More evidence for dark matter-another cluster collision

[marcus]

more evidence for dark matter---another cluster collision

http://arxiv.org/abs/0806.2320

this has some similarities to the famous Bullet Cluster collision
where the ordinary matter gas had collided and gotten stopped and the galaxies with their DM had passed on thru

it's reassuring to get a repeat of that

 

[matt.o]

This is not the only one - there are a number of other similar clusters, perhaps not as dramatic as the bullet cluster, but similar nonetheless. The other papers I can think of are:

http://adsabs.harvard.edu/abs/2005ApJ...634..813J"

http://adsabs.harvard.edu/abs/2005ApJ...618...46J"

and a couple in here:

http://adsabs.harvard.edu/abs/2008PASJ...60..345O"

 

[Wallace]

It's looking increasingly less likely, given these and other results, that DM can be completely explained away by modified gravity of any sort.

 

[shalayka]

It's looking increasingly less likely, given these and other results, that DM can be completely explained away by modified gravity of any sort.

I find it odd that you say this, considering that Moffat and Brownstein's theoretical work on modified gravity relies on observations exactly like those seen in the Bullet Cluster collision. Not that I necessarily agree with their model, but still... I don't think the statement is entirely accurate.

 

[marcus]

It's looking increasingly less likely,..

I find it odd that you say this,..

"increasingly less likely" is not a flat statement, I hear it more as a subjective weighing of the probabilities.

I know some Moffatt and Brownstein work. I've read papers by them and watched one of them give a seminar talk on PIRSA (the Perimeter Institute recorded seminar archive), and I certainly give consideration to the notion that modified gravity might provide an alternative explanation. But as I watch the evidence piling up and try to weigh the odds, I find that I heartily agree with what Wallace said. Also to me it is looking increasingly less likely.

there is a certain amount of play or flexibility in modified gravity models and people like Moffatt have room to adjust and adapt and try to fit each new observation as it comes in. And that is what we expect and want them to do. On the chance they might after all be right. but if it goes as I now expect, they will eventually run out of flexibility and be unable to adjust. We will see.

 

[smallphi]

Let's not forget the 'cosmic train-wreck cluster' which doesn't show the expected behavior of dark matter in cluster collisions:

http://space.newscientist.com/article/dn12497-cosmic-train-wreck-defies-dark-matter-theories.html

 

[marcus]

thanks Smallphi,
here is the article about that, published in Astrophysical Journal
http://arxiv.org/abs/0706.3048
A Dark Core in Abell 520
A. Mahdavi (UVic), H. Hoekstra (UVic), A. Babul (UVic), D. Balam (UVic), P. Capak (Caltech)
10 pages, 5 figures
(Submitted on 21 Jun 2007)

"The rich cluster Abell 520 (z=0.201) exhibits truly extreme and puzzling multi-wavelength characteristics. It may best be described as a "cosmic train wreck." It is a major merger showing abundant evidence for ram pressure stripping, with a clear offset in the gas distribution compared to the galaxies (as in the bullet cluster 1E 0657-558). However, the most striking feature is a massive dark core (721 h_70 M_sun/L_sun) in our weak lensing mass reconstruction. The core coincides with the central X-ray emission peak, but is largely devoid of galaxies. An unusually low mass to light ratio region lies 500 kpc to the east, and coincides with a shock feature visible in radio observations of the cluster. Although a displacement between the X-ray gas and the galaxy/dark matter distributions may be expected in a merger, a mass peak without galaxies cannot be easily explained within the current collisionless dark matter paradigm. Interestingly, the integrated gas mass fraction (~0.15), mass-to-light ratio (220 h_70 M_sun/L_sun), and position on the X-ray luminosity-temperature and mass-temperature relations are unremarkable. Thus gross properties and scaling relations are not always useful indicators of the dynamical state of clusters."

My take on it is: here's another cluster collision aftermath where they have now mapped where the dark matter is, and mapped where the X-ray gas is, etc. And they are trying to reconstruct what happened during the collision.
they tried to fit MOND but said it was hopeless
I guess "trainwreck" could be considered as more evidence for the existence of dark matter, giving still more information about how it behaves. Still don't know what it is or the extent of its interaction. every weak-lensing map they make of it gives additional information

and yes, it doesn't always behave the way the simple theory of its behavior says it should, apparently----so the plot thickens

 

[matt.o]

The trainwreck is exactly that - a trainwreck - it's an absolute mess! The other examples provide "cleaner" mergers and are more suited to this type of dark matter/baryonic matter separation experiment.

 

[Chronos]

Admitting to the existence of any amount of cold dark matter is a nail in the MOND coffin. The entire concept was premised as an alternative to CDM.

