Dark Matter Exists!

Monday, August 21, 2006
Many people have heard that at the end of the 19th century there were small anomalies in the observed location of the planet Mercury. It was apparently not quite where it should have been, had Newton's theories of gravitation been entirely correct. Einstein proceeded to modify Newton'ts laws somewhat and it was discovered that with these modifications the observed locations of Mercury were now spot on. Score one for Einstein.

Toward the end of the 20th century, small gravitational anomalies have been once again observed, this time in the behavior of very large objects in space. Rotating galaxies, for example, are not quite rotating as they should if the Newton-Einstein formulas are entirely correct. Likewise, orbital velocities of galaxies in clusters are not as they should be. Two possible explanations have been proposed, either that there is a better theory of gravity to be had (though nobody seems to have a good idea of what this might be) or that there is a lot of matter, type unknown, out there which would account for the anomalies observed without any change in the (extremely accurate and well-tested) Theory of Relativity. This is the so-called dark matter, the idea being that there's some sort of unknown stuff out there which we can only observe at the cosmic scale, because for reasons unknown it doesn't show up in anything around us we know of.

Now the evidence is apparently in, as a collection of astronomers using a constellation of our most advanced telescopes has announced direct observation of dark matter. Your tax dollars at work.

Don't worry, there's still the even more perplexing mystery of dark energy to ponder....


chuck said...

More links here with a short discussion. Also, more on the Poincare conjecture. Apparently Perelman is now living with his mother, taking unemployment payments of $50/mo. Why does this remind me of Godel, who starved himself to death for fear of eating poisoned food after his wife died, or of Paul Erdos who never owned a home but wandered about staying with one mathematician or another?

truepeers said...

Hmm, no doubt I don't fully get it, but after reading the paper abstract, it seems that the paper title - "direct empirical observation" - misleads. They observe and then rely on deductive reasoning to argue that only the existence of dark matter can explain what they have observed.

terrye said...

All I know about this, I heard on Star Trek.

But it is fascinating how there is always more to know.

Like Merlin said in the Once and Future King, there is always something to learn.

chuck said...


Here is a good explanation of the of the observations. The bottom line is that dark matter doesn't interact with itself or ordinary matter except through gravity. Thus the importance of the colliding galaxies: the ordinary matter piled up like a car crash while the dark matter passed on through and continued on its merry way. That's why gravitation lensing had to be used to find it, as the orbiting stars gambit fails when the stars are stripped out.

truepeers said...

Chuck, thanks for the link; it elucidates the dark matter p.o.v. clearly. But it doesn't convince me there couldn't be other explanations for what is observed, as yet unimaginable. I have no way of judging the plausibility of the existence of dark matter, but it seems clear these guys have not observed it, merely inferred it as the only plausible explanation for observations at present. I got a laugh out of the self-promoting commenter who said the observations could all be the working of a Matrioshka Brain But really, who knows?

chuck said...

...merely inferred it as the only plausible explanation for observations at present.

Well, they have inferred the existence of something that is curving space. Matter, in other words. The question is whether or not it is ordinary matter or something new. The sweeping out effects of the colliding galaxies seems to settle that part of the question. But this all reminds me of one of my favorite book titles Experimental Astrophysics. Sure, and we should all whip up a few new stars whenever we have an new question, eh?

Anyhoo, no one knows what this dark matter is. It is a bit like the neutrino whose existence was inferred in 1930 but not observed until 1956.