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TooMany
2011-Oct-23, 01:21 AM
Can someone explain why this study makes dark matter more mysterious?

http://www.google.com/url?sa=t&rct=j&q=dark%20matter%20mystery&source=web&cd=1&ved=0CCsQqQIwAA&url=http%3A%2F%2Fwww.wired.com%2Fwiredscience%2F20 11%2F10%2Fdark-matter-mysterious%2F&ei=V2qjTqqJBYr8iQKH66E2&usg=AFQjCNH8jO4UVDskjck3H3zLLRxAzM-G3A

antoniseb
2011-Oct-23, 02:37 AM
Can someone explain why this study makes dark matter more mysterious?...

Only that one popular model for DM in galaxy formation seems to be contradicted by too local examples. I'm personally thinking that is Fornax and Scultor were the results of a few mergers of even smaller galaxy units in their early history, the observed results wouldn't contradict this model, but I haven't run models, so this is a gut response... not criticism or fact.

TooMany
2011-Oct-23, 05:07 AM
Thanks antoniseb.

My understanding is that DM is important in explaining galactic rotation curves, galaxy cluster dynamics, galaxy formation and the structure of the universe on large scales.

Also I've read that DM interacts only very weakly with matter and itself except through gravity. What properties of DM allow it to explain the formation of galaxies and large scale structure? If it helps in the "condensation" of matter, how does it do that without significantly interacting with matter or itself (except through gravity)?

The ratio of DM to visible matter in the dwarf galaxies is very high. The article appears to be saying that the DM should be more clumped. Why would it be expected to be clumped if it barely interacts with itself or with visible matter?

In other words, I don't understand exactly why a distribution similar to visible matter is surprising and hence why that makes DM more mysterious.

TooMany
2011-Oct-23, 03:00 PM
Thanks antoniseb.

I've read that DM interacts only very weakly with matter and itself, except through gravity. My understanding is that DM is important in explaining galactic rotation curves, galaxy cluster dynamics, galaxy formation, and the structure of the universe on large scales.

What properties of DM allow it to explain the formation of galaxies? If it promotes the "condensation" of matter, how does it do that without significantly interacting with matter or itself (except through gravity)? Is it expected to be more “clumped” than visible matter in galaxies? That seems inconsistent with the DM halo idea.

Cougar
2011-Oct-23, 11:20 PM
As Grey pointed out in a thread back in July 2010,




The distribution of dark matter needed to account for the roughly flat rotation curves seen in galaxies turns out to be pretty much spherical, with a mass distribution that falls off like the inverse square of the distance from the center.... So, highest at the center, dropping off slowly as you move away. Note that the visible matter in a galaxy drops off in density much more quickly than this.


The fact that dark matter does not interact with normal matter, or even itself (except gravitationally), means it does not clump together when pulled toward a center of gravity (like normal matter does).

antoniseb
2011-Oct-23, 11:55 PM
... What properties of DM allow it to explain the formation of galaxies? If it promotes the "condensation" of matter, how does it do that without significantly interacting with matter or itself (except through gravity)? Is it expected to be more “clumped” than visible matter in galaxies? That seems inconsistent with the DM halo idea.

The properties in question are all gravity of large distributed objects. Concerning galaxy formation, let me wave my hands, and describe, rather than show you simulations... Some of our models result in dark matter in the early universe being created in collections of varying sizes, mostly on the scale of dwarf galaxies. If there is a lot of non-reactive mass distributed in such a space, reactive matter inside that space will follow non-Keplerian paths inside it, tending toward creating galaxies much faster than if that dark matter wasn't there.

In our simplest models you can think of the original dark-matter objects as being smoothly distributed spheres of co-orbiting dark matter particles... concentrated kind of like globular clusters, only non-luminous, thousands of times larger in scales, and with "stars" the mass of perhaps a hundred protons. Clumps of matter inside these dark matter spheres are clumpy by the accident of where star forming regions happen, and blow matter out to other places. This shouldn't affect the distribution of the dark matter. However, as I said above, if a couple of dark matter spheres should coalesce, then I would expect a somewhat lumpy distribution of dark matter in a smallish galaxy. ... again, no math or simulations here on my part, just hand-waving, so that last statement should be considered intuitive, but suspect.

TooMany
2011-Oct-24, 12:54 AM
antoniseb, I need to take a few baby steps first. http://www.bautforum.com/images/smilies/embarrassed-good.gif

Cougar said that "it does not clump together when pulled toward a center of gravity (like normal matter does)"

OK. So VM (visible matter) is actually responsible for the clumping. VM clumps through interactions and radiation. Then DM is drawn in closer by the VM clumping. I suppose the DM kinetic energy increases with the density and DM KE losses are insignificant. The increasing DM density probably induces some further clumping of VM. The DM density profile however does not follow the VM profile because DM does not interact (significantly) with either. Correct?

Now, in these particular dwarf galaxies the DM and VM seem to follow the same distribution instead of the different ones seen in the Milky Way. That is not expected because the DM should not be as "clumped" (for lack of a better word) as it appears to be?

If the photo or illustration of one of the dwarf galaxies in the article is accurate, the appearance is that the VM distribution is quite smooth and symmetrical. It looks like an ellipsoid. In other words, it has the appearance of having settled into a regular form.

antoniseb
2011-Oct-24, 03:12 AM
... So VM (visible matter) is actually responsible for the clumping. VM clumps through interactions and radiation. Then DM is drawn in closer by the VM clumping. I suppose the DM kinetic energy increases with the density and DM KE losses are insignificant. ...

I don't imagine DM as clumping WITH the visible matter clumps. Visible matter is 10-20% of the mass involved, and CAN have detectable influence on perturbing the orbits of the DM particles, but they aren't going to get the DM to just stop and hang out with the visible matter.

TooMany
2011-Oct-24, 06:29 AM
Is it correct that visible matter "clumps" or "condenses" because it sheds kinetic energy (KE) through radiation (caused by interaction), but DM does not because it does not shed KE to any significant degree?

Given an initial distribution of DM and DM particle velocities, the particles may eventually settle into some other distribution. However once DM reaches that distribution it is stuck because if cannot shed KE (significantly). Hence, on it's own, DM will only "clump" in some limited way, but will not be able to further condense. Does that make sense? :confused:

Given the lack of interaction of DM, it seems that clumping must be driven by VM even though it is only 10-20% of the available mass.

antoniseb
2011-Oct-24, 11:52 AM
Is it correct that visible matter "clumps" or "condenses" because it sheds kinetic energy (KE) through radiation (caused by interaction), but DM does not because it does not shed KE to any significant degree?

Given an initial distribution of DM and DM particle velocities, the particles may eventually settle into some other distribution. However once DM reaches that distribution it is stuck because if cannot shed KE (significantly). Hence, on it's own, DM will only "clump" in some limited way, but will not be able to further condense. Does that make sense? :confused:

Given the lack of interaction of DM, it seems that clumping must be driven by VM even though it is only 10-20% of the available mass.

I think that any observed DM clumping will be found to be the result of recent mergers of multiple DM halos, and not a result of the evolution of clumps in the DM from any other means. I think your assessment of shedding KE is a good starting point for thinking about it. I suggest taking a look at the virial theorem.

Grey
2011-Oct-24, 12:52 PM
Given the lack of interaction of DM, it seems that clumping must be driven by VM even though it is only 10-20% of the available mass.Not really. Remember that inverse square mass distribution we need? Turns out that if you have a bunch of particles that interact only through gravity, they'll naturally tend to form a rough sphere with an inverse square mass profile. You are correct that dark matter won't collapse further because it can't radiate, but that's alright, because by not radiating, it clumps in just the way it would need to in order to account for galactic rotation curves. In our current models, dark matter drives the formation of the visible part of galaxies, rather than the other way around.

TooMany
2011-Oct-24, 05:08 PM
OK Grey, that's helpful and, antoniseb, I see how the virial theorem applies. Going back to our galaxy, DM is thought to be less clumped than VM, however in these two dwarf galaxies the distribution appears to be about the same, even through the DM is even more dominant. Is that what is "mysterious"?

Looking at photos of the dwarf galaxies, both appear more or less regular (somewhat ellipsoidal). Antoniseb, you seem to be suggesting that the DM distribution in these galaxies may just be an accident due to some recent? mergers, but given their very regular appearances, that doesn't seem very likely. Do you agree?

Do these galaxies resemble globular clusters? Is there evidence of dark matter domination in globular clusters?

Another interesting question is whether DM has angular momentum like the VM in our galaxy. Rotation would surely affect DM distribution too. Are there any theories about a relation between VM and DM rotation? (Maybe this is wandering off topic.)

Grey
2011-Oct-24, 08:47 PM
OK Grey, that's helpful and, antoniseb, I see how the virial theorem applies. Going back to our galaxy, DM is thought to be less clumped than VM, however in these two dwarf galaxies the distribution appears to be about the same, even through the DM is even more dominant. Is that what is "mysterious"?I don't think that's what they're saying. My quick reading suggests that what's unusual is that the dark matter is even less center concentrated than normal. So instead of an inverse square mass profile, it's a shallower line, with the density closer to constant throughout the visible region.


Another interesting question is whether DM has angular momentum like the VM in our galaxy. Rotation would surely affect DM distribution too. Are there any theories about a relation between VM and DM rotation? (Maybe this is wandering off topic.)Dark matter should certainly have some angular momentum, and that should be coupled at least to some extent to the angular momentum of the visible matter.

I'd have to do more research to go into any more detail. ;)

transreality
2011-Oct-24, 09:57 PM
Is it possible that individual dark matter particles passing very close to the central concentration of baryonic matter could be accelerated into wider orbits? I guess this would amount to some sort of energy transfer from the baryonic matter where the effective temperature of the DM cloud is increased by the concentration of the VM component.

Cougar
2011-Oct-24, 10:47 PM
Looking at photos of the dwarf galaxies, both appear more or less regular (somewhat ellipsoidal).

Fornax is a dwarf elliptical with pretty low density. It is elongated along NE-SW, with apparent dimensions 17' x 12.6'. There is little concentration toward a nucleus. All of this would tend to make a more uniform distribution of dark matter less "mysterious."

TooMany
2011-Oct-24, 10:58 PM
Not sure why Matt Walker at the Harvard-Smithsonian Center for Astrophysics thinks it's an issue then. Guess I better find the paper and read it.

"Our measurements contradict a basic prediction about the structure of cold dark matter in dwarf galaxies. Unless or until theorists can modify that prediction, cold dark matter is inconsistent with our observational data," Walker stated.

Sounds serious, no?

antoniseb
2011-Oct-24, 11:28 PM
... "Our measurements contradict a basic prediction about the structure of cold dark matter in dwarf galaxies. ...

