Evidence for dark matter

  • Thread starter touqra
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  • #1
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Main Question or Discussion Point

The observation on the bullet cluster collision was stated as direct evidence for dark matter.
http://apod.nasa.gov/apod/ap060824.html

But could there be a possibility that it's just some molecules that are too dark to be seen ?
 

Answers and Replies

  • #2
Wallace
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A key point about dark matter that is relavant to the Bullet Cluster result is that dark matter is conlussionless. This means that whatever dark matter is made from, it does not interact with anything, not even itself. This means two 'clouds' of dark matter can pass through each other within hinderance. Any 'normal' molecules would hit each other in this process, causing large shock waves.

In the Bullet Cluster result what you see is that the 'normal' matter in the clusters, the gas, forms these shock fronts and give of the X-rays observed, while the dark matter does not. This leads to a spatial seperation of the dark and normal matter and a big headache for modified gravity theories!

There is no way known for any molecules of normal matter to act in the way we've observed dark matter to in this cluster system, given the required mass density the gravitational lensing result implies.
 
  • #3
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Do neutrinos interact with each other?
 
  • #4
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Do neutrinos interact with each other?
Since they have no mass (or a REALLY tiny mass) they wouldn't stay in the cluster.
 
  • #5
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A key point about dark matter that is relavant to the Bullet Cluster result is that dark matter is conlussionless. This means that whatever dark matter is made from, it does not interact with anything, not even itself. This means two 'clouds' of dark matter can pass through each other within hinderance. Any 'normal' molecules would hit each other in this process, causing large shock waves.
The cross section of dark matter annihilation is not zero, and with that powerful collision, there should be interaction among them ?
 
  • #6
Wallace
Science Advisor
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The cross section of dark matter annihilation is not zero, and with that powerful collision, there should be interaction among them ?
No one has any idea what the annihilation cross section of dark matter is, apart from a very small upper limit (i.e. whatever it is it is tiny). The reason we know it must be small is precisely because we don't see any self interaction effects.
 
  • #7
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No one has any idea what the annihilation cross section of dark matter is, apart from a very small upper limit (i.e. whatever it is it is tiny). The reason we know it must be small is precisely because we don't see any self interaction effects.
If it doesn't interact with itself, nor normal matter, then, it can't be falsifiable, and hence, non-existent
?
 
  • #8
If it doesn't interact with itself, nor normal matter, then, it can't be falsifiable, and hence, non-existent
?
This isn't true. Neutrino's exist, but they don't interact with matter (at least not much!), nor with each other.

Also, problems arise because we don't actually know what dark matter is. It's only something theorized because atoms apparently only make up ~4% of the universe.
 
  • #9
DaveC426913
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If it doesn't interact with itself, nor normal matter, then, it can't be falsifiable, and hence, non-existent
?
It does interact via gravity, which is what opened the whole can of worms in the first place.
 
  • #10
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If it doesn't interact with itself, nor normal matter, then, it can't be falsifiable, and hence, non-existent
?
I think the message of bullet cluster observation is more that dark matter should be rather cold (in order to condense in galaxies) and weakly interacting (like most of exotic dark matter candidates).
So this leaves lots of candidates.
But, it is not behaving as gas which is already a very important news.
 
  • #11
I think the message of bullet cluster observation is more that dark matter should be rather cold (in order to condense in galaxies) and weakly interacting (like most of exotic dark matter candidates).
So this leaves lots of candidates.
But, it is not behaving as gas which is already a very important news.
It's not that dark matter should be cold - but more interestingly just that it doesn't glow/light up if you warm it.
 
  • #12
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It's not that dark matter should be cold - but more interestingly just that it doesn't glow/light up if you warm it.
Hello mike,

by cold, I was meaning it is not particle of high velocity like neutrinos which are often considered as hot dark matter candidates and which dilute too quickly.
What did you mean ?
 
  • #13
Ah ok, I thought you meant in a literal temperature sense.

Silly me! :)
 

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