Saturday, February 09, 2008

The Dark Stuff Conundrum Goes On

For every fact there is an infinity of hypotheses. ~Robert M. Pirsig

Another day, another rationalization for why we can't find dark energy.
If it seems that I talk a lot about dark energy and dark matter, it's because I do. As soon as scientists stop issuing their theories du jour about the stuff, I'll stop. Today's stab in the dark is brought to us by Dr. HongSheng Zhao of the University of St. Andrews. If you recall, a big chunk of our universe appears to be missing, around 96% of it according to the article (it used to be 75%, but we've apparently lost even more lately). Physicists, cosmologists, and assorted other scientists have decided that this means there is something out there we can't see, taste, smell, or otherwise detect. They started out calling this "dark matter".

Along the way, the theorists decided that dark matter didn't account for everything that was missing, so they decided there was something called "dark energy" out there, too. It is even more mysterious than dark matter. Dark matter has been indirectly detected by examining the motion of stars around distant galaxies. Dark energy, on the other hand, hasn't been detected at all. In fact, it's only inferred effects have been found to not be working exactly as they ought.

Well, says Dr. Zhao, your problem is that dark energy and dark matter are actually the same thing. Now, the perceptive reader might well say, "Well, duh. Einstein demonstrated energy-mass equivalence a century ago, as in E=mc2. Nothing to see here, move along." Well, that does not appear to be what Dr. Zhao means. He means that, in fact, they are manifestations of the same thing, like, say, water and ice. He calls it "dark fluid." Furthermore, says the doctor, dark energy has already shown itself by masquerading as dark matter. Because, after all, they are equivalent.

Yes, that sounds like a rather circular argument to me, too.

The problem is that we need are this dark whatever-it-is because of what is perceived as the expansion rate of the universe, a rate determined by examining the distances to distant objects and the speed at which they appear to be receding. Unfortunately, the apparent speed is dependent on the apparent distance, which is dependent on a lot of assumptions about how we can tell how far away something is.

And those assumptions have changed enormously over time.

So scientists are searching for something that could conceivably not be nearly as abundant as they currently think it is, should the reigning "standard candle" (the type I-A supernova) turn out not to be such a standard after all.

And then there's this. According to what passes for the standard model of the universe these days, all galaxies are surrounded by dark matter. As noted above, this has been determined by studying the motions of stars in galaxies and finding out that stars well out from the galactic nucleus are going faster than they should. In fact, the only thing that would keep such fast-moving stars in the galaxy would be some unseen amount of mass in a halo around the galaxy.

Unfortunately, there's a galaxy with the prosaic name NGC 4736. This stars in this galaxy behave exactly as one would expect if there were no such thing as a halo of dark matter. So what, you say? What's one exceptional galaxy in a universe full of them? That's the problem. The universe is full of galaxies; we've looked at an incomprehensibly small number of them. Yet we've managed to find one that violates the idea that galaxies must be surrounded by dark matter.

The standard model (this week) considers dark matter as an essential construct in galactic formation. It is not good to find a perfectly normal galaxy out there that doesn't have any. Of course, many scientists aren't buying in to this announcement, primarily because they have too much invested in the dark matter-energy model (it's still a little early for the fluid part to be included).

What remains to be seen, of course, is whether any other galaxies will be found that appear to have no dark matter. If some are found, the naysayers will have to start coming up with theories to explain where the dark stuff went. It may even lend credence to the dark fluid interpretation.

They'll be able to say it floated away.

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