In post #12 above you checked with me about the recent merger idea, and yes, that is a key part of a hypothesis I have that explains this/these dwarf's dark matter distribution. Walker's basic prediction assumes that the dwarf has always been one single DM halo and galaxy. Other modeling has indicted that there should be (have been) on the order of a hundred dwarf galaxies orbiting the Milky Way, and now we think that many of them have merged with us. I expect some small set have merged with each other. I haven't read any simulations about the relaxation time of two dark matter halos merging and settling into one dm halo with the expected distribution, but I'll bet it is on the scale of or long in comparison to the age of the universe (again, my speculation, not backed up by simulations or facts).

Grey
2011-Oct-25, 12:05 PM
Guess I better find the paper and read it.The paper referenced in the article is here (http://arxiv.org/abs/1108.2404), and there's another paper reporting a similar finding here (http://arxiv.org/abs/0910.3538).

TooMany
2011-Oct-25, 03:59 PM
I did read the paper (to the extent that I could understand it). It wasn't too clear to me how detailed the analysis of the DM distribution really was, but I'll re-read it. Thanks for the other reference Grey.

Is this the same as the "cuspy halo problem"? From the Wikipedia article with that name, that problem even applies to normal galaxies like ours. Perhaps the point is that in a highly dark-matter dominated dwarf galaxy, the absence of the cusp is even harder to explain and therefore the properties of DM used in various cosmological simulations are incorrect.

If this is true, then someone has to come up with some new proposal for DM physics that fits observations. I can see where that may present problems if more interaction is required. I'm not sure how Warmer Dark Matter would help, but there must be some reason that it is proposed to be cold in the first place.

Antoniseb may well be correct that it is a result of recent mergers. The intuitive problem I have with that is the appearance of the galaxies does not suggest recent mergers, because of the regularity of the distribution of stars in both galaxies. Moreover the "cuspy halo problem" apparently already exists independently of these new observations.

One issue about the missing mass really bothers me and I wonder if someone can settle this. The most obvious conjecture is the DM is just that, dark ordinary matter, particularly molecular hydrogen. It certainly seems cold enough for H2 to exist in clouds. I've read that H2 is extremely difficult to detect and that most estimates of it's abundance are inferred from the presence of CO which is used as a proxy. Are we really sure that large amounts of H2 aren't there? How do we know that?

TooMany
2011-Oct-26, 12:04 AM
Grey, the second paper you mentioned here (http://arxiv.org/abs/0910.3538) was very interesting because it covers a lot of studies and discusses the issue in terms that are relatively clear to a layman, like myself. It addresses mergers to some extent as well.

It's inspiring to see how hard scientist are working on these difficult issues. There are lots of papers. I was looking into evidence of gas in galactic collisions when I stumbled accross this paper http://arxiv.org/abs/0904.4638v1. The paper proposes that the rotational curves, morphology and stability of spiral galaxies can be explained by assuming there is up to 5 times more bayronic matter (that we do not see) in the form of small, dense clouds of H2 in the outer parts of the galaxies. They perform simulations in which the H2 distribution follows that of the detected HI.

A while back, I read a couple of papers about "extreme scattering events" (in photo-metric observations) which were proposed to be caused by dense H2 clouds on the order of 1AU or less in size in our galaxy. However these papers were about 10 years old and I could not find any newer research.

trinitree88
2011-Oct-26, 11:05 AM
Grey, the second paper you mentioned here (http://arxiv.org/abs/0910.3538) was very interesting because it covers a lot of studies and discusses the issue in terms that are relatively clear to a layman, like myself. It addresses mergers to some extent as well.

It's inspiring to see how hard scientist are working on these difficult issues. There are lots of papers. I was looking into evidence of gas in galactic collisions when I stumbled accross this paper http://arxiv.org/abs/0904.4638v1. The paper proposes that the rotational curves, morphology and stability of spiral galaxies can be explained by assuming there is up to 5 times more bayronic matter (that we do not see) in the form of small, dense clouds of H2 in the outer parts of the galaxies. They perform simulations in which the H2 distribution follows that of the detected HI.

A while back, I read a couple of papers about "extreme scattering events" (in photo-metric observations) which were proposed to be caused by dense H2 clouds on the order of 1AU or less in size in our galaxy. However these papers were about 10 years old and I could not find any newer research.

TooMany. Interesting paper. I'll bet a hot fudge sundae that successive years will find molecular hydrogen to be the complete culprit, though we need observational evidence to be sure. It's too unsettling to believe that stuff completely eludes billion dollar particle physicists and their detectors while dominating the universe. pete

StupendousMan
2011-Oct-26, 11:35 AM
I'll bet a hot fudge sundae that successive years will find molecular hydrogen to be the complete culprit, though we need observational evidence to be sure.

Big Bang nucleosynthesis calculations yield ratios of light elements -- hydrogen, helium and lithium -- which agree well with observations ... if the amount of ordinary, baryonic matter is much lower than the amount needed to account for dark matter's gravitational effects. If you believe that there is no non-baryonic dark matter, how do you explain the agreement of nucleosynthesis calculations with observations?

TooMany
2011-Oct-26, 04:13 PM
This is not an ATM thread. We are just exploring possibilities based on scientific papers. Based on a number of papers (a couple of good ones cited above) the current CDM theory has unresolved problems. One issue is that the proposed new particle has not been directly detected. Another is that the proposed CDM properties should produce a pronounced cusp at the center of any virial concentration but evidence for such a cusp is lacking in most cases.

The paper most recently sited investigates the possibility of an obvious alternative to non-bayronic dark matter in explaining observations of spiral galaxies. The paper simply makes the supposition that CMD is plain old H2 in small clouds and then does simulations to show that it's a realistic possibility. In fact the paper claims that H2 explains spiral arms better than non-bayronic matter. Admittedly it is just a supposition unless the H2 can somehow be directly detected. However, the papers on "extreme scattering events" provide some evidence that may support the existence of such clouds. I'm really surprised that more followup work is not apparent on these scattering events.

We have conflicting evidence about things. Why should BB nucleosynthesis arguments be consider conclusive and eliminate alternative explanations of observations when 1) they are based on careful choices of a number of primordial parameters and 2) the agreement with observations is exaggerated, since the lithium (and He3?) abundances do not match with the favored parameters. Lithium is off by at least a factor of 3. That does not agree well with observations.

As of right now, we do not have a nice neat theory that fits all the evidence, so alternatives must be explored.

Grey
2011-Oct-26, 04:41 PM
Another is that the proposed CDM properties should produce a pronounced cusp at the center of any virial concentration but evidence for such a cusp is lacking in most cases.I disagree with this statement. There are some cases where the predicted density profile does not match the observed profile, but for most cases, observations do support that CDM behaves much as we suspect it should. The exceptions may have specific explanations. It's possible that they may not, but at the moment, the CDM model works very well for explaining galactic dynamics and cluster dynamics. For myself, I expect that issues like the dark matter profile in some dwarf galaxies will be addressed by some specific effect that we haven't considered, or by a small modification to the CDM model, not by a complete replacement of that model.

TooMany
2011-Oct-26, 06:16 PM
Grey,
Sure, it could be that some explanations will make the non-bayronic CDM problems go away. But at least in the context of this paper The Core-Cusp Problem (http://arxiv.org/abs/0910.3538), you seem to be minimizing the seriousness of the issue .

In scientific theories, matching most of the evidence is not enough. For example, Newton's theory of gravity explained the motions of the planets very nicely until careful measurements of Mercury's orbit indicated something was amiss. This was eventually explained by a new theory of General Relativity which was quite revolutionary. The problem with the abundance of lithium could lead somewhere. There are some efforts to explain it but AFAIK none are really a slam dunk. Most attempts to approach this problem start from the view that the predicted lithium was in fact present but somehow it got destroyed or hidden. That's fine, but there is also the possibility that the BB nucleosynthesis arguments are wrong.

When you do science, which involves the systematic investigation of nature through observation, it is not so much the things that fit a theory that are interesting. It is the things that don't which lead to progress. In other words, substantial effort should be concentrated on that which does not fit a theory (and indeed quite a lot of effort is being made).

It's fine to accumulate substantiations of a theory. General Relatively has been substantiated again and again within the environment in which we are able to do precision experiments. But if someone found serious disagreement with that theory, it would be important to explore the issue. You just don't stand around saying "it fits most things" or "it's the best theory we have" and minimize problems to an extent where they receive insufficient attention.

Many people have made this point. Having read lots of threads in Bad Astronomy, it seems that some of the regulars here are highly interested in defending LambdaCDM by pointing to successes and not so interested in the things that cast doubt. That's too bad because those are the real problem areas, not the points of agreement.

I think we can all agree that the study of the cosmos is fascinating and exciting, but it's hardly all figured out. I'm getting along in years but I'm really excited about what JWST might reveal. Seems that the launch date keeps moving out. The last I heard is 2018. I can't image why seven more years are needed before it can be launched, unless it's due to funding cuts. I have my fingers crossed that the launch will be a success and that I will live to see some of today's theories confirmed or shot down. Great stuff.

Cougar
2011-Oct-26, 06:17 PM
As of right now, we do not have a nice neat theory that fits all the evidence....



"It is rather ironic that the job of a scientist is to understand nature, and if the scientist completely succeeds, the reward is unemployment. But of the many things that concern me in the day-to-day existence of a scientist, waking up one morning and discovering that there are no problems to solve is rather low on the list." -- Rocky Kolb (http://astro.uchicago.edu/~rocky/)

Cougar
2011-Oct-26, 06:23 PM
....but there is also the possibility that the BB nucleosynthesis arguments are wrong.



"When the temperature dropped far below one billion degrees [three minutes after the big bang] this 'primordial nucleosynthesis' stopped and, according to the standard model, we should be left with roughly 25% helium by mass and 2 x 10-5 parts deuterium. It may seem like a miracle that astronomers in fact do measure about 25% helium in the real universe, but it is a miracle squared that they also measure something like 2 x 10-5 parts deuterium." -- Tony Rothman (http://www.physics.princeton.edu/~trothman/)

matt.o
2011-Oct-26, 09:18 PM
It's worth pointing out that Revaz et al. are NOT claiming that the dark matter halo can be replaced by H2 gas. They merely claim that it is possible to hide a lot of baryons in the outer parts of the disk (but no more than that allowed by the well constrained cosmic baryon fraction) without significantly affecting the disk dynamics or the disk stability.

TooMany
2011-Oct-26, 11:23 PM
matt.o,

You are correct and I'm afraid I have somewhat misrepresented the paper. They still assume a portion is non-baryonic. They multiply the detected gas portion of galaxies by a factor of 3 to 5 and assume that amount is baryonic. I'm not sure how much of the missing mass would be baryonic versus non-baryonic in that case. I'll try to find out. On the other hand, when you read the conclusion it appears that they may have intended to leave enough non-baryonic matter in the mix to stay within some bounds that would be consistent with LambdaCDM.

