Friday, 8 November 2013

Astrophysics Corner, Part 4 – Dark Matter

In The Witches’ Stones – Book One - Igniting the Blaze, Coryn and Steph bring up the topic of dark matter during their rescue of the amarto-sensitive Sarah Mckenzie:

“How’s it look, Steph,” Coryn asked the pilot.  “Are you picking up anything besides the beacon from the Beth?”

“We’re still a long way off.  All I get is the beacon”, Steph replied, with a shake of his head.  “But something else isn’t right.  I’m picking up slight gravitational anomalies in this system – I don’t recall them being there when we were here before.  Maybe they mean nothing but extremely dark asteroids, or some peculiar effect of dark matter, due to this being so far out on the fringe of the galaxy.  But….”  He sighed.

“You don’t like it, at all,” Coryn finished for him.

“Recent omega-hops can also leave traces like that.”  Steph sighed again.

So, just what is dark matter?  As of this time, nobody really knows, though there are, of course, theories.  What is known, is that the rotation rates of galaxies can’t be explained by the visible matter that we see in them.  The rotation velocity of stars nearer the edges of galaxies should be much slower than the rotation velocity of stars nearer the center, similar to planets in the solar system, where for example the Earth travels around the sun at about 4.7 km/second, while Neptune’s speed is only about 0.9 km/second.  But that doesn’t seem to be the case for galaxies, as indicated by Doppler shift measurements of their stars’  radial velocities.   In fact, the stars in the outer parts of galaxies move much faster than expected – their speeds aren’t much different than the inner stars.  So, there seems to be much more kinetic energy in galaxies than can be accounted for by the gravitational potential energy in their visible matter (these are connected by something called the viral theorem, which says that the kinetic energy ought to be about half the potential energy).

The best way to account for this discrepancy is by assuming that there is much more matter in galaxies than we can see, and that this dark matter is in a halo around the galaxy, rather than concentrated in a disk as most visible stars are.  Both theoretical density profiles and computer simulations support this conclusion.  There are also other observational phenomena that support the notion of dark matter, such as motions within galaxy clusters, some observed gravitational lensing and cosmological considerations (literally, the big bang theory).

So, while the evidence for dark matter’s existence is good, we don’t have much idea of what it actually is.  There are some who say it is just regular (baryonic) matter that is hard to see from any great distance  – black holes, neutron stars, cold brown dwarfs, rogue planets, asteroids and so forth.  But so far, searches for these objects (via gravitational micro-lensing for example) haven’t turned up nearly enough of them to account for the estimated missing mass.

That leaves new exotic particles, so called WIMPS (weakly interacting massive particles) which are thought to interact only weakly with regular matter, basically via gravity.  If they exist, there are probably millions going through your body every second.  But so far, there haven’t been compelling observations of these particles either.   Maybe in five or ten centuries (as in The Witches’ Stones series) we will know more about this astrophysical mystery.

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