements of the cluster's gravitational structure.

I can't come up with any plausible laboratory experiments for dark matter. High-energy particles do not seem to have any interaction, or the effect would show up in particle accelerators. It would probably also eliminate the high-energy cosmic rays that we observe, which helps rule that out. If big stars capture dark matter, then it might affect the neutrino brightness curve of a nearby supernova. We could measure that with a terrestrial instrument, but nearby supernovae are once-in-a-lifetime events.

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