Evidence for Dark Matter: Bullet Cluster X-Ray & Weak Lensing Study

In summary, the authors of the paper "Catching a bullet: direct evidence for the existence of dark matter" present observations of the merging cluster system 1E0657-556, showing three mass components: X-ray plasma, electrically neutral matter, and dark matter. Weak lensing reveals that dark matter is the most massive component, while X-ray plasma is displaced from the present position of the bullet galaxy cluster. This evidence for dark matter challenges the current understanding of gravity and raises questions about its nature. Other evidence and theories about dark energy and dark matter continue to be investigated.
  • #1
Science Advisor
Gold Member
Here is the latest in the 'bullet cluster' series on evidence for the existence of dark matter:

Catching a bullet: direct evidence for the existence of dark matter
Authors: D. Clowe (Ohio University), S. W. Randall, M. Markevitch (CFA)
Comments: 4 pages, 1 figure, to appear in the proceedings of the 2006 UCLA Dark Matter Symposium. Accompanying the paper is a partial data release at this http URL

We present X-ray and weak lensing observations of the merging cluster system 1E0657-556. Due to the recently collision of a merging subcluster with the main cluster, the X-ray plasma has been displaced from the cluster galaxies in both components. The weak lensing data shows that the lensing surface potential is in spatial agreement with the galaxies (~10% of the observed baryons) and not with the X-ray plasma (~90% of the observed baryons). We argue that this shows that regardless of the form of the gravitational force law at these large distances and low accelerations, these observations require that the majority of the mass of the system be some form of unseen matter.
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  • #2
Read the article and answer my question.

They say:
This was done by observing how light from distant galaxies is bent by the gravitational pull of dark matter in a process called gravitational lensing.

Ok, great, but doesn't this mean that the only places where we can actually observe the lensing done by dark matter _is_ the area around visible galaxies, and we shouldn't be surprised that that's exactly where it was calculated to be?
Sounds a bit like circular reasoning.
  • #3
hi SF,

The idea of gravitational lensing is that the lens is in the foreground and the lensed objects are in the background. No matter where on the sky you look there should be enough background galaxies to make a measurement of the amount of matter in the foreground.

The obstacle comes in resolving the distant background objects because the stretching of the images of these galaxies is what is used to measure the foreground matter distribution.
  • #4
The article by D. Clowe, S. W. Randall and M. Markevitch at
http://arxiv.org/PS_cache/astro-ph/pdf/0611/0611496.pdf shows that the
“bullet” galaxy cluster had three mass components.

The first is the X-ray plasma component ( 90% of the visible matter) that was
stripped out of the bullet galaxy cluster as it interacted (electromagnetically?) with
the plasma in the “target” galaxy cluster. This component is displaced from the
present position of the bullet, which is marked by:

The second component (10% of the visible matter), is electrically neutral
matter --- dust grains, molecular vapours and stellar stuff — as ordinarily
observed in galaxies. In the same location is:

A third invisible component is, ”dark matter”, “some type of matter (that) does
not emit, absorb, or deflect light in any observed bandpass”. This component,
unlike the plasma, is not displaced from the stellar stuff that marks the
bullet’s present position. It is revealed by weak lensing, which if based on
“ordinary” general relativity, shows that it is the most massive of the three

The first two components are called “baryonic matter”. What observational
evidence is there for restricting this classification to the first two
components? Or is this done simply because the calculations of nucleosynthesis and elemental abundances cannot explain the now observed preponderance of dark matter?
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  • #5
Could the gravitational field self gravitate? Can't the existence of the gravitational field be considered an energy source in itself which would cause a gravitational field too? Then it would be understandable that it would have its own momentum so that when the galaxy it's attached to suddenly stops the field would keep moving for a bit before recentering itself on the galaxy. Wouldn't this be just another form of gravitational wave where sudden changes in the momentum of the mass can cause its gravitational field to propagate out even further?
  • #6
Self gravitating fields of such a nature would spiral into a feedback loop.
  • #7
Wouldn't the curvature due to the two galaxies cancel somewhere in between them?