Somewhat disappointing in that I thought I had found a paper in which a hypothetical completely-baryonic composition was actually investigated. The paper expresses concern over stability and makes an argument in favor. That makes me wonder if, completely aside from BBT, there are reasons to believe that galaxies cannot be primary composed of baryonic matter. If there are I'd like to know about them. Also I previously asked how we know that there is not a lot of undetected H2, especially in the outer parts of galaxies but no one has responded yet.

Thanks for the observation and I apologize for misleading any readers that the conclusions applied to completely baryonic galaxies.

StupendousMan
2011-Oct-27, 01:25 AM
How do we know there isn't a lot of H2 in the outer parts of galaxies? That's a fair question. How would we detect it? Not easy, since H2 is a symmetric molecule, and thus doesn't emit radiation efficiently.

Turn the problem around for a moment. Assume that you can place an H2 cloud into the halo of the Milky Way. What will conditions in the cloud be -- temperatures, density? Will the cloud be in pressure equilibrium with the ambient medium? Will the cloud be stable, or will it tend to collapse?

If there's enough H2 to be dynamically important, how many clouds will there be along a random line of sight outside of the Milky Way? Could we detect these clouds in absorption in front of quasars? Do we?

TooMany
2011-Oct-27, 05:47 AM
(I think one of my posts got dropped or maybe it's still awaiting approval from a moderator.)

StupendousMan,
I can only conjecture, but here you go. It's way cold out there. It's even way cold in this part of our galaxy if you are not close to a star. I'm not sure what assumptions are made to argue that H2 is unstable in the interstellar medium since it is quite stable here on earth which is 100 times hotter and within 1AU of a star. The hypothesis is that these hard to detect clouds of H2 are very condensed. The suggestion is that these clouds exceed the usual density of the interstellar medium by a factor of 10^3. These clouds are small, 1AU or less and maybe as small as 0.1AU. There may even be frozen H2 in them. What little UV radiation can reach them is absorbed by the outermost parts so the H2 inside is safe. If they are somewhere way out there in the galactic plane and it's easy to imagine that they would be shielded by dust and gas from galactic UV radiation.

Can you tell me how stars can form at all in our galaxy, even in regions of dense stellar population, without very cold clouds of H2? I believe that current theories of stellar formation require very cold molecular clouds that are dominated by H2. Is that incorrect?

I mentioned these "extreme scattering events" as a possible signature of these clouds. Here's a relatively recent paper Extreme Scattering Events: insights into the interstellar medium on AU-scales (http://arxiv.org/abs/astro-ph/0610737v1). It concludes that a substantial portion of galactic mass is in these clouds. And yes, the detection occurs in quasar observations.

What happens inside these clouds? I don't know, but they are not massive enough to condense into stars. Do they condense into sub-stellar objects? I don't know. I do know that H is the primary mass component of the Universe that has actually been detected. To speculate that there might be a heck of a lot more around, because it's very difficult to detect in the H2 form, is just a common sense hypothesis.

By the way, I previously indicated that these were photo-metric observations. The paper I referenced above refers to radio wave frequencies (still photons or course, but I want to make that clear). I don't think the research is extensive enough to be draw high accuracy conclusions, but it certainly is food for thought. Does anyone care or do we already know what's what?

antoniseb
2011-Oct-27, 06:15 AM
... I'm not sure what assumptions are made to argue that H2 is unstable in the interstellar medium ...

One thing out in interstellar space that is in short supply on Earth is ionizing radiation. One cosmic ray whizzing through a sparse cloud of Hydrogen will send the two protons off in various directions, too energetically to pair up with other free atoms in the cloud. The general trend is toward ionization without let up or remorse.

StupendousMan
2011-Oct-27, 11:47 AM
The hypothesis is that these hard to detect clouds of H2 are very condensed. The suggestion is that these clouds exceed the usual density of the interstellar medium by a factor of 10^3. These clouds are small, 1AU or less and maybe as small as 0.1AU. There may even be frozen H2 in them. What little UV radiation can reach them is absorbed by the outermost parts so the H2 inside is safe. If they are somewhere way out there in the galactic plane and it's easy to imagine that they would be shielded by dust and gas from galactic UV radiation.

In order for this idea to be taken seriously by the astronomical community, someone must show that the proposed H2 clouds are in a stable hydrostatic equilibrium in these conditions. Can you do so? What is the timescale for the clouds to be photoionized by the ambient UV radiation field in, say, the plane of the Milky Way? If these clouds have the standard mixture of heavier elements, how luminous should they be in CO? Does that luminosity fall below our detection threshold?




Can you tell me how stars can form at all in our galaxy, even in regions of dense stellar population, without very cold clouds of H2? I believe that current theories of stellar formation require very cold molecular clouds that are dominated by H2. Is that incorrect?

Sure, current models of star formation do indeed feature cold molecular clouds of H2. But those clouds are concentrated in small regions of the galaxy, not spread out evenly everywhere; that claim is backed by both observation and by theory.




I don't know. I do know that H is the primary mass component of the Universe that has actually been detected. To speculate that there might be a heck of a lot more around, because it's very difficult to detect in the H2 form, is just a common sense hypothesis.

Fair enough. Now, in order to progress from common sense to science, please proceed from speculating to computing quantitatively the consequences of your speculation.



By the way, I previously indicated that these were photo-metric observations. The paper I referenced above refers to radio wave frequencies (still photons or course, but I want to make that clear). I don't think the research is extensive enough to be draw high accuracy conclusions, but it certainly is food for thought. Does anyone care or do we already know what's what?
That paper was published in 2006. If there has been no followup since, it is likely that few astronomers take the idea seriously.

TooMany
2011-Oct-27, 03:10 PM
One thing out in interstellar space that is in short supply on Earth is ionizing radiation. One cosmic ray whizzing through a sparse cloud of Hydrogen will send the two protons off in various directions, too energetically to pair up with other free atoms in the cloud. The general trend is toward ionization without let up or remorse.

On earth, ionizing radiation is absorbed by a dense layer of gas with thickness =~ 0.000001AU. You are assuming a very low density that would not promote recombination. In the article I mentioned, the density of clouds is thought to be around 1000 times that of the warm interstellar medium that we can measure. If you doubt the viability of cold clouds of hydrogen in the coldest parts of the galaxy, then how do you explain ongoing formation of stars from cold molecular hydrogen clouds in much warmer parts of the galaxy that have been subject to ionizing radiation for billions of years?

TooMany
2011-Oct-27, 03:58 PM
In order for this idea to be taken seriously by the astronomical community, someone must show that the proposed H2 clouds are in a stable hydrostatic equilibrium in these conditions. Can you do so? What is the timescale for the clouds to be photoionized by the ambient UV radiation field in, say, the plane of the Milky Way? If these clouds have the standard mixture of heavier elements, how luminous should they be in CO? Does that luminosity fall below our detection threshold?


See my reply to antoniseb. My guess would be that these clouds do not have the "standard mixture" of metals. My guess would be that metals form mostly in the parts of the galaxy where stars (and supernovas) occur, in the visible disk. Around this lies a more primordial mix that is nearly all H and He. With that conjecture I would not expect to see much CO. H2 distributions have historically been determined using CO as a proxy. That may be a mistake in less developed parts of galaxies.

The billions of stars in our galaxy have had billions of years to photo-ionize H2. Nevertheless there is still plenty around and stars are still forming in the general vicinity of intense radiation. How has all this H2 survived?


Sure, current models of star formation do indeed feature cold molecular clouds of H2. But those clouds are concentrated in small regions of the galaxy, not spread out evenly everywhere; that claim is backed by both observation and by theory.

Did you miss that the proposed distribution of this matter is dense clouds of cold H2?


Fair enough. Now, in order to progress from common sense to science, please proceed from speculating to computing quantitatively the consequences of your speculation.

I'm not engaging in that game that is played on the ATM threads. I am making an intuitive argument but you may find some of the details in the paper I referenced. If not, maybe I can find you something that can explain how stars can still be forming and what mass and density is required for molecular clouds to survive in the very coldest and darkest reaches of galaxies.


That paper was published in 2006. If there has been no followup since, it is likely that few astronomers take the idea seriously.

More original work was done much earlier. I can only guess why this has apparently not been followed up. Maybe no funding for observations? Maybe the idea that cold clouds of hydrogen could exist in the coldest parts of galaxies is just too dumb, hence no further effort? Maybe the evidence evaporated? Maybe astronomers don't take it seriously because it might contradict the theory that they are committed to. Objectivity of scientist is overrated. They are human beings and are liable to form beliefs just like everyone else.

This is an issue I was talking about earlier. You can support a theory by finding confirmatory evidence. I could measure the orbit of every asteroid in the solar system to verify Newtonian gravity but that would not advance understanding. What is also important is to actually investigate alternatives to and problems with a theory.

Shaula
2011-Oct-27, 04:57 PM
Objectivity of scientist is overrated. They are human beings and are liable to form beliefs just like everyone else.
There are also a lot of young or up and coming scientists who would LOVE to be known as the one who changed the way we understand things. So please don't bother with the "it's a belief system" or any similar arguments. They don't hold water if you know even one group of scientists. Scientists are human and recognition, the desire to understand and a need to tear down the old ideas are certainly no less represented among them.

TooMany
2011-Oct-27, 06:05 PM
Shaula,
I really can't make a detailed argument about exactly how scientist behave. I'm sure there are plenty of young people who would love to make new discoveries and I'm all for encouraging them.

Here's what I see from an outsiders point of view. There seems to be relatively little effort made to explain galaxies in terms of things we know to exist like H2. Instead the focus is on explaining galaxies through non-baryonic CDM. The focus is on non-baryonic explanations because of a general belief in the BBT which does not allow the missing matter to be normal matter (in it's present form). After more than twenty years of trying, direct detection of this non-baryonic matter still alludes us.

I would have to get into a PhD program in various Universities to discover whether there is some bias that might discourage students from studying unpopular theories. If you read some threads in the ATM forum, you will see that suggestions that current theory might be wrong often cause an unduly indignant response. Such responses are not encouraging for those attempting to think "out of the box". If such an attitude exists to some degree in the scientific world itself, then it is not unreasonable to suggest that many of those bright young open minds may be discouraged. That's not something we should argue endlessly about (IMO) because it strays from physical science into psychology. I don't think it's a productive issue to discuss. I only brought it up because there appears to be little research into the possibility of the dominance of baryonic matter.

Getting back to science, I think I've tried to make a decent case that the missing mass might just be plain old H2. I've pointed out the difficulty of detection. When I look at pictures of spiral galaxies, the bright part is obvious, but in many pictures a much fainter surrounding part that is larger in area is also visible. Anything that is visible either has stars or is hot for some reason. I cannot help but imagine that even this large faint part may also be surrounded by another large component that is too cold to generate visible radiation and too transparent to affect the visibility of background galaxies. So perhaps when we observe galactic rotation curves that imply large amounts of dark matter, the reason is that only a portion of the mass is concentrated in the visible part and most of it consists of surrounding, dark, but perfectly normal matter.

