Just because you can’t see the dark matter doesn’t mean you can’t take a picture of it. Via Universe Today, here’s a press release from Johns Hopkins announcing a beautiful new image of the reconstructed dark matter density in cluster CL 0152-1357 by Jee et al. (I couldn’t find the paper online, but you can get a higher-resolution version of the picture at Myungkook Jee’s home page.) The dark matter is in purple, the galaxies are in yellow.
How do you do that? It’s not because we’ve detected some form of light coming from the dark matter. Rather, we’ve detected (once again) its gravitational field — this time, via the tiny distortions in the shapes and positions of background galaxies (weak lensing). This is a form of gravitational lensing that is so subtle you could never detect it happening to a single galaxy — it would be impossible to distinguish between lensing and the intrinsic shape of the galaxy. But if you have a large number of background galaxies (which the universe is kind enough to provide us), you can use statistics to reconstruct the gravitational field through which the light travels, and hence figure out where the dark matter must be.
Of course we’re still trying to detect the dark matter, both directly (in ground-based experiments) and indirectly (looking for high-energy radiation produced by annihilating dark matter particles), not to mention using particle accelerators to actually produce candidate dark matter particles. Over the next ten or twenty years, probing the properties of dark matter is going to be one of the top priorities at the particle/astrophysics interface.
But if you modify gravity so rotation curves come out right without dark matter, weak lensing should follow too, eh?
A little roundup of recent material for the freethinkers out there:
http://arxiv.org/abs/astro-ph/0506370
http://arxiv.org/abs/gr-qc/0511026
http://arxiv.org/abs/hep-th/0511260
Or perhaps modifications aren’t even needed, just plain old GR? The lack of response to
http://arxiv.org/abs/astro-ph/0512048
is interesting.
dear dissident,
it is not enough to explain away galaxy rotation curves, clustering, and the evolution of the universe with modified gravity, one also has to explain the structure of the accoustic peaks in the cosmic microwave background. these peaks would not have the right amplitudes were it not for dark matter amplification of the primordial fluctuations. as far as i’m aware, no modified gravity approach to date can simultaneously explain all of these features, whereas dark matter, through the so called lambda-CDM model does so very precisely. of course, sean and mark, who have worked on modified gravity models themselves (astro-ph/0410031) may have some interestnig insights on that count.
of course this is not to say that we know what dark matter is, we only know what it ain’t. and it probably ain’t going to be something that accounts for some, but not all of the features that dark matter accounts for. of course if something comes up that does account for all the above features, and then some , especially if it predicts something different from dark matter that is observable, then that would certainly be something! i’m sure many theorists are plotting away as we type…
subdoh, surely you are aware of the many problems with our current understanding of the CMB. Things like the Axis of Evil and most recently http://arxiv.org/abs/astro-ph/0510160 indicate that there’s something going on with it that’s not included in any of the models to which it’s being fitted. The support apparently lent to this or that model by such fits should therefore be taken with a large scoop of salt.
Dissident,
Why did you throw in hep-th/0511260? It doesn’t seem to have any bearing on your point. (BTW, I have some prior familiarity with it.)
Chris, it does actually; it’s just the latest in a series of papers by Reuter et.al. on their non-perturbative approach to QG, which leads to things like
http://arxiv.org/abs/hep-th/0410119
http://arxiv.org/abs/hep-th/0410117
http://arxiv.org/abs/astro-ph/0509163
I’m actually going to interview Dr. Lee for my Podcast tomorrow, so I should get more details out of him. Oh… and thanks for the link.
Dissident,
Thanks; this is fabulous stuff. The similarity of their conclusions about the sub-Planckian scale structure of spacetime to those recent arrived at in work with Causal Dynamical Triangulations (CDT) is also extremely interesting.
