Friday, April 20, 2012

And now a mystery on a mystery

Nobody knows what dark matter is, though it seems pretty clear that it is there, even in our galaxy. But one group says there's not much around where the Sun is. They come up with limits on the local density that suggest that Earth-based dark matter searches have poor prospects.

The preprint explains their assumptions. They're looking at the velocities of stars above and below the galactic plane, and using those to estimate the mass density that would induce that distribution. They're not assuming spherical symmetry. I haven't studied the paper closely yet, but their result is that "Only the presence of a highly prolate (flattening q >2) DM halo can be reconciled with the observations". Click on that link and think what the rest of the galaxy looks like. Hmm. Interesting if true; almost as though dark matter doesn't like normal matter, or that the galactic black hole had something like a dark matter magnetic field or ...

DAMA won't like this result. They claim to have a signal with evidence for dark matter.

UPDATE: 4-May. I've talked to two people about this one. One said his team considered the star velocity measurements to be very tricky, and was dubious of the result. The other said that their group was non-committal except for one fellow (knowledgeable about such things) who said that the calculation model was "dumb." No explanation of why: maybe there are stability problems with the calculation. Or maybe not, I don't know the gentleman.


Texan99 said...

I remain confused about what it is we're detecting that makes us posit the existence of dark matter and dark energy. Well, I guess I have a vague notion what the deal is with dark matter: we assume there must be something out there that's exerting a gravitational force, even though we can't see it. That silly TV show I was making fun of the other day seemed to be saying that, at the same time, there's something exerting a repulsive force that we can't see. I assume a lot got lost in translation there, since there doesn't seem much point in positing both an invisible attractive force and an invisible repulsive one. Or are some areas shrinking too much and some expanding too much? If you're ever inspired to put up a post aimed very low on the scientific sophistication scale on this subject, I'd appreciate it.

james said...

The dark matter is fairly straightforward. We've known for a hundred years that stars in galaxies orbit too fast given the amount of matter we can see and estimate. If the mass of the galaxy were only what we see, the force on the stars would be less and their orbits would be much more leisurely.

In recent years the Einstein imaging around a couple of colliding galaxies was very clear: there was something that was bending light just the way matter does, except that all the rest of the two galaxies were pretty plainly somewhere else. The gas and stars had slowed down as they passed through each other, but "something" had barreled on through and was now separated from the rest of the galaxies.

So we don't know what it is, but there's something there that gravitates like ordinary matter but we can't see it and it doesn't interact much with ordinary matter.

Dark energy is a different kettle of fish--it is much more model-dependent. The idea is that with the current model, galaxies should be mostly receding from each other, but this rate should be slowing down (sort of the way that a ball slows down as it rises in the air). Instead, at earlier eras (very distant galaxies), they were receding more slowly than now: so something is speeding them up.

Something has to push to make this happen, and that means energy (force times distance), and because we can't see it anywhere else it is called "dark energy." Which means we haven't a clue what it is.

Einstein added a constant term to his famous equation about gravity in order to make it capable of describing a steady-state universe. He later said it was the biggest mistake of his life, since it was found that the universe was expanding.

But now it turns out that his kludge term can be used after all, to describe our accelerating sort of universe. The only problem is: what does such a term represent physically? It is nice to have equations that describe how things work, but we'd really like to know what they mean.

Yes, some things are pulled too tightly: dark matter around galaxies pulls the stars around rapidly. And some things are expanding too much: inter-galactic whoknowswhat that pushes galaxies apart. At least we think so.

Texan99 said...

Is dark matter generally considered to have something to do with black holes, or are those two completely different notions? As for dark energy, maybe the other galaxies just don't like us.

Action at a distance is more mysterious than people often give it credit for. Feynman remarked that people used to think that angels pushed the planets around in their orbits, but now we know better: the angels are invisible now, and they push at right angles to what we used to think.

james said...

Some researchers think dark matter consists of lots of tiny (early universe?) black holes, yes. How you look for them depends on how big they are.

I think you may be onto something: the Galactic Skunk explanation of expansion.