Do simulations of such conditions exist? Has theoretical work been done to determine how much normal matter so distributed would be required to explain the rotation curves?

You can't argue that it's impossible because it must be non-baryonic. Maybe there are some other factors that have been missed in BBT that would allow the matter to be baryonic and still preserve the overall BB idea. The BBT already depends on speculation that transcends known physics, in particular the inflation field. If the theory permits that kind of speculation, then surely it should permit much more conservative speculation like "the universe is largely composed of normal matter" just like we find on earth, in stars and in gas clouds.

Grey
2011-Oct-27, 06:48 PM
Do simulations of such conditions exist? Has theoretical work been done to determine how much normal matter so distributed would be required to explain the rotation curves?

You can't argue that it's impossible because it must be non-baryonic.Yes, such simulations have been done, and to some extent the problem is that it can't be baryonic, entirely apart from questions of the big bang. That is, if it's normal matter and can interact via the electromagnetic force, than as the galaxy coalesces, it will tend to collapse to the center (just like the visible matter does). That would make the core-cusp issue that is seen in some dwarf galaxies even worse. Even CDM models predict some concentration of matter toward the center (that inverse square mass distribution profile that we talked about), which is what's needed to explain a flat galactic rotation curve. But if your dark matter can shed heat through radiation as it collapses like normal matter can, you don't get an inverse square density profile, you get a much heavier concentration of mass at the center. That shouldn't be much of a surprise, since that's exactly what we see the visible matter in a galaxy doing: it's much more concentrated in the center than in the outer regions.

So if your concern is the core-cusp problem, switching from CDM to baryonic matter makes the expected situation worse, not better. For that issue, we need to explain why there's even less of a concentration in the very center than our models predict, at least in some galaxies, whereas baryonic matter should be even more concentrated in the center than something that only interacts through gravity would be.

TooMany
2011-Oct-27, 10:20 PM
Grey,

Can you site a reference that claims that baryonic galaxies are not possible, based on mechanics alone? :question:

Your argument is not entirely convincing. We live in a 4-5 billion year old solar system and yes 99% percent of the matter is dead center because it condensed into a star, but the rest of the matter in the system has not collapsed into the sun. We have planets, we have an asteroid belt, we have a Kuiper belt (all of which are stable) and possibly an enormous Oort cloud.

Galactic dynamics are much more complicated than just rotation curves and models based on the behavior of uniform distributions of gases. I gather from abstracts I read today that we really still do not understand precisely how stars form or even how cold molecular gas clouds form and sustain themselves. It would seem that your argument might imply that all the baryons should be in stars by now or perhaps even in one big black hole. But that is not the case. Some ongoing processes prevent all gas from collapsing into the center or into stars in spite of the passage of at least 12 billion years. The most obvious thing that keeps spiral galaxies from contracting into steep density profile is their angular momentum, the same reason that the planets don't plunge into the sun.

The density profiles that you refer to are based on visible matter. What about the normal matter that we cannot easily see? How do you know that cold H2 is not there if you cannot detect it? I've always found the assumption strange that much of the matter in galaxies is visible and that there is no larger portion that is too cold to glow.

Cold lumps of matter in the Kuiper belt have only recently been detected because they are so dim. Now we know that about 20 to 200 times as much mass exists there than in the asteriod belt. This cold dark matter is only 20AU to 50AU from us. What if it were light years away and not near a star to illuminate it? Would there be much chance of detecting it?

One of the cool things about the non-baryonic CDM theory is that DM behavior is rather constrained because it doesn't interact in a way that can shed kinetic energy or affect matter much besides helping it clump. So any departure from it's predicated behavior must be presumed to result from transitory (dynamically unstable) conditions, like mergers. That should make it relatively easy to verify or disprove. Because baryonic matter interacts, fuses (in stars), has phases, charge, chemistry, a spectrum of masses etc., it's behavior is much more complicated and not entirely figured out at the level of galactic dynamics.

Here's an interesting question I'll try to research. Do all galaxies studied require large amounts of dark matter to explain their rotation curves?

antoniseb
2011-Oct-28, 01:19 AM
... Do all galaxies studied require large amounts of dark matter to explain their rotation curves?

Yes, but the dark matter, luminous matter ratios vary (within a range) for different galaxies. Also, galaxy clusters have considerable dark matter outside of any galaxies.

Side note: That much molecular Hydrogen (even if it could somehow stay cold and bound) should present very strong absorption features in the IR spectrum between 1 and 4 microns. These features are in a range that we've studied pretty extensively, and are not reported.

Tensor
2011-Oct-28, 06:18 AM
Shaula,
I really can't make a detailed argument about exactly how scientist behave.

But that didn't stop you from making derogatory comments about their behavior as a group, did it?


I'm sure there are plenty of young people who would love to make new discoveries and I'm all for encouraging them.

There are plenty of old people who would love to make new discoveries also. The Nobel prize is worth about a 1.5 million US dollars. Your saying that a legitimate scientist would give that and the prestige of making a completely revolutionary discovery because they don't want to accept the data.


Here's what I see from an outsiders point of view. There seems to be relatively little effort made to explain galaxies in terms of things we know to exist like H2. Instead the focus is on explaining galaxies through non-baryonic CDM. The focus is on non-baryonic explanations because of a general belief in the BBT which does not allow the missing matter to be normal matter (in it's present form).

So, you're an outsider and claim you can't make detailed arguments about their behavior. So what was it exactly that led you to the conclusion that the reason the focus was on non-baryonic explanations was the science and not because of the scientist's general belief in the BBT?


After more than twenty years of trying, direct detection of this non-baryonic matter still alludes us.

You mean, like the neutrino? Postulated in 1930, detected in 1956. Or how about Helium? First detected on the Sun in 1868, first isolated on Earth in 1895. Or how about the problem with Mercury's advance of perihelion? First detected in 1859, not explained until 1915.


I would have to get into a PhD program in various Universities to discover whether there is some bias that might discourage students from studying unpopular theories. If you read some threads in the ATM forum, you will see that suggestions that current theory might be wrong often cause an unduly indignant response. Such responses are not encouraging for those attempting to think "out of the box".

The reason for those indignant responses is due to the OP. The flat out ignorance of the OP's argument, the lack of proper evidence for the OPs idea or the fact that the OP's idea "is not even wrong". Or do you think those responses are just put out there with no thought to their posts? If so, perhaps you can point out a thread in ATM exactly where the the majority of the indignant responses were incorrect.



If such an attitude exists to some degree in the scientific world itself, then it is not unreasonable to suggest that many of those bright young open minds may be discouraged.

As has been pointed out many times here, anyone going through the process of getting their PhD or proposing new ideas (even those that get accepted) as a post-doc or even later, go through questioning much, much worse than anything that happens here. It's not a question of whether the process is discouraging, it's a question of the person being able to defend their idea with data and observational or experimental support


That's not something we should argue endlessly about (IMO) because it strays from physical science into psychology. I don't think it's a productive issue to discuss. I only brought it up because there appears to be little research into the possibility of the dominance of baryonic matter.

So, instead of discussing the whys of the science, you went to accusing the scientists of forming belief systems, or of their inability to accept new ideas. Have you looked into the issue of micro-lensing (http://en.wikipedia.org/wiki/Gravitational_microlensing), details here (http://arxiv.org/pdf/astro-ph/0604278v1)? How about the Bullet Cluster (http://arxiv.org/pdf/astro-ph/0608408v1)? The thing is, dark matter isn't just dependent on one thing. It's several different things (nucleosynthesis, rotation curves, cluster movements, micro lensing, failure of other ideas) that combined, make for the explanation for all of the observations.


Getting back to science, I think I've tried to make a decent case that the missing mass might just be plain old H2. I've pointed out the difficulty of detection. When I look at pictures of spiral galaxies, the bright part is obvious, but in many pictures a much fainter surrounding part that is larger in area is also visible. Anything that is visible either has stars or is hot for some reason.

And this is exactly the kind of post that gets those indignant (as you call them)posts in ATM. Pictures look like this. My intuition tells me that. So, this is what it has to be. It doesn't work like that. We had one guy here who insisted the Sun was made of iron, because he saw a picture of the sun and in the picture, the sun was green. We get people who argue against relativity because it doesn't make sense to them. What are the numbers? Give us the calculations, the simulations, the observations. But you don't want to do that, you want to do it intuitively. Try doing Quantum Mechanics intuitively, ignoring the math, and see how far you get.

In this case, it's simply a question of you don't know, what you don't know. Antoniseb points out the 1-4 micron IR. It doesn't appear you knew about either micro lensing or the Bullet Cluster. The micro lensing part is important because if you put the dark matter where it needs to be to account for the rotation curves, you end up with the observed micro lensing. If the dark matter isn't where the micro lensing observations say it is, then the dark matter isn't in the right place to produce the observed rotation curves.


I cannot help but imagine that even this large faint part may also be surrounded by another large component that is too cold to generate visible radiation and too transparent to affect the visibility of background galaxies. So perhaps when we observe galactic rotation curves that imply large amounts of dark matter, the reason is that only a portion of the mass is concentrated in the visible part and most of it consists of surrounding, dark, but perfectly normal matter.

If it was perfectly normal matter, it wouldn't stay dark (see the 1-4 micron observations). It would absorb energy (at least the observed CMB) and would glow at some temperature at or above the CMB. Those 13.7 billion years for the universe give it plenty of time to warm up. Yes, there are area that are colder than the CMB, but not many.

Shaula
2011-Oct-28, 06:49 AM
Here's what I see from an outsiders point of view. There seems to be relatively little effort made to explain galaxies in terms of things we know to exist like H2.
There is little apparent current effort because there was a long period of trying just about everything astronomers could think of. Modified gravity (still being tested), compact objects, diffuse objects etc etc. The observations gave us a very specific set of parameters that other models have struggled to produce. With effort I am sure you can get a tweaked distribution of matter that might just about explain what is going on - but then there is no method to get the matter into this configuration. Or you can put this matter in at the start and let it evolve - and you get nothing like what we see today. So far the only model that mostly fits our models of how the universe evolved AND our observations is the one we are using. It is always being tested and checked, other things are being looked at. But they just don't work. Dark matter was not a first choice at all. It is where the evidence led us, rather reluctantly.

And cold H2 fails because if there were enough to explain rotation curved you would see it spectroscopically. Simple as. It does not need to glow - just be in the way of something that does. Like a honking great galaxy, for example.

The indignant tone? Because, for example, I spent about ten years reading and studying physics. Then another ten years working in the field. Then someone comes up and says "Uh, duh, have you not thought that it could be (insert simple idea)" and then dismisses any and all of my arguments - claiming that my specialist knowledge is biased or blinkered. I think I have a right to be annoyed! And there are many, many people who have studied harder and longer than me, know more and are cleverer, that get the same or worse treatment. If people listen, take suggestions and read up on what they are proposing, take the time to understand what they are trying to overturn rather than assume everyone in the field are a little dense, then they get a better response. They assume I am stupid and/or a stooge of the establishment who are promoting an idea to maintain their grant funding then I am going to assume they are worthy of a little scorn. I will end this meta discussion on my part now with an apology to the mods for continuing it in the first place.

TooMany
2011-Oct-29, 04:35 PM
I did not intend to make a derogatory comment about scientists. I was just pointing out that they are human. I have great respect for scientists. I'm a long time fan of the late Richard Feynman as a role model. Shaula, for me it's a privilege to have someone with your training comment on issues. Indeed I can understand your frustration when people simply don't listen.

Tensor, I have to take a few deep breaths after that salvo. You are putting words in my mouth that did not come from me. You are not arguing with a crackpot who thinks the sun is made of iron because it's green. I'm just a guy with some physics and math training who is interested in cosmology. I'm not inventing these DM issues, I'm reading papers and thinking about them. Can we please drop the emotional subject now?

Can you assert that there is no H2 because we would have seen IR absorption? First of all I haven't seen reference to IR absorption as a means of detection. I've have seen references to IR lines used to detect warm molecular H (>~ 300K). Here is a typical statement about H2 detection:

"Molecular hydrogen is difficult to detect by infrared and radio observations, so the molecule most often used to determine the presence of H2 is CO (carbon monoxide). The ratio between CO luminosity and H2 mass is thought to be constant, although there are reasons to doubt this assumption in observations of some other galaxies."

Also, suppose the molecular hydrogen is in small cold dense clouds on the order of 1AU. If we looked for absorption (e.g. FUV) along some line of sight, what would be the probably of detecting one? The papers about extreme scattering events indicate that large amounts of H2 could exist in that form and yet only be detected rarely by it's effects on eclipsed sources like QSOs and pulsars. Is it impossible for very cold clouds of H2 to be stable, or continuously form? If so, how do stars form amid all the radiation from existing stars?

So I'm skeptical that it's a slam dunk that H2 is not there because we haven't seen IR absorption. I don't think you can sweep away the possibility that easily.

There are lots of papers in the literature that explore alternative theories of galactic dynamics. Cooperstock, a GR expert, claims that analysis of galactic rotation using Newtonian approximations leads to incorrect results and that GR theory must be applied. He concludes in this paper (http://arxiv.org/abs/astro-ph/0610370):


We have seen that the non-linearity for the computation of density inherent
in the Einstein field equations for a stationary axially-symmetric pressure-free
mass distribution, even in the case of weak fields, leads to the correct galactic
velocity curves as opposed to the incorrect curves that had been derived on the
basis of Newtonian gravitational theory.

If he is correct, then there is no need at all to propose large amounts of missing matter to explain the rotation of galaxies. Isn't it quite important to investigate this possibility? He is in effect saying, we did the gravitation analysis wrong in the first place.

When you tell me that everything is figured out and that all alternative possibilities have be thoroughly investigated and ruled out, I am a bit skeptical.

I'm well aware of the Bullet Cluster. It seems to indicate that in a cluster collision the diffuse gaseous matter collides, but the dark matter does not. Could the dark matter be small clouds of H2? What is the probability that they would collide? (I don't know.) On the other hand, there is the Train Wreck Cluster that does not appear to be consistent with what is seen in the Bullet Cluster.

I am aware of micro-lensing studies but not familiar the scope of the conclusions that can be drawn from the observations so far. I'll read the paper. Thanks for the reference.

StupendousMan
2011-Oct-29, 05:13 PM
There are lots of papers in the literature that explore alternative theories of galactic dynamics. Cooperstock, a GR expert, claims that analysis of galactic rotation using Newtonian approximations leads to incorrect results and that GR theory must be applied. He concludes in this paper (http://arxiv.org/abs/astro-ph/0610370):



If he is correct, then there is no need at all to propose large amounts of missing matter to explain the rotation of galaxies. Isn't it quite important to investigate this possibility? He is in effect saying, we did the gravitation analysis wrong in the first place.


You have read one paper by Cooperstock. Good. Now go read another 10 or 20 papers by other authors, including some of the papers which Cooperstock criticizes. At that point, you may be able to discuss the subject with some understanding.

At the moment, you don't have the information you need to hold this discussion. Some of us could spend time trying to summarize results of other papers for your benefit, to bring you up to speed. Why should we do that, unless you demonstrate that you are willing to put time and effort into this study yourself?

TooMany
2011-Oct-29, 07:16 PM
Should we see who's stack of papers weighs more or should we evaluate each paper for what it is? :confused:

Yes it's just one paper, but what's interesting about it is that an expert in GR is asserting that the Newtonian approximations that are employed to argue for missing mass are just plain wrong. That's a very significant paper. On the face of it is not speculative at all (no missing matter, no exotic particles, no unusal claims about the distribution of matter in galaxies and no new physics). It is simply applying the accepted theory of gravity (GR) more rigorously by not relying on the Newtonian approximation which we already know is wrong under some circumstances (e.g. orbit of Mercury).

The paper I pointed out is not the first he has written. Earlier papers he published have been criticized and he has answered those criticisms. There may be newer critiques, I will look for them.

It would be great if I could evaluate his conclusions directly, but I can't because I don't have the depth of understand of GR and mathematics needed. Perhaps you do and you can explain exactly why he is wrong or identify the 10 to 20 papers that prove him incorrect. :neutral:

Don't imply that I'm lazy. I am reading papers as best that I can understand them. The papers are published by competent scientists, not crackpots.

Hornblower
2011-Oct-30, 03:55 AM
Should we see who's stack of papers weighs more or should we evaluate each paper for what it is? :confused:

Yes it's just one paper, but what's interesting about it is that an expert in GR is asserting that the Newtonian approximations that are employed to argue for missing mass are just plain wrong. That's a very significant paper. On the face of it is not speculative at all (no missing matter, no exotic particles, no unusal claims about the distribution of matter in galaxies and no new physics). It is simply applying the accepted theory of gravity (GR) more rigorously by not relying on the Newtonian approximation which we already know is wrong under some circumstances (e.g. orbit of Mercury).

The paper I pointed out is not the first he has written. Earlier papers he published have been criticized and he has answered those criticisms. There may be newer critiques, I will look for them.

It would be great if I could evaluate his conclusions directly, but I can't because I don't have the depth of understand of GR and mathematics needed. Perhaps you do and you can explain exactly why he is wrong or identify the 10 to 20 papers that prove him incorrect. :neutral:

Don't imply that I'm lazy. I am reading papers as best that I can understand them. The papers are published by competent scientists, not crackpots.

It always has been my understanding that the difference between the Newtonian approximation and the GR calculation is almost vanishingly small at the velocities and energy levels here, and that the discrepancy between the orbital velocities in the outer parts of typical galaxies and what we expect from the mass inferred from starlight we can see is very large. What does this author say that is contrary to my line of thought?

Tensor
2011-Oct-30, 06:00 AM
I did not intend to make a derogatory comment about scientists.

snip...

Tensor, I have to take a few deep breaths after that salvo. You are putting words in my mouth that did not come from me.

Putting words in your mouth?

From your post #35


Maybe astronomers don't take it seriously because it might contradict the theory that they are committed to. Objectivity of scientist is overrated. They are human beings and are liable to form beliefs just like everyone else.

From your post #37


The focus is on non-baryonic explanations because of a general belief in the BBT which does not allow the missing matter to be normal matter...

You want to explain how I put words in your mouth?


You are not arguing with a crackpot who thinks the sun is made of iron because it's green.

I didn't say you were a crackpot. But, again, from post #37:


When I look at pictures of spiral galaxies, the bright part is obvious, but in many pictures a much fainter surrounding part that is larger in area is also visible. Anything that is visible either has stars or is hot for some reason. I cannot help but imagine that even this large faint part may also be surrounded by another large component that is too cold to generate visible radiation and too transparent to affect the visibility of background galaxies.

He imagined that the sun was made of iron because it was green in a picture. You imagine that there has to be a large cold component because there is a large faint part of something else in a picture. Exactly how are they different, they're both intuitive based, not observation based, right? I also said "What are the numbers? Give us the calculations, the simulations, the observations."


I'm just a guy with some physics and math training who is interested in cosmology. I'm not inventing these DM issues, I'm reading papers and thinking about them. Can we please drop the emotional subject now?

I'm just reading the words you wrote. Seeing the comments about scientists, without qualifiers, but I'll be happy to drop it.


Can you assert that there is no H2 because we would have seen IR absorption? First of all I haven't seen reference to IR absorption as a means of detection. I've have seen references to IR lines used to detect warm molecular H (>~ 300K). Here is a typical statement about H2 detection:

"Molecular hydrogen is difficult to detect by infrared and radio observations, so the molecule most often used to determine the presence of H2 is CO (carbon monoxide). The ratio between CO luminosity and H2 mass is thought to be constant, although there are reasons to doubt this assumption in observations of some other galaxies."

Typical for wikipedia, which usually isn't the greatest as a source for anything scientific. Unless, of course, you use it as a starting point to look at the papers it references. Molecular Hydrogen has gotten much easier to detect since Spitzer went up. For instance here (http://arxiv.org/pdf/1005.3832) or just type in molecular hydrogen, IR, Spitzer, arxiv, and a whole lot of papers come up.


Also, suppose the molecular hydrogen is in small cold dense clouds on the order of 1AU. If we looked for absorption (e.g. FUV) along some line of sight, what would be the probably of detecting one?

Pretty good (http://arxiv.org/abs/astro-ph/0003132) actually, but it's not just IR. Oh, here's another (http://arxiv.org/pdf/1011.2951v1), especially note the conclusion on H2[/url]. And if you get some extra free time, here (http://arxiv.org/cits/astro-ph/0003132) is a list of papers that reference that first one.


The papers about extreme scattering events indicate that large amounts of H2 could exist in that form and yet only be detected rarely by it's effects on eclipsed sources like QSOs and pulsars. Is it impossible for very cold clouds of H2 to be stable, or continuously form? If so, how do stars form amid all the radiation from existing stars?

I'm willing to bet that if you took some time to look, you might find out answers to all those. It took me all of ten minutes to find those papers and list of papers above.


So I'm skeptical that it's a slam dunk that H2 is not there because we haven't seen IR absorption.

It's not just IR absorption. But we have seen IR absorption, we just haven't seen enough of it. See above.


I don't think you can sweep away the possibility that easily.

And your support, other than supposition, incredulity, and demanding we provide evidence for a negative, is what exactly?

As I said, you simply don't seem to know, what you don't know. If you want to ask questions fine, but to me it appears that you don't like the answer you are being given and are trying to push molecular hydrogen as the needed mass for rotational curves. Which sounds ATM.