 

[Buckethead]

I searched to find this thread so I wouldn't have to start a new one about Abell 520, but I am baffled by something:

The Bullet cluster was presented as being all but final proof of Dark Matter being the victor in the wars with MOND and other theories. The reason was that the lensing (DM) followed the large matter and not the gas in the center of the collision. Then comes along Abell 520 and suddenly we have the opposite state of affairs. The DM now seems to be in the core. I would now have expected the cosmology community to retract their claims about the Bullet cluster being the final proof of DM, but instead have come to find that the Bullet cluster is somehow still proof wheras Abell 520 is swept away as just being evidence of some new property of DM that adds to the puzzle. I'm sorry, but this attitude baffles me. Had Abell 520 come first, the reaction to the bullet cluster I expect would be quite different! Has the momentum for DM become so strong that no one is willing to even consider that it can't be right, even in the face of contradictory evidence?

 

[DaveC426913]

Has the momentum for DM become so strong that no one is willing to even consider that it can't be right, even in the face of contradictory evidence?

I does have a fair amount of elegance.

While different manifestations of colliding clusters may need different explanations of the distribution of DM, those explanations do not require a modification to the theory itself, only to the mechanical circumstances of the clusters.

On the other hand, every time a new cluster is found, the modified gravity theory itself is needing to be adjusted.

 

[Buckethead]

I does have a fair amount of elegance.

While different manifestations of colliding clusters may need different explanations of the distribution of DM, those explanations do not require a modification to the theory itself, only to the mechanical circumstances of the clusters.

This may be true, but it seems to me that this (approaches) ad hoc science, adjusting the initial conditions or whatever of Abell 520 until they support the theory. I'm not saying that this is not necessary and it may prove fruitful, but my real complaint here is that the Bullet Cluster is a shining star, the HOLY GRAIL because it supports the theory so beautifully, and Abell 520 is seen as simply a bastard child. Shouldn't someone come up with a theory that follows Occum's Razor for both the Bullet Cluster and Abell 520?

On the other hand, every time a new cluster is found, the modified gravity theory itself is needing to be adjusted.

Just so you know, I don't think MOND is the correct theory either. I'm rooting for Mach's Principle to lead the way to finding a theory that becomes the eventual winner.

 

[Wallace]

The nice thing about the Bullet Cluster is that we are lucky in that we see the collision almost perfectly edge on, and that the two clusters involved were both spherical, relaxed clusters without significant substructure, they were of similar mass, and we see the collision on the 'first pass'. All of this means that quite simple modes for the clusters (in terms of the radial density profiles etc) are good approximations, making the modelling relatively straight forward.

Abell 520 is, on the other hand, a complete mess, with all kinds of sub-structure and the fact that we are seeing a merger that has been going on for a while, rather than the first pass of two clusters. This makes the modelling tricky and the results unclear. It just means that the Bullet Cluster is a nice clean 'lab' for testing DM and gravity while Abell 520 is a lot messier, for whatever theory you are considering.

Now to address your misrepresentations of how cosmology works. No one has suggested that the Bullet cluster a shining star or holy grail and/or that Abell 520 is somehow an anomoly to be ignored. If you wanted to look for the strongest evidnce for DM, you'd be much more likely to point to the CMB data or cluster counts than you would to the Bullet Cluster. Its more of a satisfying vindication than a cornerstone.

In addition, while one could try and play 'tweak the initial condtions' in order to try and re-produce Abell 520 specifically, it would be pretty tricky and not very illuminating IMO. Instead, what we have done and continue to do is create mock Universes given the parameters of the model we have, and then look at the kinds of objects we get. By doing mock observations you can then find whether something like Abell 520, or something like the Bullet Cluster, appear in the mock observations. So far, this kind of approach has given a lot of vindication to the standard model (i.e. DM and DE), with some issues to do with the densities of cluster cores, although modelling the complex astrophysics to get these regions correct is a challenge.

 

[Buckethead]

Thanks for the detailed response. I apologize if I came off as implying cosmologists are sweeping data under the rug which of course is not the case. Certainly I can see the enourmous amount of data being extracted and used. I just got finished reading the excellent book "Einsteins's Telescope" by Evalyn Gates and am just amazed at just how much has already been accomplished. It's truly monumental. My comments about the Bullet Cluster being a Holy Grail however are based on repeated comments I have read in books and forums. I'm sure you've read them yourself about how the "BC shows that DM without a doubt exists" and other such "nail in the coffin" comments being thrown around.

Your thoughts about Abell 520 are appreciated and I've seen some animations of the simulations you describe and can see how this applies to Abell 520 and that's a satisfactory explanation for me.

