Galactic Pancake Mystery Solved

mOoZik writes "According to the BBC, Astronomers have figured out why a series of small galaxies surrounding the Milky Way are distributed around it in the shape of a pancake. Theorists believed that the eleven dwarf galaxy companions should have a diffuse, spherical arrangement, but a University of Durham team used a supercomputer to show how the galaxies could take the pancake form without challenging cosmological theory."

Comments

Remind you of anything?

[Daxx_61]

This sounds a little like planetary formation. What if these 'halos' were really rings, due to some sort of spin in the original setup? Do they have to be a 3-dimensional halo? I am not an astronomer, but it sounds reasonable to me - could someone please explain this?

Silly scienticians!

[Flamekebab]

without challenging cosmological theory
Isn't it supposed to be about challenging current theories?

A Quick Question

[MankyD]

I've got a slightly more than average knowledge base on cosmology (though maybe not more than your average slashdotter). I've read a few books, but one thing I've never cleared up: Why do galaxies form in flat spirals and pancaks, and not in gravitationally stable spheres? Is there a simple reason I'm missing?

Re:A Quick Question

[Pedrito]

I'll grant that I'm no a math whiz, so I don't know the mathematics involved, but the reasons seem so obvious as to make me wonder how this could possibly challenge "cosmological theory." Most galaxies are pancake shaped. Most solar systems are pancake shaped. As you mentioned, nothing is really "gravitationally stable." I mean, if there's any movement at all, things are going to coalesce into a pancake shape eventually, unless all the movement is completely canceled out by opposite forces/movement, which is just statistically incredibly unlikely.

Re:A Quick Question

[Bastian]

So what you're saying is that you have to have the matter in the galaxy orbit (more or less) around a common axis, like in our solar system.

What would cause this to happen, instead of there being a bunch of randomly-oriented orbits?

(I suppose I am making the critical assumption that the distribution of matter immediately after the big bang was uniform, and I'm sure any cosmologist would be happy to smack me down over that, but I'll ask anyway.)

Re:A Quick Question

[ph43drus]

IAAP (I am a physicist), and the gravitationally stable spheres is a problem. That's what this study was looking at.

The bright, visible, normal matter forms into a disk in every galaxy we see. This cannot be explained with Newtonian gravity (or Einsteinian, for that matter). You see, when you just stick the normal matter in a simulation to check the evolution of a galaxy, it doesn't stay in the disk shape. To get the simulations to work (meaning, predict disk galaxies), you have to put a spherical halo of dark matter around the galaxy. With the dark matter there, it works perfectly.

The other option is that we're completely screwed up in terms of our beliefs about gravity. However, we haven't gotten any clean results using alternate formulations of gravity.

So the answer is, to the best of my knowledge, galaxies are disks because they either have (a) halos of dark matter or (b) our formulation of gravity is extremely flawed. Both of these answers are the current big embarrassments in physics. Both are bad answers. I haven't seen an alternate to gravity that works, and dark matter is a cop out.

From looking at the article, it looks like they've done a huge simulation taking the dark matter halo bit to the extreme and finding that it correctly predicts a flat arrangement for the satelite galaxies.

This is not proof of dark matter, it merely shows that it is the appropriate adjustment to gravity to explain some of the phenomena in the sky. What this actually means is that the correct answer for "why are galaxies disks?" will likely be the same answer for the pancake mystery. So, the authors did not actually solve the problem, they just showed that the current, in vogue kludge works for the pancake mystery too.

I'm, personally, quite sick of hearing it held up as an explanation as if it is the end all be all of fixing cosmology. It is a stupid cop-out with nothing to back it up. There is no experimental evidence for it. We have yet to find anything that could be dark matter (and there are people looking very hard to find it). Other theories don't have anything in them which could be dark matter (no particle theorist has come up with anything, and certainly not the high energy experimentalists). It is a big ol' strap of duct tape to fix a gaping hole in cosmology. There is nothing backing it up. Quite frankly, it's starting to sound like the Æther to me. And we all remember where that got us.

Jeff

Re:A Quick Question

[Bastian]

I am actually glad to hear that. . . I don't know enough about current physics to really make any sort of valid judgement on the theory. (I frequently get the feeling that I don't even understand the basic ideas behind relativity very well.) But the idea of dark matter always seemed to me to be rather fishy, sort of like the old crystal spheres that held the planets aloft.
I mean, they're both explanations for physical phenomena that were unexplainable under the model for how the universe worked at the time they were created, and they are both these sort of hand-wavey firmaments that don't seem to be something we can see or touch, but that we know to be there because they happen to magically make everything work. And dark energy feels like another hasty band-aid on top of the whole mess, like epicycles. My intuition is to think that the last few times people found problems with the theory, it turned out that the fundamental model was wrong, not that we needed to fill the universe with more crap (crystal spheres, aether, thunder gods, what have you), and to think that maybe it isn't so silly to try and extrapolate that pattern.

