Questioning dark matter

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  • #1
nicksauce
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Recently there has been a lot of discussion in the science media about this article, and other similar ones.
http://www.physorg.com/news160726282.html

Could someone knowledgeable about this please write about what is actually going on here? I don't really trust the way this is being reported.
 

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  • #2
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We all know that Newtonian gravity isn't correct. Firstly, because tests within our solar system obey GR rather than Newtonian gravity, and secondly because galactic dynamics don't appear to be obeying Newtonian gravity. Although we refer to those second phenomena by the label "dark matter", we've really never been certain whether those phenomena are actually due to there being more more matter than what we can see, or are just due to the inapplicability of Newtonian gravity laws. Evidence of bullet clusters supports the former, but the article is just saying that some other evidence (galaxy formation modelling) favours the latter.
 
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  • #3
Chronos
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The bullet cluster is pretty much the smoking gun for DM, as is virial theorem - which requires it to account for rotational curves of galaxies. The vast majority of scientists are currently convinced DM is the correct explanation.
 
  • #4
Chalnoth
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but the article is just saying that some other evidence (galaxy formation modelling) favours the latter.
Well, it doesn't really. Galaxy formation is currently so poorly understood that it can't realistically be said to support any particular view.
 
  • #5
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“The physicists do belief that this phenomenon can only be explained if the satellites were created a long time ago through collisions between younger galaxies.”

Think of the dark matter ‘cloud’ around a galaxy as a motor vehicle and the galaxy itself as a passenger. When two galaxies collide, then the galaxies themselves get ejected and the dark matter cloud is separated. There was an image of such an empty area of deep space that lensed background stars by its gravity, like DM would. So Professor Kroupa and his colleagues ‘believe’ that the 11 dwarf galaxies they studied are such galaxies formed by collisions that have no DM. The contradiction is that these dwarf galaxies spin rapidly as though they did have DM. However, galaxies can also form directly from a cloud of gas and so would not lose their DM in any collision mergers. If the scenario one describes creates a contradiction, then most likely this is because it is based on a belief.

http://map.gsfc.nasa.gov/
http://www.nasa.gov/home/hqnews/2006/aug/HQ_06297_CHANDRA_Dark_Matter.html
http://www.uslhc.us/LHC_Science/Questions_for_the_Universe/Dark_Matter
 
  • #6
Chalnoth
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Given that galaxies are comprised of mostly empty space, I can't imagine how the stars and the dark matter could be separated in a merger. You might strip the gas from the galaxy through a merger, but I can't imagine stripping the stars.
 
  • #8
Chalnoth
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Well, a lot depends upon the specifics of the modeling here, but in any case the simple picture that you paint is, well, too simple. Basically, the dark matter doesn't collide. It passes right on through in a collision, so the dark matter being a vehicle in such a collision is just an inaccurate analogy.

Regardless, these systems are complex beasts, and the difference may just be caused by gas interactions of the galaxies with respect to the cluster medium.
 
  • #9
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Regardless, these systems are complex beasts, and the difference may just be caused by gas interactions of the galaxies with respect to the cluster medium.
You are right that this merger is complex, but there is no known plausible mechanism by which interactions with the galaxies and the ICM can cause the galaxies and dark matter to separate, whilst the ICM remains with the dark matter core. This may happen dynamically ('gravitational slingshots'), but simulations have not been able to reproduce the effect yet.

In any case, the answer is probably simpler as shown in the dynamical analysis of http://adsabs.harvard.edu/abs/2008A&A...491..379G" which leads them to conclude that the dark core is caused by a filament running along our line-of-sight.
 
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  • #10
Chalnoth
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You are right that this merger is complex, but there is no known plausible mechanism by which interactions with the galaxies and the ICM can cause the galaxies and dark matter to separate, whilst the ICM remains with the dark matter core. This may happen dynamically ('gravitational slingshots'), but simulations have not been able to reproduce the effect yet.
Simulations have yet to reproduce a whole lot of things about galaxy dynamics. And we already know of many complexities that are either poorly-understood, or that we think we do understand but lack the computing power to calculate. So if we're going to detect new physics, we really are going to have to detect it in a much simpler scenario than this. Right now our understanding just isn't up to the point that we can infer new physics out of such a complicated event.

In any case, the answer is probably simpler as shown in the dynamical analysis of http://adsabs.harvard.edu/abs/2008A&A...491..379G" which leads them to conclude that the dark core is caused by a filament running along our line-of-sight.
Wouldn't surprise me.
 
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  • #11
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Right now our understanding just isn't up to the point that we can infer new physics out of such a complicated event.
Tell me about it. Understanding these complex events is my job!
 
  • #12
Chalnoth
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Tell me about it. Understanding these complex events is my job!
Yeah. And let me say, I have enormous respect for those who attempt to delve into understanding the dynamics of galaxies and galaxy clusters. It's not my cup of tea, but those of us interested in other aspects of cosmology really depend upon you guys.
 

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