My bottom line concern still holds however in that the search for an alternate solution is waning and I'm not sure it's for a good enough reason. After all, there is still no satisfactory description of what DM and DE is or could be. What if in 20 years DM is still not detected, then what? I'm worried that by that time it will have become the true standard model and alternate theories will go the same route as alternate theories to GR.

There are other routes that have to be considered. MOND and other changes to gravity is one, but not my favorite. As an armchair physicist I would like to see some truly alternate routes being explored. My current favorite would be to try and tie in Mach's Principle (especially as a local, cluster sized phenomenon) to all this which no one seems to be exploring. Finding the true nature of inertia (why does a body resist acceleration, but has no resistance to various velocities) may in some strange way tie into this as well. We need to take everything into account to get to the bottom of this strange and wonderful area of mystery.

 

[matt.o]

As has been stated previously, A520 is not as good an experiment for testing the dark matter-baryonic matter separation when compared with the Bullet cluster, so let's not get carried away!

A520 does show a clear DM/gas separation where expected, similar to the Bullet, although there are two irregularities in the lensing structures 3 and 5 (in Mahdavi et al.'s notation). Considering structure 3, the "dark core", Mahdavi et al. offer the explanation that this may be a filament aligned along the line of sight (remember weak lensing maps show the projected surface mass density), not so unlikely given clusters form at the nodes of filaments. Along came http://adsabs.harvard.edu/abs/2008A%26A...491..379G" who used cluster member spectra to study the dynamics of A520. Lo and behold, they find two high velocity structures coincident with lensing peak 3, indicating it is very likely that we are indeed observing a line-of-sight filament which is causing the "dark core's" lensing signal.

Structure 5 is more of an issue. In Mahdavi et al. they find a significant overdensity in the stellar luminosity, but no corresponding lensing signal. Since both the galaxies and the dark matter are collisionless, they should stick together during the merger which doesn't appear to be the case here. However, http://arxiv.org/abs/astro-ph/0702649" did detect a corresponding lensing signal (although there may have been some issues with the data and analysis) which I believe has been independently confirmed by others and presented at a conference (and as yet unpublished) . I think I'll wait this out until further imaging and analyses have been done before making strong conclusions about the existence/non-existence of a lensing signal for structure 5.

 

[Wallace]

What if in 20 years DM is still not detected, then what? I'm worried that by that time it will have become the true standard model and alternate theories will go the same route as alternate theories to GR.

You mean constantly talked about and tested? The theoretical basis of gravity is far from settled and all kinds of extensions and/or modifications to GR or even complete re-writes from scratch are constantly proposed and considered. This is true both in the realms of changing gravity vs dark matter and changing gravity vs dark energy.

I think it would be hard to argue that the mix of research time dedicated to alternative to the standard framework vs working out the finer details of the standard models predictions is out of whack. Any cursory glance over astro-ph reveals a constant stream of work looking at 'non-standard' models in cosmology, and it's not as though this comes from fringe players. Some very distinguished cosmologists currently work almost exclusively on non LCDM ideas, Wayne Hu comes to mind as a good example.

In any case everyone is well aware that even the standard model is simply a parametrised ignorance, with the underlying physics unknown. The fact is that the model appears to work so well for some reason, so we'd be silly to ignore it just because we don't have a solid theoretical basis for it at this point. By working on the standard model, evidence either grows for it, demanding the theorist work even harder to explain why it should be so, or potentially small but significant deviations from model predictions emerge, which could be the crucial hint towards some theoretical breakthrough.

In any case the menagerie of theories and effort in the cosmological community is more diverse now that at any time in history, so I wouldn't worry about alternative theories being shut off. The only reason we (for instance) don't talk about MOND as much as was done 20 odd years ago is that the evidence has consistantly gone against it. But this is only one of many modified gravity models out there, and taken together the field of modified gravity is extremely active.

 

[Buckethead]

Thanks again for another very satisfying response. As an amateur I only get my information from forums, popular books and popular science programs and DM seems to be the only thing that really gets much attention in these formats. It's good to hear that so many are working on alternate theories.

Can you tell me if modified gravity theories are the only other theories being worked on or are there even more abstract theories being tossed around that don't include DM or modified gravity?

 

[Wallace]

I can't think of any other possibility on logical grounds; the problem is a mismatch between mass and gravity, so the solution must address this issue hence must either introduce mass or change gravity.