There are lots of papers in the literature that explore alternative theories of galactic dynamics. Cooperstock, a GR expert, claims that analysis of galactic rotation using Newtonian approximations leads to incorrect results and that GR theory must be applied. He concludes in this paper (http://arxiv.org/abs/astro-ph/0610370):

We have seen that the non-linearity for the computation of density inherent
in the Einstein field equations for a stationary axially-symmetric pressure-free
mass distribution, even in the case of weak fields, leads to the correct galactic
velocity curves as opposed to the incorrect curves that had been derived on the
basis of Newtonian gravitational theory.

If he is correct, then there is no need at all to propose large amounts of missing matter to explain the rotation of galaxies. Isn't it quite important to investigate this possibility? He is in effect saying, we did the gravitation analysis wrong in the first place.

You want to explain why they derive their galactic density from the rotation curves? Shouldn't they they take the density (cause after all, there is no missing matter, right?), and then find what the rotation curve is? Or how about explaining what's wrong with this this (http://arxiv.org/pdf/0811.1478v1) objection? Or maybe this one (http://arxiv.org/pdf/astro-ph/0602519v3). Or, how about this (http://arxiv.org/pdf/astro-ph/0508377v2) and this (http://arxiv.org/pdf/astro-ph/0510750v1). How about explaining why they haven't addressed micro-lensing in their papers? Or how do galactic cluster motions agree with their GR calculations?


When you tell me that everything is figured out and that all alternative possibilities have be thoroughly investigated and ruled out, I am a bit skeptical.

And color me skeptical if someone starts throwing out scatter-shot objections, without obviously reading the pertinent papers.


I'm well aware of the Bullet Cluster. It seems to indicate that in a cluster collision the diffuse gaseous matter collides, but the dark matter does not. Could the dark matter be small clouds of H2? What is the probability that they would collide? (I don't know.)

And H[[sub]2 doesn't collide, why? Intuition?


On the other hand, there is the Train Wreck Cluster that does not appear to be consistent with what is seen in the Bullet Cluster.

And, that is a problem why? Did you read the original paper on the Train Wreck Cluster? If so, then you know why that probably isn't a problem and you are intentionally ignoring the conclusions. If you did not read it, then why even comment on it?


I am aware of micro-lensing studies but not familiar the scope of the conclusions that can be drawn from the observations so far.

And yet, you are more than willing to insist that H2 can be a replacement for dark matter.


I'll read the paper. Thanks for the reference.

Your welcome. I've given you links to a few more.

Tensor
2011-Oct-30, 06:33 AM
Should we see who's stack of papers weighs more or should we evaluate each paper for what it is? :confused:
Evaluate each one. But, you don't think there's anything wrong with pointing out you should read papers that support and criticize the one paper you read, do you?


Yes, it's just one paper, but what's interesting about it is that an expert in GR is asserting that the Newtonian approximations that are employed to argue for missing mass are just plain wrong. That's a very significant paper. On the face of it is not speculative at all (no missing matter, no exotic particles, no unusal claims about the distribution of matter in galaxies and no new physics). It is simply applying the accepted theory of gravity (GR) more rigorously by not relying on the Newtonian approximation which we already know is wrong under some circumstances (e.g. orbit of Mercury).

snip...

It would be great if I could evaluate his conclusions directly, but I can't because I don't have the depth of understand of GR and mathematics needed.

So, how exactly did you determine it was a very significant paper? How did you know it was correct? How did you know the criticisms were wrong? How did you know Cooperstocks rebuttals to those criticisms were correct?


Perhaps you do and you can explain exactly why he is wrong or identify the 10 to 20 papers that prove him incorrect. :neutral:

How about you explain exactly why he is right, in the face of so much criticism? But, I'll start with the fact that they used the rotational curve to determine the mass that should go into the equations that determine the rotational curve. You see something wrong with that? Or how about explaining how their calculation explain the micro lensing, since they don't and haven't. Funny how you so strongly want us to explain exactly why dark matter is wrong, but you intuition is fine for explaining why everything else, but dark matter, seems to be right.


Don't imply that I'm lazy. I am reading papers as best that I can understand them. The papers are published by competent scientists, not crackpots.

and so are the rebuttals.

TooMany
2011-Oct-30, 03:52 PM
It always has been my understanding that the difference between the Newtonian approximation and the GR calculation is almost vanishingly small at the velocities and energy levels here, and that the discrepancy between the orbital velocities in the outer parts of typical galaxies and what we expect from the mass inferred from starlight we can see is very large. What does this author say that is contrary to my line of thought?

That's what I thought too. But according to the papers that assumption is wrong. The author explains in a qualitative way that the result is because the mass is distributed and self-gravitating (like at gas when looked at from the galactic scale). The details of the mathematics are way beyond me. I believe he first published this finding in 2006 and has since refined his models and answered his critics. This is the most recent paper I've found submitted in December 2010. Except for the math, it is easy to read and very direct in it's assertions.

General relativistic dynamics applied to the rotation curves of galaxies (http://arxiv.org/abs/1101.3224v1)

Here are some other papers cited:

[1] F. I. Cooperstock and S. Tieu, astro-ph/0507619.
[2] F. I. Cooperstock and S. Tieu, astro-ph/0512048.
[3] F. I. Cooperstock and S. Tieu, Mod. Phys. Lett. A. 21, 2133 (2006).
[4] F. I. Cooperstock and S. Tieu, Int. J. Mod. Phys. Mod. Phys. A. 22, 2293
(2007).

This is pretty interesting because if true it would eliminate a lot of mysteries about galaxies and confirm GR (as opposed to MOND or MOG). I believe he is extending his results to the case of clusters but I'm not too clear what the status of that is. I'll have to reread the papers.

StupendousMan
2011-Oct-30, 06:44 PM
Cooperstock's assertion is that if one considers a self-gravitating fluid which is rotating, and applies general relativity in a proper manner, one will find an extra term in the field equations which introduces behavior which does not appear under a Newtonian treatment of gravity.

This assertion would invalidate decades of work by many astronomers and physicists. It cannot be tested easily on Earth. It does claim to introduce a new region of parameter space in which GR differs from Newtonian gravity -- a region in which all the gravitational forces are very small.

I'm not a GR theorist, so I can't comment on the claim itself. There have been some comments on this theory from experts, to which Cooperstock and colleagues respond in their more recent papers. I know of no other astronomers outside the Cooperstock group who have accepted the claim.

Cooperstock suggests that measurements of galactic rotation curves at different heights above the plane of a galaxy would distinguish between his models and the models containing Newtonian gravity and dark matter. These measurements would be difficult to make, but not impossible, for a relatively small number of relatively nearby galaxies. Cooperstock asks astronomers to make these measurements, but, as far as I know, has not applied for time to do so himself.

I hope that others will correct me if I have misrepresented things in this posting.

In my opinion, Cooperstock has not made a convincing case.

TooMany
2011-Oct-30, 10:28 PM
Putting words in your mouth?

You want to explain how I put words in your mouth?

No I don't because it will just lead to an emotional debate. The statements I made and you quoted accuse scientists of being human.


He imagined that the sun was made of iron because it was green in a picture. You imagine that there has to be a large cold component because there is a large faint part of something else in a picture. Exactly how are they different, they're both intuitive based, not observation based, right? I also said "What are the numbers? Give us the calculations, the simulations, the observations."

I admit it's just a subjective impression. In the case of the sun, it's very well understood what it's made of. The cold parts of galaxies are much more difficult to observe, don't you agree?


I'm just reading the words you wrote. Seeing the comments about scientists, without qualifiers, but I'll be happy to drop it.

Agreed.


Typical for wikipedia, which usually isn't the greatest as a source for anything scientific. Unless, of course, you use it as a starting point to look at the papers it references. Molecular Hydrogen has gotten much easier to detect since Spitzer went up. For instance here (http://arxiv.org/pdf/1005.3832) or just type in molecular hydrogen, IR, Spitzer, arxiv, and a whole lot of papers come up.

Wikipedia was an easy place to find a brief statement although it was not very detailed. I have seen better descriptions of the detection problems in papers.

I skimmed the above paper. It's about detection (in energetic star-burst galaxies) of H2 warm enough to radiate in IR from rotational emissions. Is it really all that relevant to the issue of detection of very cold clouds of H2?


Pretty good (http://arxiv.org/abs/astro-ph/0003132) actually, but it's not just IR. Oh, here's another (http://arxiv.org/pdf/1011.2951v1), especially note the conclusion on H2[/url]. And if you get some extra free time, here (http://arxiv.org/cits/astro-ph/0003132) is a list of papers that reference that first one.

I'm willing to bet that if you took some time to look, you might find out answers to all those. It took me all of ten minutes to find those papers and list of papers above.

I'll have a look at these. It's fairly easy to find papers that might be enlightening, it's much harder to digest them all and evaluate their conclusions. Sometimes it's even hard (for a layman) to discern exactly what the conclusions are.:) Quite honestly I have been looking for definitive papers that would exclude all forms of H2 from contributing significantly. I did see a quote from a cosmologist claiming that FUSE had ruled out large amounts of H2 (by looking for FUV absorption lines) but as yet I haven't found a paper that asserts that.


It's not just IR absorption. But we have seen IR absorption, we just haven't seen enough of it. See above.

And your support, other than supposition, incredulity, and demanding we provide evidence for a negative, is what exactly?

Who is this "we". Would you care to explain your affiliation and motive for responding in this way? What demand are you referring to?


As I said, you simply don't seem to know, what you don't know. If you want to ask questions fine, but to me it appears that you don't like the answer you are being given and are trying to push molecular hydrogen as the needed mass for rotational curves. Which sounds ATM.

No that's not exactly what's happening here. I don't see your offered answer (if you are referring to IR detection) as conclusive because it's used to detect warm H2, not cold. I'm not dismissing your response as untrue. I just don't yet see it as conclusive.

What is happening here is that I'm being skeptical about the existence of exotic CDM. I have read papers that offer possibilities for the existence of large amounts of hydrogen in cold clouds. Now you are threatening to marginalize the issue by declaring it ATM?

Can you explain what this "mainstream" is. Is it perhaps LambdaCMD as described by e.g. Ned Wright? Is then any scientific paper that does not fully support LambdaCDM "Against the Mainstream"?

Pardon me, but who the heck are you to declare what's correct and what's not? You think there is no debate in the scientific community about the existence of exotic dark matter? Or you know exactly who's right and who's wrong? Is that it?


You want to explain why they derive their galactic density from the rotation curves? Shouldn't they they take the density (cause after all, there is no missing matter, right?), and then find what the rotation curve is?

They explain their reason for that approach in the paper. From my point of view, we have measured the rotation curves (as far out as we can measure them from the galactic centers), but we are trying to determine what distribution of matter explains the rotation. We know more about the rotation curves than we know about the galactic density distribution. Why would we start with some speculative density distribution? We are trying to explain the rotation, are we not?