Been there, done that.
hi dissident,
i didn’t meant to say that our understanding of dark matter through the lambda cdm model is complete. i meant to say that it is more complete than most of the proposed alternatives. modified gravity models can explain some, but not all of the features that we observe. in the final analysis, these models do not account for more than a subset of the features that lambda cdm accounts for (to boot, galaxy rotation curves, clustering, and the structure of the accoustic peaks of the cmb).
lambda cdm, inspite of being quite an ugly hodge-podge, explains the data very well, but of course, possibly not quite, as you point out. in fact this ‘not quite’ is where one can do some real science, and i’m looking forward to finding out what the resolution of the dark matter problem is in my lifetime (i hope!).
for the record, i share the same bias as you that it probably has to do with some aspect of gravity/ extra dimensions that we do not quite understand yet, its just that i’m not conviced by anything on the table at the minute.
parenthetically, i’m not an astrophysicist, so i can’t really understand the papers you’ve refered me to, but i should say that many ‘observations’ either go away once the systematic errors of an experiment are better understood, or they get stronger as more and more eperiments are done. ‘pentaquarks’ are a good example of the former, lambda CDM is a good example of the latter. it’s ugly, no-one wants it, but it works (so far), and its borne out by many different observations working in many different regimes.
Jacques, I didn’t want to invoke you in vain, but my first thought upon seeing http://arxiv.org/abs/hep-th/0511260 was actually “aha, retort!”…
subodh: you mention “galaxy rotation curves, clustering, and the structure of the accoustic peaks of the cmb”. Modified gravity in its various shapes can easily handle the first; the second I don’t think has been analysed thoroughly yet – but dark matter is already known to have a problem with clusters, as noted e.g. right here on cosmicvariance:
http://blogs.discovermagazine.com/cosmicvariance/2005/09/05/dark-matter-and-extra-dimensional-modifications-of-gravity/#comment-2932
As for the CMB, we need to get a better handle on whatever it is that’s affecting it after its purported “last” scattering. Let me put it this way: if you have a model which you know to be incomplete, with some important contribution missing, and yet it seems to fit the data well, what will happen to the fit when you put in that missing important contribution? Will the fit get better – or worse?
P.S. Drat! The link to my old comment is wrong; it was meant to go to Marks’s post at the beginning of that thread, not to my comment in it. Here’s the correct link:
http://blogs.discovermagazine.com/cosmicvariance/2005/09/05/dark-matter-and-extra-dimensional-modifications-of-gravity/
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Then you, like me, eagerly downloaded it, and read it through.
And then, disappointed, you scratched your head and went back to whatever it was you had been doing …
Jacques,
Thanks for taking a look at these papers and saving the rest of some effort in figuring out if they make sense. Your posts are quite useful.
The continued claims that beta functions are “nonperturbative” are bizarre. Hasn’t anyone pointed out to Reuter that this is just not true?
On the other hand, if the perturbative truncation Reuter studies really did get the qualitative behavior right, is there some sort of lattice study someone could do to show that it really is true nonperturbatively? Is anyone trying?
— a. krug
I have discussed Reuter et al’s work with people (whom I otherwise respect greatly), who — despite the obvious objections — continue to be enthusiastic about it because it “agrees” with what is found in the Causal Dynamical Triangulations (CDT) approach.
Specifically, the anomalous dimension found by Reuter et al “agrees” with the CDT calculation, indicating that the spectral dimension of spacetime, ds=2, at short distances. (You’ll note that those are not, in any obvious way, the same thing, but don’t think too hard about that.)
I’ve been meaning to write a blog post discussing of CDT, for a while now. Someday, I’ll get around to it.
It’s indeed disappointing when people exploring different approaches to a problem seem to ignore each other. The public, who’s ultimately paying for it all, just might get the idea that those failing to respond have no answers…
A debate between Distler and Reuter (and Loll?), now that would be something! As would one between, say, Sean and Cooperstock & Tieu. Why not right here on cosmicvariance? Maybe the recent Krauss guest-blogging/debate invitation could serve as a model?
Dissident,
in context of the future book, I agree:)
Jacques, I will be looking forward to that discussion. I am utterly confused about both claims of two-dimensionality. One my usual confusion about measurability: is the spectral dimension a measurable quantity, does it express some property of measurment or series of measurments? it does not seem to be phrased that way. Also, RNG flow interpolating between two and four dimensions seems to me a strange phenomena, contradiciting general intuition about such flows losing degrees of freedom between the UV and the IR.
This is probably a stupid question.
Could the 2d regime be associated with (non-critical) strings ?
And even more stupid.
Could the 4d reflect some sort of Hausdorff dimension of non-critical strings?
From a simple generalized view.
For those who want to explore the greater potential of space travel, what would lensing tell you about the routes to take? Of course, I am speculating 🙂
Okay, I’ve completed the interview with Dr. Jee. You can access it here: Podcast: Dark Matter Maps