Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

A Why should the Bullet Cluster focus light?

  1. Oct 10, 2015 #1
    In the Bullet Cluster, we have two well defined galaxy clusters that exhibit gravity lensing. Between them is a mass of gas (mostly hydrogen and helium) that is roughly eight to nine times the stellar (observable) mass of the two galaxies. The mass of gas doesn't exhibit any lensing. My question is, why do we expect the gas to act like a lens? For a lens to work, no matter how massive, it needs to focus the rays of light coming from behind it in a coherent way. Since this cloud could be shaped like a box, or a cone or have any shape at all, why are we so convinced that it can focus light? There's no question it can bend light, the question is how is it able to focus light.

    Bullet X-Ray Contour.png

    Here's the X-Ray contour map of the Bullet Cluster. From this it's easy to see that we're not dealing with a relaxed system (there's absolutely no symmetry). I don't see any possible way to de-project the surface brightness to a 3D density profile without an assumption of a relaxed system. Without a density profile, I don't see how you can make any predictions at all about how this mass should bend light. What am I missing?
     
    Last edited: Oct 10, 2015
  2. jcsd
  3. Oct 10, 2015 #2

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    Where does that number come from?
    Most of the mass of the system is dark matter.
    It does not have to be coherent - we can see multiple images of the same object taking different ways around the galaxy.
     
  4. Oct 10, 2015 #3
    Thanks for pointing that out. I updated the question to specify the observable (stellar) mass.
    Yes, it does have to be coherent to focus. The multiple images are a result of the magnification factor and the distance from the actual object to the observer and the distance from the lens to the observer, but the light is still focused.

    A glass marble and a class cube can have the same mass. One will act like a lens, the other won't.
     
    Last edited: Oct 10, 2015
  5. Oct 10, 2015 #4

    phinds

    User Avatar
    Gold Member
    2016 Award

    You are misunderstanding gravitational lensing, I think. Light from a galaxy, say 8 billion light years from us shows gravitational lensing around a galactic cluster, for example, that is 4 billion light years from us. The distribution of matter in the galactic cluster is irrelevant; what matters is the total mass, which for the Einstein Rings acts the same as if that amount of gravity existed at the center of mass of the cluster.

    Light passing THROUGH the cluster has the problem you describe but that has nothing to do with Einstein Rings.
     
  6. Oct 10, 2015 #5
    No, I'm pretty sure that the source of the gravity lensing must bend space the same way an optical lens does with light. There's LOTS of mass between the source of the photon and the observer, but if the space isn't curved in a coherent way (e.g. a sphere), then the light won't be magnified. If we could create a concave galaxy cluster then we would see the light scatter instead of focus.
     
    Last edited: Oct 10, 2015
  7. Oct 10, 2015 #6

    phinds

    User Avatar
    Gold Member
    2016 Award

    But it IS "curved" in a coherent way, just as I described. The lensing effect isn't THROUGH the cluster, it's AROUND it, so convexity/concavity is utterly irrelevant.
     
  8. Oct 10, 2015 #7
    Then could you please explain to me, like I'm five, how the Bullet Cluster demonstrates the existence of Dark Matter. If what you say is true, then the separation of gas from stellar material isn't going to be a factor and we should look outside of the combined cluster for any conclusions about the gravity well.
     
  9. Oct 10, 2015 #8

    phinds

    User Avatar
    Gold Member
    2016 Award

    Most of the mass of the cluster, as with all clusters, is dark matter. The significance of the bullet cluster is that it shows the flaws in the so called "modified gravity" theories (MOND) that try to explain dark matter. It does this by showing that the dark matter masses of each of the two colliding galaxies went well past the intersection of the two galaxies and formed blobs on the other sides of the collision. In order to do that, it had to be shown that the dark matter was in fact forming those blobs. It did that because of gravitational lensing. That is, the only way to "see" the dark matter blobs was by gravitational lensing. This is completely explained in the Wikipedia article on the bullet cluster, which I recommend you read.

    EDIT: better still, you should get clear about what gravitational lensing IS. Your concept of concavity/convexity of the galaxies doing the lensing seems to show that you have a fundamental misunderstanding of what it is / how it works.
     