Or how about explaining what's wrong with this this (http://arxiv.org/pdf/0811.1478v1) objection? Or maybe this one (http://arxiv.org/pdf/astro-ph/0602519v3). Or, how about this (http://arxiv.org/pdf/astro-ph/0508377v2) and this (http://arxiv.org/pdf/astro-ph/0510750v1).[QUOTE]

I need some time to examine these papers. I did read the first paper you sight a couple of days ago. I have to admit I did not understand what point they are trying to make about Post-Newtonian methods. I had to look it up. Here's what I found as a definition in Wikipedia:

[QUOTE]Post-Newtonian formalism is a calculational tool that expresses Einstein's (nonlinear) equations of gravity in terms of the lowest-order deviations from Newton's theory. This allows approximations to Einstein's equations to be made in the case of weak fields.

I think the whole point of Cooperstock's papers is that these approximations do not give accurate answers in the context of a gaseous matter distribution. He repeatedly states that the non-linearity is important. I'll try to decipher the paper about Post-Newtonian again and have a look at the others.


How about explaining why they haven't addressed micro-lensing in their papers?
Because it's not relevant to their papers. They are not arguing about mass contributions from MACHOs.

BTW I did read the micro-lensing paper. Although the events are very rare, they do conclude conservatively that no more than 20% percent additional mass can be accounted for in the "halo". However, the conclusions seem to be drawn using the LMC and SMC as the background sources. Both galaxies appear to be well away from the plane of the Milky Way, so it seems to me that the conclusions can only apply to masses at altitudes significantly above/below the galactic plane. That's not where one would expect to find a lot of matter in a spiral galaxy (look at photos of edge-on spirals). (I haven't been able to find the exact altitudes relative to the MW plane, but in a wide angle picture of the night sky they appear to be substantially out of a line of sight through the MW plane. Maybe they wouldn't even be visible if they were in the plane?)


Or how do galactic cluster motions agree with their GR calculations?

I think the authors are looking into that. Besides it's not really relevant specifically to the issue of galactic dynamics. In other words, you can solve one problem without necessarily solving the other.


And color me skeptical if someone starts throwing out scatter-shot objections, without obviously reading the pertinent papers.

You will define which papers are "pertinent"? The ones I've indicated are not pertinent?

No matter how well LambdaCMD matches the CMB, cluster behavior or large scale evolution, we must also explain galaxies. Luckily we have at least 300 close by ones that we can study. The big questions are, 1) is exotic CDM really consistent with what we can observe in nearby galaxies and 2) are there alternative explanations that do not require exotic CDM? Why ask these questions? Because there is evidence that exotic CDM does not fit what we observe (in galaxies) and the proposed form of matter is as yet undiscovered. (You are probably aware that LHC hasn't found exotic DM yet and is getting close to excluding the most popular models for these particles.)

Those kinds of questions are what "mainstream science" is all about.


And H[[sub]2 doesn't collide, why? Intuition?

Yep, 1AU is infinitesimal on the galactic scale (~10^-10), let alone on the scale of a cluster. If the clouds are dense, there would not need to very many. That's a hand waving argument that I can try to back up with some calculations.


And, that is a problem why? Did you read the original paper on the Train Wreck Cluster? If so, then you know why that probably isn't a problem and you are intentionally ignoring the conclusions. If you did not read it, then why even comment on it?
(My emphasis.)

No I don't know much about either cluster collision, but a quick look showed that the Train Wreck was not as nice a package as the Bullet Cluster that's been so highly touted. That's the reason I brought it up. It shows some dark matter in the middle whereas in the Bullet Cluster it is separated. (Admittedly I know very little about it.) Which is "the original paper on the Train Wreck Cluster"? I suppose there are lots of papers on it.

Facts and conclusions are different things. Agreement with facts does not always result in agreement with conclusions. That's because to some extent conclusions are based on assumptions and facts are incomplete.


And yet, you are more than willing to insist that H2 can be a replacement for dark matter.

I willing to believe that it's an open possibility. 90 percent of the nuclei in the universe are protons (H). It's what the stars are made of. Looking for more to explain missing mass is just, well sensible. It's very hard to prove that something does not exist. A fact that works in favor of exotic CDM. I'm not claiming to have proved that exotic CDM does not exist. Are you claiming to have proved that a very substantial hidden amount of H2 does not exist?


Your welcome. I've given you links to a few more.
Thanks for the links, the literature is vast.

TooMany
2011-Oct-30, 11:31 PM
Cooperstock's assertion is that if one considers a self-gravitating fluid which is rotating, and applies general relativity in a proper manner, one will find an extra term in the field equations which introduces behavior which does not appear under a Newtonian treatment of gravity.

This assertion would invalidate decades of work by many astronomers and physicists.

Correct. It's a very bold assertion and, if proved incorrect, he will have major egg on his face. He's sticking his neck out since 2006 about the issue. If you read what appears to be the latest paper, you'll notice he claims to have addressed all formal criticisms and, as yet, heard no further.


I'm not a GR theorist, so I can't comment on the claim itself. There have been some comments on this theory from experts, to which Cooperstock and colleagues respond in their more recent papers. I know of no other astronomers outside the Cooperstock group who have accepted the claim.

Yes, I have not seen much said about this. I imagine it requires deep expertise in GR to refute his claims.


Cooperstock asks astronomers to make these measurements, but, as far as I know, has not applied for time to do so himself.

I'll check, but I don't think he's an astronomer. I think he's a physicist so naturally he would want those with expertise to do the observations. I don't think Einstein measured Mercury's orbit or the positions of stars during an eclipse.


In my opinion, Cooperstock has not made a convincing case.

No offense I hope, but it's a theoretic argument about the application of GR so far. If your not an expert, your opinion is not very convincing.

Myself, I am hopeful, because it would be a major breakthrough concerning an issue that has been puzzling scientists for decades. The implications could be far reaching. I think there has been at least one independent supportive investigation. Shouldn't this be easy to blow out of the water? Just a question of whether the math is right or wrong to decide whether it should be taken seriously.

StupendousMan
2011-Oct-31, 12:30 AM
No offense I hope, but it's a theoretic argument about the application of GR so far. If your not an expert, your opinion is not very convincing.


No offense taken. I am certainly not an expert on GR.



Myself, I am hopeful, because it would be a major breakthrough concerning an issue that has been puzzling scientists for decades. The implications could be far reaching.

No offense, but since you're not an expert, your opinion doesn't really matter to anyone but you.

Cougar
2011-Oct-31, 01:50 AM
It always has been my understanding that the difference between the Newtonian approximation and the GR calculation is almost vanishingly small at the velocities and energy levels here, and that the discrepancy between the orbital velocities in the outer parts of typical galaxies and what we expect from the mass inferred from starlight we can see is very large. What does this author say that is contrary to my line of thought?

That was exactly my thought. Our solar system is orbiting the galaxy center at about 220 km/sec. Assuming a roughly circular orbit, that velocity defines the necessary mass interior to our orbit, hence our depth in the galaxy's gravitational well. As Hornblower says, neither of those things call out for any significant relativistic correction. The difference from the Newtonian approximation would be small. I would note that the "mass inferred from starlight" already incudes all the gas and dust that we do know about. I mean, did you know that the clouds of gas near the center of our galaxy contain enough ethyl alcohol, aka vodka, to fill more than 10,000 goblets the size of earth?

Anyway, it still comes up way short. As Rocky Kolb (http://en.wikipedia.org/wiki/Edward_Kolb) put it:




"We find that the galaxy has a much larger mass than the sum of all the stars, dust, and other things we "see." The shortfall is not just a few percentage points, but most of the mass of our galaxy seems to have been left unaccounted."



Cooperstock's assertion is that if one considers a self-gravitating fluid which is rotating, and applies general relativity in a proper manner, one will find an extra term in the field equations which introduces behavior which does not appear under a Newtonian treatment of gravity.

As I understand it, in effect, the "gravitational field" in GR imparts an additional bit of gravity just from the field itself. But again, I expect the effect of this calculation is going to be even more negligible than the other GR considerations.


I did not intend to make a derogatory comment about scientists. I was just pointing out that they are human.

That's also a good reason to realize that a single paper should typically be taken as still highly questionable. Remember when Garrett Lisi published his "An Exceptionally Simple Theory of Everything"? (http://en.wikipedia.org/wiki/Antony_Garrett_Lisi) He's a brilliant mathematician, but his model apparently had a few poor matches to the 'real world'. But when several papers come out confirming different aspects of a claimed finding, especially when they're from independent measurements, those are some of the qualifications for mainstream candidacy.


I have great respect for scientists.

I'm not a scientist. I just read a lot. I am constantly amazed at how absolutely clever are our contemporary scientists. Most scientists deserve the respect of an assumption of cleverness.

TooMany
2011-Oct-31, 03:08 PM
No offense taken. I am certainly not an expert on GR.

No offense, but since you're not an expert, your opinion doesn't really matter to anyone but you.

Agreed. I certainly cannot say whether he's right or wrong.

I'm reading another paper (http://arxiv.org/PS_cache/astro-ph/pdf/0602/0602519v3.pdf) (pointed out by Tensor) that strongly supports the claim that GR analysis makes a substantial difference. However in this paper, they conclude that galactic mass is overestimated by only 133% using Newtonian analysis compared with GR. So they argue that exotic matter is still needed. They claim that Cooperstone's analysis is "unphysical" because it does not include pressure. There are many assumptions involved that I cannot evaluate. However one that jumps out a bit is found in this fragment concerning a parameter β which is key to the Newtonian overestimate in their analysis:


However, it is important to realize that globally β can only be chosen once. Because usually one requires consistency
between Newtonian and GR predictions in the region where luminous matter dominates, i.e., in the linear regime (V ∝ r), let us now fix β such that...

Sounds a little like they are waffling on using GR, but I could be misinterpreting.

Grey
2011-Oct-31, 03:13 PM
Grey,Sorry I haven't responded earlier. I didn't have time to log on over the weekend. I'll respond to this, and then try to catch up with the rest of the thread. :)


Can you site a reference that claims that baryonic galaxies are not possible, based on mechanics alone? :question:I haven't had much success finding on online article. I'll keep looking around, but I'm not optimistic, because it's not really news. We've known for quite a while that none of the forms of baryonic matter that we've been able to think of can account for all of the mass we can see is in a galaxy from its gravitational effects. Note that this is true from multiple lines of evidence: galactic rotation curves, cluster dynamics, gravitational lensing. They all confirm that most galaxies have a lot more mass than we can see. Some of that is very definitely just things like gas clouds, dust, massive objects like rogue planets, brown dwarfs, or even micro black holes. These latter categories were initially promising, because like stars, small numbers of largish objects are essentially collisionless. But as Shaula notes, if there were a bunch of rogue planets, we'd expect to see microlensing events as they pass in front of background stars. We've done studies, and we do indeed detect those microlensing events, and that lets us constrain how much mass exists as MACHOS (MAssive Compact Halo ObjectS). It's not anywhere near enough to account for all the extra mass.

