Direct detection of dark matter tendrils?

ivorax

Hey folks, so perhaps some of you may have heard/read of Dietrich, et al, recent paper on dark matter "tendrils" being "directly detected" between the galaxy clusters Abell 222 and Abell 223 (see a synopsis by Nature here: http://www.nature.com/news/dark-matt...vealed-1.10951).

My question is with regards to the specific nature of the detection. What makes it, in particular, a DIRECT detection of dark matter? I thought that direct detection could only be achieved by experiments--not through the inference of missing mass (no matter how robust their methodology is).

Can someone clarify this?

Thanks

SpaceTiger

"Direct detection" of dark matter usually implies that detection and identification of particles/objects that make up the dark matter, so in that sense the article's wording is misleading. Also, I don't think the authors are making the claim that this system is useful for distinguishing dark matter from modified gravity theories.

Rather, the main result here has to do with the distribution of the mass in the universe. Cosmological simulations have long predicted that dark matter will align itself into a "cosmic web" of filaments by the present day, with galaxy clusters tending to lie at the filament intersections. Although we have previously observed these filmaents in the distribution of galaxies and in the intergalactic medium, it has been difficult to observe it directly in the overall mass distribution. This new result claims the discovery of a filamentary structure in the mass distribution, which is observed through gravitational lensing.

The authors conclude that the mass in this filament is dominated by dark matter due to the fact that they do not observe enough luminous matter to account for what they see with gravitational lensing. The result is not at all surprising, but is still an important step in confirming our theoretical models of the distribution of matter in the universe.

ivorax

Thanks SpaceTiger, so I guess the wording in the Nature article is indeed misleading. Either way, as you mentioned, the point (which I seem to have missed) was that this is the first time we have observed (via indirect detection) these particular filaments in clusters of galaxies (as some of the large scale structure Bolshoi simulations--and others-- have shown before).

twofish-quant

They aren't making the claim because they are observationalists, and observationalists tend to just report what they see and let the theoreticians work out what it means, but if you can detect dark matter filaments with gravitational lensing and if those filaments don't have any visible signature, this puts a pretty big nail in the coffin of modified gravity theories.

SpaceTiger

I'm definitely playing devil's advocate here (I tend to think dark matter is proven beyond reasonable doubt at this point), but modified gravity theories have their own predictions for gravitational lensing that generally agree with observations. I don't know if they have modeled this system specifically, but I wouldn't be surprised if they can reproduce the observed lensing signature.

The statement in my previous post was intended to contrast this work with studies like Clowe et al. 2006, where they specifically observed a system that was out of dynamical equilibrium to look for spatial offsets between the visible and dark matter, and therefore demonstrate the presence of dark matter.

Also, I disagree with your statement that observers simply "report what they see." The whole point of experimental/observational science is to design your study to test a particular theory, not just to collect observations like you were collecting bugs. In this case, the observers were trying to test the cosmological simulations (a theoretical prediction) by observing a filament that was uniquely aligned such that relatively small amounts of dark matter would produce a lensing signature. Clowe et al., by contrast, were observers who were attempting to test the theory of dark matter as compared to modified gravity.

twofish-quant

There are two types of "modified gravity". Modified gravity as an explanation for dark energy and cosmic expansion, and modified gravity as an explanation for galaxy rotation curves. These results don't change dark-energy modified gravity theories, but it's a very strong argument against things like MOND.

I think it's actually the second nail in the coffin. The first nail was a few years ago when they detected the filament in X-rays. I think it *might* be possible to claim that the filament is all baryonic and to explain the lensing with modified gravity, but I really don't see how you can claim that the matter is all in "bright" sources, which sort of nails the MOND coffin. One thing about MOND is that it's a "phenomenological" theory that just looks at galaxy rotation, and things like this forces MOND to try to develop a gravity theory that covers the intergalactic medium. Just matching rotation curves isn't good enough any more.

I think we have to be careful about the term "dark matter". You can use it to mean "non baryonic matter" or "baryonic matter that isn't obviously accounted for". One curious thing about LCDM is that even if you look only at the baryonic matter, most of it is unaccounted for, and one thing about these filaments is that it suggests the likely possibility that most of the baryonic matter in the universe is outside of galaxies, and hasn't undergone star formation yet.

On the other hand, ideally you'd like to minimize the assumptions that go into reducing your observations. It's bad if you are trying to test a theory, but it turns out that you are including the assumption that the theory is true in interpreting the results, which as far as I can tell *isn't* happening here.

However, piecing together what the data shows and doesn't show can be tricky.

MihaiM

twofish-quant, MOND is not nailed, I don't understand why you try so hard to play it down - I interpret that as an irrational rejection of uncertainty, craving for simplicity and closure. But:
- MOND is not limited to the galaxy rotation curves. In its current shape, it was applied to more than that. And remember that MOND is just an umbrella term, refuting one formula doesn't mean you refuted MOND altogether.
- Intuitively, I find that MOND could explain the phenomenon occurring between the two Abel cluters, paperwork aside. Think that the light bending produced by that so-called dense filament can be the resultant of the remote gravities of the two clusters, in correlation with the stray rest of matter, eventually - and there are probably few other things to consider. I see no problem with that. The slight bending of the filament may be the result of the rotation of the two masses of matter - I'm just speculating here, lacking this data, but the rejection of this hypothesis requires it, too.

In general, these filaments look like a merging of the gravitational fields of two or more clusters of baryonic matter. Otherwise why, for instance, are these filaments always bridging these gravitationally bound heaps of visible matter on around the shortest path, and we can find none strongly disturbed, like forming a large arch, wave, or even having a free end?

It is suspicious that this so-called dark matter is always correlated with the visible matter, iincluding the Bullet Cluster; and no, it was never detected directly; direct detection necessitates completely isolating its effects from those of normal matter - it was never observed a concentration of DM in the middle of nowhere - or finding its particle. Supporters of this theory could play the card of homogenity, but first of all, at small scales - small galaxies - this DM magically vanishes! Then this argument (of homogenity) comes in contradiction with the "most direct evidence for dark matter", namely the Bullet Cluster phenomenon, where it is explained that the weaker interaction between the DM with BM than between BM and BM made them separate that easily.

I think we should keep an open mind, I'm not saying DM does not exist, just that it was imagined by man, it has no direct proof, it is not falsifiable (you can imaginary add or substract DM anywhere your heart desires to get your result) and its strict adherence to the BM is suspicious, being supported by speculative, sometimes contradictory arguments.

Chronos

I suspect particle physics will resolve the dark matter dilemma at some point.