That being said, the other possibility I can think of beyond postulating the existence of some new form of matter, is the idea that DM is really just regular matter, but in some unexpected form that means we don't see it. Some possibilities that have been suggested are many small black holes, many many low mass stars (which are too small to start nuclear reactions) or cold clouds of gas. All of these have some serious problems with them on theoretical or observational grounds, for instance take the low mass stars; it is actually possible to hide enough mass to be the missing mass in galaxies in this form, but then the CMB peaks would be all wrong, since they also require a lot a DM compared to the total amount of baryons. That being said, I can recall a handful of papers on some of these ideas that have come out in the last few years, so its still something that is considered at some level.

On the gravity side, extra dimensional theories predict that gravity 'looks' different in our 4D 'brane' than you'd expect if there was only our 4 dimensions, even if the underlying gravity theory is the same, just extended to the extra dimensions. You could argue I guess that this is not really a 'modified gravity' theory, but modified dimensions, but that comes done to a matter of semantics.

 

[Buckethead]

I can't think of any other possibility on logical grounds; the problem is a mismatch between mass and gravity, so the solution must address this issue hence must either introduce mass or change gravity.

I think this statement might be more accurate if you say perceived mismatch between mass and gravity. It is possible that what we are measuring to be a mismatch from our frame of reference might from the frame of reference of the clusters in question not be a mismatch. I am basing this on the problem of Newton's bucket (which is my favorite area of study right now and is why I chose my forum name as I did). This theory works perfectly when applied to the problem of flat galaxy rotation, and works to some degree when addressing the lensing phenomenon (although there are still some things I can't wrap my brain around) and I have not yet tried to apply it to the CMB issue since I don't yet understand that problem. Do you know if there is anyone studying the DM problem from this perspective of a mismatch not really being a mismatch? I started a thread awhile back called Newton's Bucket;
https://www.physicsforums.com/showthread.php?t=223264&highlight=Newton's+bucket
which describes my idea of local rotationally or linearly moving frames of reference around large clusters causing a perceived mismatch between mass a gravity, but I'm curious to know if anyone has thought about this and either dismissed it entirely as just not working or studying it as being something worth looking into. I'd hate to waste my time on it if it just doesn't make sense.

 

[Wallace]

No, using either Einsteinian or Newtonian gravity you cannot describe galaxy rotation curves without DM. What you are suggesting is either that we are using the laws of gravity incorrectly, or that you are proposing a different gravity theory. You can't wave your hands and say 'reference frame' because both of these theories tell you how to convert between frames and how to interpret observations (i.e. how light travels between us and the galaxy). Either we are all simply making a common mistake or what you are proposing is a different theory of gravity.

Now, if we consider for a moment not DM but DE there is such a third way, i.e. that we maybe don't need to introduce more stuff (DE) or modify GR to explain the apparent accelerated expansion. The issue is that possibly the simple averaging that we do in order to model our universe by a smooth Friedmann model is invalid, and a correct averaging procedure would show that regular GR actually predicts what we observe. There a very good reasons to suspect this isn't the case, but the full calculation has yet to be done in full generality, so there is a chance that in fact we are just doing it wrong.

However, in the case of galaxies and cluster and dark matter this is not true. Apart from one paper (look for authors Cooperstock and Trieu (sp?)) that claimed a GR solution for galaxies without DM, but was shown to contain some serious errors, there is no way in which standard gravity theories could explain the rotation curves.

You are using 'reference frame' in a strange way that I can't make sense of? I strongly suspect that what your talking about indeed doesn't make sense, though it doesn't mean you're wasting your time thinking it through. If you can try and explain how your idea relates to say the Milky Way rotation curve then it might be a little more clear.

 

[twofish-quant]

Either we are all simply making a common mistake or what you are proposing is a different theory of gravity.

And by the way. That's not necessarily a bad thing. There's been a huge amount of work done to try to resolve the dark matter/dark energy problem by proposing a different theory of gravity. The problem with proposing a different theory of gravity, is that our theories of gravity seem to work in situations *without* dark matter, so most of these theories start with the idea that GR is "almost right" but will be wrong in some particular situations.

You are using 'reference frame' in a strange way that I can't make sense of? I strongly suspect that what your talking about indeed doesn't make sense, though it doesn't mean you're wasting your time thinking it through. If you can try and explain how your idea relates to say the Milky Way rotation curve then it might be a little more clear.

Also one general problem that I find when you have people new to a field is that people within the field use terms to mean something very specific, and that if you want to communicate well, you have to understand what they mean when people say something. One problem is that often the newbie has something useful to say, but is just using the language in ways that the people in the field consider incorrect.

I had a conversation with a linguist in which he was saying things that I thought were total nonsense because he was talking about "applying quantum ideas to linguistics" It turned out that he was making a pretty good point once I realized that he was really talking about "applying nondeterministic ideas to linguistics."

 

[Chronos]

Back to the drawing board for modified gravity. The biggest leg has been cut off at the pelvis, imo.