Similarly, clouds of molecular hydrogen would block starlight (as you've noted, such a cloud has to block it, because if it doesn't, starlight can penetrate the entire cloud, heating it up and ionizing it, and then it's no longer something we can't see). We know such things exist, but we can constrain how much mass is present in that form. Dust has the same issue. So we add up the maximum amount of mass present in all of these forms. There's a lot of it, it turns out, in many cases there appears to be more dark baryonic matter than there is luminous matter. But it's still usually nowhere near the mass we need to account for the gravitational observations.


Your argument is not entirely convincing. We live in a 4-5 billion year old solar system and yes 99% percent of the matter is dead center because it condensed into a star, but the rest of the matter in the system has not collapsed into the sun. We have planets, we have an asteroid belt, we have a Kuiper belt (all of which are stable) and possibly an enormous Oort cloud.Your argument here doesn't make much sense. The solar system is exactly the kind of mass distribution we expect from a collapsing cloud of gas, and it's also exactly the kind of mass distribution that won't work to explain a galactic rotation curve. As you note, nearly all the matter ends up in the center, with the rest arranged in a rough disk orbiting the center. The visible matter in the galaxy is in exactly this sort of arrangement as well, although less concentrated (I believe that something like half the visible matter int eh galaxy is concentrated in the central bulge). If the dark matter, whatever it may be, follows a distribution that's even roughly similar to the visible matter, we'd end up with a nice Keplerian distribution of orbital velocities, just like we see in the solar system. Here, if you look at the speeds of the planets, there's a sharp drop in velocity as you move farther from the Sun. That's exactly what astronomers expected to see for the galactic motion.


Some ongoing processes prevent all gas from collapsing into the center or into stars in spite of the passage of at least 12 billion years. The most obvious thing that keeps spiral galaxies from contracting into steep density profile is their angular momentum, the same reason that the planets don't plunge into the sun.Well, of course. Nobody is suggesting that all of the matter of the galaxy should spiral in and form a giant black hole. The flattened disks we see fit well with what we expect.


The density profiles that you refer to are based on visible matter. What about the normal matter that we cannot easily see? How do you know that cold H2 is not there if you cannot detect it? I've always found the assumption strange that much of the matter in galaxies is visible and that there is no larger portion that is too cold to glow.We absolutely know there is some cold molecular hydrogen out there. If there were enough of it to account for the gravitational observations, though, it would dim starlight more than observations indicate. And that doesn't even address the question of why it should have a mass distribution that's so different from that of visible clouds of gas.


Cold lumps of matter in the Kuiper belt have only recently been detected because they are so dim. Now we know that about 20 to 200 times as much mass exists there than in the asteriod belt. This cold dark matter is only 20AU to 50AU from us. What if it were light years away and not near a star to illuminate it? Would there be much chance of detecting it?Nope, but we'd see it through microlensing incidents. And we do see such things (since we're observing them through microlensing, we can't really tell exactly what they are, just how much mass they have). Just not enough to explain the galactic rotation curve.


One of the cool things about the non-baryonic CDM theory is that DM behavior is rather constrained because it doesn't interact in a way that can shed kinetic energy or affect matter much besides helping it clump. So any departure from it's predicated behavior must be presumed to result from transitory (dynamically unstable) conditions, like mergers. That should make it relatively easy to verify or disprove. Because baryonic matter interacts, fuses (in stars), has phases, charge, chemistry, a spectrum of masses etc., it's behavior is much more complicated and not entirely figured out at the level of galactic dynamics.Yes indeed. That's one of the reasons it's still around as the leading model. It really does work well to explain many of the things we see. There certainly are some cases that are still a little mysterious. You are correct that someone may be able to explain those apparent anomalies through some dynamic effect.


Here's an interesting question I'll try to research. Do all galaxies studied require large amounts of dark matter to explain their rotation curves?No, there are some galaxies that seem largely devoid of dark matter. Conversely, there appear to be cases where there are galaxies which have a very large amount of ark matter, but almost no stars or gas. This lends support to the notion that the issue can't be resolved by some modification of the gravitational laws. If we were just wrong about gravity, that shouldn't vary from galaxy to galaxy. But if there's some other stuff around, it actually makes sense that some galaxies would have more and some less.

Reading ahead a bit, I see that you and Shaula brought up the Bullet Cluster. This is an interesting case study, since it supports the notion that dark matter is largely collisionless. That works well for some weakly interacting particle, and it would even work well for rogue planets or some such, but it doesn't work at all well for clouds of gas. If there were numerous invisible clouds of gas in the Bullet Cluster, those should exactly like all the other clouds of gas we see there: they collide, heat up, and slow down.

Physicists and astronomers tend to be conservative. They didn't come up with the CDM model because they thought it sounded cool. They came up with it because they tried explaining the observations with lots of more mundane things, and weren't able to make them work. But it's not really that outlandish to suggest that there's an unknown particle that doesn't interact through the electromagnetic or strong force. We know one class of particles just like that: neutrinos. Dark matter can't be made of neutrinos (at least not all dark matter) because they're so light that they move quickly if you give them any energy at all, so they tend to disperse rather than clumping. But there's no particular reason that a similar but more massive particle might not exist. It was 26 years after the neutrino was first proposed that someone finally detected one.

Grey
2011-Oct-31, 03:27 PM
Yep, 1AU is infinitesimal on the galactic scale (~10^-10), let alone on the scale of a cluster. If the clouds are dense, there would not need to very many. That's a hand waving argument that I can try to back up with some calculations.I actually think this would be a really good plan. I think you might learn quite a bit if you figured out an appropriate mean density, and then calculated how many clouds on the order of 1 AU in size (and with what distribution) would be required to account for the missing mass.

Cougar
2011-Oct-31, 04:43 PM
I'm reading another paper (http://arxiv.org/PS_cache/astro-ph/pdf/0602/0602519v3.pdf) (pointed out by Tensor) that strongly supports the claim that GR analysis makes a substantial difference. However in this paper, they conclude that galactic mass is overestimated by only 133% using Newtonian analysis compared with GR. So they argue that exotic matter is still needed.

As you say, that paper makes a number of assumptions that are not easily evaluated, and they're working on a toy galactic model. But accepting the assumption of cleverness, they seem to know what they're talking about. And using their methods and assumptions, they find that the Newtonian approximation yields an expectation that is 1/3 greater that the GR calculation operating on a perfect fluid. That's certainly more than I would have expected, but it is still about 15 - 27 times too small to account for observed galactic dynamics. This treatment, while interesting, remains a far cry from solving the dark matter 'problem'.

TooMany
2011-Oct-31, 06:12 PM
Here's a news article (http://www.sciencedaily.com/releases/2008/09/080919075530.htm) containing an interesting Hubble photo of a very unusual coincidence in which one steeply-inclined spiral is back lit by another much larger spiral.

If you enlarge the photo and look very carefully to the left of the foreground galaxy, you will see that dust lanes in the smaller galaxy extend to a radius about 2-3 times the radius of the visible part of the galaxy. Notice that to the right, where the smaller galaxy is not back lit, no dust lanes are visible. I wonder how much mass that dust and the attendant gas represents. Moreover, this dark matter is only visible due to dust content. If it were just cold H2/He, it would be transparent.

Of course this is just one spiral galaxy but interesting nevertheless.

TooMany
2011-Oct-31, 06:34 PM
That's certainly more than I would have expected, but it is still about 15 - 27 times too small to account for observed galactic dynamics. This treatment, while interesting, remains a far cry from solving the dark matter 'problem'.

Why do you just assume that their analysis is correct and Cooperstone's is not? Where did you get 15-27 times too small when baryonic matter is currently thought to be about 20% of the total mass? Here's what I get from their very conservative admission - baryonic matter is now 26% of the predicted mass.

That is of course assuming that the baryonic estimate is correct, that their analysis is correct and Coopstone's is not. See my previous post for a photo of some baryonic dark matter.

Cougar
2011-Nov-01, 01:11 PM
Where did you get 15-27 times too small

Well, check me on this, but I've seen a few different estimates for the amount of dark matter associated with the Milky Way. One conservative estimate has 5 times more dark matter than baryonic matter. Another stated 9 times more dark matter. (I notice that wiki puts it at 10 times more dark matter.) The Balasin and Grumiller paper concluded that calculating via Newton yielded 0.3 more gravitational effect than the GR calculation. 5 times more dark matter is about 15 times more than the figure in the Balasin and Grumiller paper. 9 times more dark matter is about 27 times more than the Balasin and Grumiller paper figure. (OK, that's actually 30 times more - I 'rounded' .3 up to an even 1/3 to get 27. Math/poetic license. At least that's not off by 23 orders of magnitude. :rolleyes:)

The fact remains, working out our galaxy's dynamics using the Einstein field equations falls way short of explaining what the scientific community have come to conclude is the effect of some type of dark matter permeating the galaxy.

TooMany
2011-Nov-01, 04:11 PM
The fact remains, working out our galaxy's dynamics using the Einstein field equations falls way short of explaining what the scientific community have come to conclude is the effect of some type of dark matter permeating the galaxy.

Cougar, that is not a fact, that is a conclusion from the paper you refer to. You choose to believe that paper, perhaps because it continues to support the theory that most astronomers are committed too.

Cooperstone first published two years prior to that paper and has published two years since. Who has more expertise? I don't know, do you? I mentioned a key statement in that paper that they used to arrived at their conclusions: "usually one requires consistency between Newtonian and GR predictions in the region where luminous matter dominates[/I]. That statment makes me wonder if their analysis is correctly applying GR.

Here's a question for those who are GR experts. I suppose there are differential equations that specify GRT precisely. These equations are very difficult to work with except in very simply cases (e.g. point mass and uniform distribution). I believe that both papers use simplifications that allow them to produce an analytic solution (equations). Such simplifications can always be held in doubt.

Would it be possible to do the following instead? Build a computer model of a galaxy as a gaseous fluid. Subject the model to the known rotational curves and the known distribution of baryonic matter. Then apply GR to this distribution of matter to check if the model is stable (over time). If it is not, then, using "sensible" conjectures about the distribution of unseen baryonic matter, adjust the model until a simulation using GR is stable. The idea is too eliminate approximations of GR in the analysis. Of course there would still have to be an underlying model of the galaxy as a distribution of gas. But we might derive a much more convincing answer to whether a baryonic galaxy without exotic CDM is possible within the GR theory of gravity and how much (if any) undetected baryonic matter is required.

antoniseb
2011-Nov-01, 06:39 PM
Cooperstock didn't discuss strong gravitational lensing, or the Bullet Cluster. Instead he just decided to test out a properly tuned Bessel function as a replacement for a real General Relativity solution. His paper shows no genuine simulation. I take him to be less credible than Milgrom, who at least tells you up front about his fudge factors.

TooMany, If you want to take this further, please assert something in the ATM section and defend it. Here you are using ATM rhetorical techniques to put an ATM idea in mainstream... I point especially to your statement that you don't know if Cooperstock has more or less knowledge about the distribution of baryonic matter in galaxies than all of the mainstream published astronomers.