Re:A Quick Question

[Anonymous Coward]

I am a physicist too, and I suggest you review some astrophysics.

Dark matter is more than a "cop out". If dark matter only explained one thing, it might be a cop-out, but it simultaneously explains observations in cosmology, observations of galactic rotation curves, and large-scale structure formation -- all independent phenomena. This is a nontrivial accomplishment.

It is also not true that no particle theorists have come up with anything which could be dark matter. Quite the opposite -- they have too many theories. The leading candidates are axions (thought to be needed to solve problems with CP symmetry in QCD) and supersymmetric partners. Experiments are underway to try to detect these.

As for discs, spheres of matter with some net angular momentum will form into rotating discs without dark matter, once there's a region locally dense enough to collapse. But dark matter is needed to explain our observations of the specific way in which disc galaxies rotate. It is also needed to explain how the universe got clumpy enough for matter to start collapsing into galaxies in the first place.

Dark matter is the "new aether"? You could have said the same to people who proposed that an unseen body (Neptune) was perturbing the orbit of Uranus. But people looked, and they eventually found that there really was something there. And this was based on only one kind of observation, the orbit of Uranus, not cosmological observations plus galactic rotation observations plus galaxy cluster observations, all of which are explained by dark matter.

Re:A Quick Question

[nimblebrain]

Hear, hear.

I've been quite surprised at the influx of "odd" observations over the past few years; I certainly wasn't expecting local pancake structures.

You raise a pretty good point, though, on the structure of disks, large and small, in the first place.

Plasma physicists jump up and down that the in-vogue theories treat large-scale magnetic fields and currents as non-existent, as though charge must cancel out on the large scale, therefore it has no effect. Sometimes, they make a good point - some of the disk systems do resemble dynamos.

Some of the papers I've read in passing on "push" gravity theories estimate that the force of gravity is proportional to 1/d**2 locally, but trends to 1/d on the outsides of the galaxy. Otherwise, there's a lot of unseen matter there (and we haven't seen anything resembling the high-velocity clouds gathering on the edges of the galaxy)... or, alternately, we're ignoring a dynamo effect.

Or... etc. (Assuming we stop before postulating that angels sit on the edge fanning galaxies with their wings ;)

It's the bank of poorly-explained pieces that will lead us to our next big theoretical breakthrough (or revolution) - but it takes some special vigilance to keep track of what hasn't actually been explained properly, and what's been merely papered over.

Too many tweaks. They should have realized something was wrong sometime between inflation theory, and dark-energy-requiring ever-increasing-acceleration theory. Plenty of duct tape on things already :)

By the way, speaking of aether... ;)

I can understand the establishment position somewhat... it's either duct tape or anarchy. There's got to be a standard to measure against, but if the explanations start stretching thin, they need an exit strategy.

If that day comes, they will need to exit to something, though. What's out there that can explain the pancakes at multiple scales of the universe and other phenomena as well?

Perhaps they need to take a page out of other research and development, and apportion some funds to "blue sky" research.

The biggest dividends will come from research that's reviewed for logic, self-consistency and explanation of phenomena without regard to how well it fits into prior patterns. Pro-Ams and people in fields with more easily measureable results (applied sciences, for one) realize these benefits, but being in a field where so many assumptions have to be made to interpret the results in the first place make this next to impossible for the theoreticians to condone dissent.

Everybody's MMV :)

-- Ritchie

Re:In depth ...

[foobsr]

Asking why is metaphysical in (at) the end :)

Anyway, there they have some more readable info

CC.

The most irritating part of being an astronomer

[Simonetta]

The most irritating part of being an astronomer must be constantly defending the allocation of millions of dollars of public funds on whatever it is that they do.

A major new theory in regards to the shape and spacing of galaxies; what difference does it make to anyone?

Any bible-thumping corrupt two-bit schmuck of a politician can come up with a reason why the millions of dollars spent on astronomical research would better be directed towards one of his campaign contributers. And there are lots of those politicians nowdays.

So how actually do the astronomers keep all this money flowing their way? I would suspect that astronomy is 80% math and computer programming now instead of primarily star-gazing.

In the past, it wasn't this hard to justify the astronomers. Gods ruled the stars; kings ruled the people by the grace of the gods; astronomers interpreted the movement of the stars to convince the people that the gods still favored the king, and the king saw to it that the astronomers got plenty of money.

Astronomical research was important in navigation and agriculture. When to plant and which direction to steer when out-of-sight of land was critically important. But real extraterrestial knowledge came slowly. It was only four hundred years ago that Westerners realized that the Earth moved around the sun.

Today the most interesting about astronomy isn't theories about objects billions of miles away, it's how astronomers justify spending millions of dollars looking at objects billions of miles away.