  10. Oct 10, 2015 #9
    I'm not sure you have the argument right (or perhaps I'm just misreading it). I've read a couple of articles about it now and you (or I) are missing an issue. The proponents of MOND claim that their modified gravity can bend light just as well as Dark Matter (better in some cases). The argument seems to be: well if that's the case, then how come the dust isn't bending light more because there's 9 times more of it? It's to that argument that my question is directed: yes, we all agree that there's 9 times more matter, but where's the argument saying a non-relaxed cloud of dust of any shape can bend the matter such that it's focused on a distant observer?

    And yes, I agree with you about lensing if you take the observation out to a radius where the entire cluster appears to be a point mass, but we are drawing our contour maps (X-Ray and Weak Gravity) right through the heart of the cluster. If I place two black holes right next to each other and look directly between them, are you saying that I'll see a magnified image?
     
    Last edited: Oct 10, 2015
  11. Oct 10, 2015 #10

    mfb

    User Avatar
    2016 Award

    Staff: Mentor

    No it does not, and that approach won't lead anywhere.
     
  12. Oct 10, 2015 #11

    phinds

    User Avatar
    Gold Member
    2016 Award

    This seems to be a continuation of your belief that gravitational lensing is in some way similar to optical (glass lenses) lensing, but as mfb pointed out, that incorrect point of view will lead you to false conclusions. Gravitational lensing is not limited to Einstein Rings, which as I see you agree, occur around the periphery of a high-mass area of the cosmos. Light is "bent" (that is the local geodesics are not straight in the context of Euclidean Geometry, which space-time doesn't follow anyway) by any-shaped large mass and the resulting distortions can be detected.
     
  13. Oct 10, 2015 #12

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    Citation, please.

    In MOND there is no dark matter, and since the bending of light is relativistic phenomenon, it can't be explained by any non-relativistic theory, like MOND. It's simply outside its domain of applicability.
     
  14. Oct 10, 2015 #13
    I may indeed have an incorrect concept of bending of space. Could you please answer my previous question about the two black holes. If you tell me they'll magnify an image when I look between them, then I'll go back and do more reading.
     
  15. Oct 10, 2015 #14

    phinds

    User Avatar
    Gold Member
    2016 Award

    No, gravitational lensing never magnifies the image. That's a property of optical lensing and for the nth time, it just doesn't apply here.
     
  16. Oct 10, 2015 #15
  17. Oct 10, 2015 #16

    Chronos

    User Avatar
    Science Advisor
    Gold Member

    Try plotting the path of rays of light [assume they are all parallel] passing the cluster and see if they converge, diverge or remain parallel..
     
  18. Oct 10, 2015 #17

    phinds

    User Avatar
    Gold Member
    2016 Award

    I can see why this article gave you exactly the impression that you have because that's basically what it says. I believe this is a misrepresentation of what is actually happening. An optical lens magnifies the SIZE of an object. Gravitational lensing magnifies the amount of light that reaches the observer, and it presents multiple images which are separated much more that the size of the object. That is, the multiple images have an angular separation that is way bigger than the angular size of the object being lensed. BUT ... none of the images looks any bigger than the object and that's where the writer of the article misunderstood the physics and made the same mistake you are making. You will note that the article was written by a public affairs officer at NASA HQ, not by a scientist at a research center.

    EDIT: you have GOT to get rid of this conflating of optical lensing and gravitational lensing or you are going to remain confused. There are zillions of things in physics that have absolutely nothing to do with gravitational lensing and optical lensing is one of them. It is very unfortunate, but understandable, that the term "lensing" is used because it causes exactly the confusion you have. Best to replace in your mind the term "gravitational lensing" with "gravitational gallumpfing" and study it as a new phenomenon completely unrelated to optical lensing.
     
  19. Oct 10, 2015 #18
    How do you perform this exercise without knowing the density profile of the gas mass? If you don't have the density profile, then you can't create a mass profile. If you don't have a mass profile, there's no way to draw the geodesics, which is basically the question I have.
     
  20. Oct 10, 2015 #19
    I posted the article because it was easy. There are volumes of information on the subject. Galaxy clusters are not my main focus, SNe IA are and I know enough about the subject to tell you without question that we couldn't detect most of the distant SNe Ia without the magnification that comes from a gravity lens.
     
  21. Oct 10, 2015 #20

    phinds

    User Avatar
    Gold Member
    2016 Award

    Exactly. The magnification of the amount of light, not any magnification of the size of the object. For the final time, this has nothing to do with optical style lensing where the apparent size of the object is increased.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook