# Galaxy defy Kepler? True or bull?

1. Nov 17, 2006

### MonstersFromTheId

I was watching a show about black holes at the centers of galaxies.

And (perhaps I misheard), but there was a one line comment made to the effect that stars in a spiral galaxy don't always move around the center of a spiral galaxy in accordance with Kepler's laws (which, by that, I assume they meant, with each star carving out equal areas in equal time, i.e. closer stars go around more quickly, father stars more slowly).

Instead, the stars, (if I heard right), circle at a nearly even rate regardless of distance from the center, like someone drew the galaxy's spiral on a piece of paper, and started turning the paper.

1) Is that even close to true (i.e, I misheard, or, a hard working writer that was a bit over his or her head trying to write the dialog for this subject, blew a line).

2) If that is true, how is it accounted for?

2. Nov 17, 2006

### Garth

Dark Matter - there are a host of threads about the subject.

Garth

3. Nov 17, 2006

### EL

Here is something relevant:
http://en.wikipedia.org/wiki/Rotation_curves

4. Nov 17, 2006

### Chronos

Dang it Garth, beat me to it. You did, however, neatly leave open the 'what is dark matter' door! That remains a fascinating issue. I blame it all on deuterium. The stuff is promiscuous and blithely disregards most laws of nucleosynthesis, IMO.

5. Nov 17, 2006

### MonstersFromTheId

Fascinating. Absolutely fascinating.

I gotta say, however, that the whole idea of "Dark Matter" and "Dark Energy" just,.. well they make me itch.

I'm not saying I don't buy it.

It just bugs me that like 98% of the universe apparently consists of "something" (we have absolutely not the SLIGHTEST clue as to what the "something" is), that can't be directly detected, or examined, touched, smelled, kicked, caught, heard or seen, or even seen to interact, with - well - anything at all, (other than on galactic scales).

It bothers me. A lot.

Try this simple exercise. Take THE most scholarly paper you can think of on Dark Matter, or Dark Energy. Every place the terms "Dark Energy" or "Dark Matter" show up, cross them out, and substitute the term "Magic". Now re-read the paper and YOU tell ME that the term "Magic" doesn't seem to fit a bit too snugly for comfort.

Again, I want to be clear here. I'm not saying there's no such thing as Dark Matter or Dark Energy. The observed data is just too overwhelming to toss the baby out with the bath water. But the properties of Dark Matter and Dark Energy, ya gotta admit, come about as close to having their toes hanging right over the edge of the line between the real and the mystical as you could reasonably expect to get.

(Then again, if you think about it, piezoelectric behavior would certainly SOUND like a crock if we weren't all runnin' around with quartz watches on our wrists. Try to imagine how well a paper proposing the possible existence of a substance like quartz would seem if the only place quartz could possibly form was out past the orbit of Pluto. Makes you wonder what kinds of truly bizarre materials are out there to discover and revolutionize our thinking, just in our own back yard, our own solar system, let alone 70,000 light years from here.)

Last edited: Nov 17, 2006
6. Nov 17, 2006

### Janus

Staff Emeritus
Dark matter is simply matter that does not emit electromagnetic radiation. This does not mean that it can not be detected otherwise.

It bothers me. A lot.

Try this simple exercise. Take THE most scholarly paper you can think of on Dark Matter, or Dark Energy. Every place the terms "Dark Energy" or "Dark Matter" show up, cross them out, and substitute the term "Magic". Now re-read the paper and YOU tell ME that the term "Magic" doesn't seem to fit a bit too snugly for comfort.

[/QUOTE]

Except that we already know that some types of "magic" do exist. Neutrinos are an example of known dark matter. They do not interact with the electromagnetic spectrum at all. It is not unreasonable to assume that there is other matter out there of the same ilk.

7. Nov 17, 2006

### Garth

You have to distinguish between Dark Matter and Dark Energy.

There is a lot of observational evidence to say DM exists, galaxy rotation curves, galaxy cluster dynamics, cluster gravitational lensing of more distant quasars, large scale structure formation in the early universe and cosmological constraints. It seems that DM consists of about 27% of the critical density (critical density: $\Omega = 1$.)

However what is it? The standard model of Big Bang Nucleosynthesis can only allow about $\Omega = 0.04$ maximum baryonic matter and so the rest $\Omega = 0.23$ has to be exotic non-baryonic matter that has not been identified in the laboratory.

Note we see only about $\Omega = 0.003$ in the form of gas and stars so the standard model has about 10X the amount of hidden (dark) ordinary baryonic matter over the observed matter. Change the model (to the linear expanding model) and the rest could be baryonic as well.

The requirement for Dark Energy on the other hand is more problematic. It is needed to make up the rest of the critical, or near critical density, that Inflation and CMB analysis requires. However this is model dependent, and that might be questionned.

It is also required to provide 'cosmic acceleration' to explain why distant Type Ia supernovae are fainter than expected, however this interpretation requires such SN to be of standard luminosity over cosmological time and this too can be questionned.

If these assumptions on which the standard $\Lambda$CDM model is based are wrong then, until exotic DM and DE are discovered in the laboratory, the inclusion of DE and exotic non-baryonic DM may be seen to be an example of modern cosmology adding extra 'epicycles' to 'save the appearances', just as the Ptolemaic theory did in Copernicus' day.

Garth

Last edited: Nov 17, 2006
8. Nov 17, 2006

### Office_Shredder

Staff Emeritus
Would kepler apply here? His laws are based (both experimentally and theoretically) on the object in question interacting with the central force generator. In a galaxy, it's not a bunch of things orbitting a black hole. It's a bunch of things orbitting a black hole while pulling on each other. So it's not surprising the math works out differently (at least from what I can see. I may be wrong)

9. Nov 17, 2006

Staff Emeritus

Well of course when we say Kepler here, we really mean Newton. Kepler gave no causal account of his three laws, but Newton showed that (in modern language) they were the consequence of conservation of angular momentum and an inverse square central force, all this of course in the context of his three laws of acceleration and momentum.

Newton's theory is still a highly accurate predictor and has been applied to the case of multiple sources you describe, although only in approximation, yet there are general theorems, like the virial theorem, which would be obeyed by matter gravitating a la Newton, but which are violated by the stars in the galaxies, if we assume they are the only gravitating sources. But if you allow a cloud of gravitating matter that we can't see directly, then the behavior IS Newtonian.

The recent astronomical picture of the dark matter (false colored by its graviational lensing effect) being stripped off a small galaxy as it moves through a visible resisting medium, has greatly supported the "Dark matter is matter" view in contrast to the "So-called dark matter is really a modification to the law of gravity" view.

10. Nov 17, 2006

### Office_Shredder

Staff Emeritus
Well, I know we're talking about newton, but they're called kepler's laws.

Is this online somewhere? I'd like to see the picture

11. Nov 17, 2006

### Garth

You may in the end be wrong but your line of thought is not so far off the mark. (Apart from using Kepler rather than Newton as selfAdjoint pointed out). In the Cooperstock and Tieu preprint General Relativity Resolves Galactic Rotation Without Exotic Dark Matter that was their thesis, i.e. as galactic rotation is determined by a gravitational field from matter that itself is in motion the non-linear effects of GR cannot be ignored and as a consequence they thought DM is not required.

However others disagreed and rejected their approach. Nevertheless, that main point needs to be kept in mind when modelling galaxy rotation, even if DM is also required.

For the 'Bullet Cluster' evidence for DM, complete with a picture, see NASA Finds Direct Proof of Dark Matter

Garth

Last edited: Nov 17, 2006
12. Nov 17, 2006

Staff Emeritus
Thanks for the links, Garth. Wouldn't you agree that whatever the fate of the Cooperstock-Thieu thesis, GR effects should be computed as the normal "default background" in professional discussions of DM hereforeward?

BTW how would you categorize reasonable speculation with hopes of detectability as constituting partial understanding? I am thinking of axions here, but other hypotheses are out there too,

13. Nov 17, 2006

### Garth

By 'GR effects' do you mean the non-linear ones? A quick back-of-the-envelope calculation would suggest they should not be significant in a typical galaxy, however a full analysis is required to be sure of that.

Without its non-linear effects GR reduces down in the first approximation to the Newtonian model that is almost universally used.
The DM particle(s) may be discovered at any time, maybe the Higgs Particle has at last been discovered.

As for which of the possible zoo of DM particles is most plausible, I am no expert. However I do make the observation that only when we do eventually identify the DM particle(s) in the laboratory, measure their properties and find those properties concordant with that required by astrophysical and cosmological observations that we will know what we are talking about. Before that time we need to keep an open mind.

Of course by its (hypothetical) nature the DM particle is going to be difficult to detect, but I do not find that a reasonable excuse, it is like explaining the disappearance of all my odd socks by the hypothesis of a shy-odd-sock-pixie. The reason you never see one is because they are too shy and so difficult to detect. The hypothesis might be correct but I do not find it convincing, until, that is, one is actually captured.

Garht

Last edited: Nov 17, 2006
14. Nov 17, 2006

### SpaceTiger

Staff Emeritus
That's a very silly analogy. Unlike "shy-odd-sock-pixies", dark matter can be motivated by other physical theories, including many models of supersymmetry. Many of these particles are predicted to be beyond detectability, so the fact that we haven't detected them cannot be used as evidence against their existence.

I agree that, until the dark matter is directly detected, we shouldn't rule out alternative explanations. However, the case is getting quite strong (as you mentioned in reference to the bullet cluster) and I think the astronomical community is seeing a decreasing need to examine the alternatives.

As for non-linear general relativistic effects, I don't think anyone would suggest that they shouldn't be explored, but such calculations are considerably less transparent and require a great deal more effort (whether analytical or numerical). I think it's perfectly reasonable that dynamicists continue to work in the Newtonian approximation until it is convincingly demonstrated that GR calculations are required.

15. Nov 18, 2006

### Garth

I was being a little tongue-in-cheek, however the fact remains that theories, either of cosmology or particle physics, that depend on intrinsically undectectable or nearly undetectable 'entities' to make them work ought to be treated with some scepticism.

I agree DM has been detected, see my post above, it is non-baryonic DM I worry about, until, that is, it is discovered in the laboratory.

We know of several kinds of baryonic DM already, such as in the standard theory that matter required to make up the observable $\Omega = 0.003$ to $\Omega = 0.04$ for a start...
Agreed.

Garth

Last edited: Nov 18, 2006
16. Nov 18, 2006

Staff Emeritus
You regard the DM in the bullet cluster as baryonic? It was detected - and imaged! - by its purely gravitational effects.

17. Nov 18, 2006

### SpaceTiger

Staff Emeritus
It is, in principle, possible for that to be purely baryonic matter. A huge collection of fist-sized rocks, for example, could cause the observed gravitational effects without emitting an observable amount of radiation. Most mainstream cosmologists disregard this possibility because WMAP and nucleosynthesis suggest that there aren't enough baryons to account for the dark matter. This, of course, assumes GR is correct, so Garth doesn't necessarily recognize those constraints.

18. Nov 18, 2006

### Garth

I would put it like this.

The standard model requires the Higgs Boson (which just may have been discovered) or the Inflaton to 'power' Inflation, together with non-baryonic DM and DE. The standard model predicts a baryonic density of $\Omega = 0.04$ so exotic non-baryonic DM is required to complete the matter density to $\Omega = 0.27$. However none of these entities have been conclusively detected in the laboratory.

An alternative possibility is that all the matter is baryonic in nature. This requires a modification to the gravitational theory that describes the expansion during BBN. I find it intruiging that the linear expanding model is claimed to produce the correct amount of helium if the baryonic density is around $\Omega = 0.2$. Could this strictly linear expansion be caused either by a modified GR or by DE operating at that early epoch?

If that DM is baryonic then its present form needs to be explained, as ST said, fist-sized rocks, but also cold Jupiters, BHs and WHIM are all possibilities.

Either option requires speculation, I just think that, until the HB, DM and DE are discovered in the laboratory, the alternative ought to be given greater attention than at present.

Garth

Last edited: Nov 19, 2006
19. Nov 19, 2006

### EL

Sorry for going slightly off topic, but I think I've now seen this claim of that the HB recently could have been discovered a couple of times. I just wonder in what experiment? Any refs?

20. Nov 22, 2006

### MonstersFromTheId

This is a simply WONDERFUL discussion, truly facinating ideas, and these links are amazing! Best reading I've had all month.

A bit off-topic here, and more of a kind of fanciful sci-fi kind of question, but, can anyone think of any practical aplications for the use of DM?