Also, doesn't the cuvature correspond to the density gradient rather than to the barycenter of mass? If so, why would one expect much lensing in the middle anyway?
  • #8
I have asked this question before in other threads with no answer. Is it possible (in theory) that the dark matter and dark energy result from a quantum interaction between "asymmetrical mass units" of matter + antimatter via an interaction involving gravity + antigravity ? Is this hypothesis in any way supported (or not supported) by the observation of "lensing" ?
  • #9
Another comment on this topic. Should it be called "dark mass" and not "dark matter" since Einstein with E = Mc^2 clearly was dealing with "mass" as M and not "matter" ? Perhaps only semantics, perhaps not if [matter ≠ mass].
  • #11
SF said:
Dark Energy might be in trouble.

This news is bad for DE, but on its own not enough to kill it. Of course, there is other evidence mounting ...
  • #12
Kea said:
This news is bad for DE, but on its own not enough to kill it. Of course, there is other evidence mounting ...

You can't keep a good epicycle down... :wink:

  • #13
Garth said:
You can't keep a good epicycle down... :wink:

Indeed! Next we'll be hearing some argument that DE, despite being remarkably uniform over the entire history of the cosmos, somehow changes at small scales...hee, hee, this is all so funny.
  • #14
I find it hard ot believe that an accurate length scale for the detection of dark energy has been calculated. The evidence that I've seen for dark energy (supernovae type 1a and the integrated Sachs Wolfe Effect) seem to show that the universes expansion is accelerating BUT to make a measurement of how fast the expansion is accelerating and connect that to a laboratory length scale seems like overextending to me. I don't know who derived the length scale of 85 micrometers, but id like to see how it was done. No doubt the experiment was done well, but all i think it can do is put some weak constraints on dark energy.

Related to Evidence for Dark Matter: Bullet Cluster X-Ray & Weak Lensing Study

1. What is dark matter and how is it different from regular matter?

Dark matter is a type of matter that makes up a significant portion of the total mass in the universe. Unlike regular matter, which is made up of atoms and can be seen and detected through light, dark matter does not interact with light or any other form of electromagnetic radiation. This makes it invisible and difficult to study using traditional methods.

2. What is the Bullet Cluster and why is it important in studying dark matter?

The Bullet Cluster is a cluster of galaxies located about 3.8 billion light-years away from Earth. It is important in studying dark matter because it is one of the most massive and energetic collisions of galaxy clusters ever observed. This collision has caused the visible matter in the two clusters to separate from each other, providing a unique opportunity to study the distribution of dark matter.

3. What is the X-ray study of the Bullet Cluster and how does it provide evidence for dark matter?

The X-ray study of the Bullet Cluster involved using the Chandra X-ray Observatory to observe the hot gas in the cluster. The results showed that the majority of the hot gas was located in between the two clusters, where the visible matter had separated. This suggests that the majority of the mass in the cluster is not made up of visible matter, providing evidence for the existence of dark matter.

4. What is weak lensing and how does it support the evidence for dark matter in the Bullet Cluster?

Weak lensing is a phenomenon in which the gravity of a massive object, such as a galaxy cluster, distorts the light of more distant objects behind it. By studying the weak lensing effects in the Bullet Cluster, scientists were able to map the distribution of mass in the cluster and found that it did not match up with the distribution of visible matter. This supports the existence of dark matter, as it suggests that there is a large amount of unseen mass in the cluster.

5. What are some alternative explanations for the evidence of dark matter in the Bullet Cluster?

Some alternative explanations for the evidence of dark matter in the Bullet Cluster include modified theories of gravity and the possibility that the visible matter in the cluster is simply distributed differently than the dark matter. However, these alternative explanations have not been able to fully explain the observations and do not have as much supporting evidence as the dark matter theory.

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