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 mass of the two galaxies. The mass of gas doesn't exhibit any lensing. My question is, why do we expect it 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. 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.

A glass marble and a class cube can have the same mass. One will act like a lens, the other won't.

Edit: The answers devolved into a discussion of ΛCDM that I wanted to avoid. Let me rephrase the question: We would expect a spherical, relaxed X-Ray cloud (without significant stellar matter) to exhibit some form of gravitational lensing. Is there any reason that after going through a collision like the Bullet Cluster, the same cloud would display the same kind of lensing (on the scale of one or two Mpcs) or would it just scatter the light in an incoherent way?


The Bullet Cluster is the after-effect of the collision of two galaxy clusters. The gas cloud in the center of the two clusters consists of baryonic particles that interacted with each other through both electrostatic and gravity forces, slowing them down. Most of the mass of the Bullet Cluster, however, consists of dark matter that passed through the collision unimpeded by electrostatic forces, and that now lies outside the central gas cloud.

Therefore, gravitational lensing of light that passes to us from behind the Bullet Cluster, is strongest on either side of the central gas cloud. This can be seen in the 7th photo down as you scroll down from the top in this link: http://www.cfhtlens.org/public/what-gravitational-lensing. Most of the lensing effect occurs when light passes through the dark matter. There is some weak lensing from the gas cloud, but its mass is far less than the dark matter, so its lensing is weaker.

Gravitational lensing does not necessarily mean that light is focused as through a well-shaped optical lens. It's effects can be distortion and multiple images. All light that comes to us undergoes some gravitational lensing, but only strong gravitational lensing produces dramatic effects.

  • $\begingroup$ The argument for the Bullet Cluster goes something like this: 90% of the matter in the two clusters is in the form of gas, so the gravity lensing effect should be strongest around the gas. I appreciate the background material you provided, but it doesn't explain why we think the non-descript shape of the gas should have the same focusing ability as a relaxed spheroid. The mass of the gas may be 9 times that of the stellar matter, but I don't see how that translate into an ability to focus light. $\endgroup$ – user32023 Oct 10 '15 at 15:40
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    $\begingroup$ @DonaldRoyAirey Only 9% of the mass is in the form of hot gas. The lensing shows that most of the mass is still in and around the visible galaxies and not associated with the hot gas. $\endgroup$ – ProfRob Oct 10 '15 at 16:24
  • $\begingroup$ Yes, if you accept a priori Dark Matter, which I do not. Nor do I believe in MOND, but I'm trying to understand each other's arguments in objective terms. MOND people claim their relativistic model can bend light just as well as DM. I find that proposition no more preposterous than a stable, electrically neutral, non-relativistic particle that has been ruled out of the Standard Model. $\endgroup$ – user32023 Oct 10 '15 at 16:53
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    $\begingroup$ @DonaldRoyAirey The gravitational lensing analysis cares not a jot what form the gravitating mass is in that is the whole point. 9% of the mass is in the form of hot gas when analysed using a GR formalism. Who are "MOND people" and which papers claim the lensing can be explained without very significant quantities of dark (as in, we cannot see it) matter that is associated with the galaxy clusters and not the hot gas? $\endgroup$ – ProfRob Oct 10 '15 at 17:32
  • $\begingroup$ I'm sure you are talking about Strong Lens analysis. You seem too informed about the subject to not know that Weak Lens analysis (which is what is used in the Bullet Cluster) depends on a statistical analysis of the background stars within the cluster and is heavily dependent on the configuration on the mass in the foreground. A small sphere on this scale will focus the geodesics better than an amorphous cloud of higher mass. $\endgroup$ – user32023 Oct 11 '15 at 14:20

We don't require that a lens focus light, only that it bend light, as you said. From optics, a concave lens disperses light, sending it out at wider angles than it comes in. From astronomy, other examples of lensing include cases where a single image is broken up into multiple images, or is smeared out into a distorted version of the original. A common lensing effect is our ability to see more than 50% of the surface of a spherical body because light from just past the horizon is bent in our direction.

As far as the shape of the gas cloud plays in, the total mass and the center of gravity of the cloud are the primary concerns in determining lensing. As long as the cloud is sufficiently compact, the shape only slightly changes the otherwise spherical distortion of the lensed images.

Edit: regarding the difference between a glass marble and a glass cube: keep in mind that those objects change the path of light through refraction, while galaxy clusters change the path of light by curving space. Refraction depends on the refractive indices of two materials and an interface between them; gravitational lensing can occur in vacuum near a point mass. The two processes are very different, and we only call the latter "lensing" because the effects are like using a lens, not because the lensing body forms an optical lens.

  • $\begingroup$ I added a contour map of the Bullet Cluster to illustrate my comment. First, it doesn't resemble a circle or a relaxed system in any way. Second, because we can't assume that the system is relaxed, we have no way of de-projecting the surface brightness to a 3D density profile. I just don't see how it's physically possible to make any assumptions about how this mass of gas should refract light. $\endgroup$ – user32023 Oct 10 '15 at 14:24
  • $\begingroup$ I think you've missed the point: gravitational lensing has nothing to do with refraction. In fact, the gas itself has an index of refraction very close to 1 so it would make a terrible optical lens. But the cloud is very massive. When it comes to gravitational lensing, the more massive spiral galaxies (which are shaped like flat disks) are more effective than the less massive elliptical galaxies (which are more lens-shaped). $\endgroup$ – Asher Oct 10 '15 at 14:48
  • $\begingroup$ You are correct to point out that I mean 'bend' instead of 'refract' in my previous comment. However, I'm unaware of any data that reflects the ability of individual galaxies to bend light. Would you mind providing a reference so I can read up on the subject. $\endgroup$ – user32023 Oct 10 '15 at 15:43
  • $\begingroup$ I see your edit (what happened to my pretty picture?). I agree with your statement only if you extend the radius to where the entire cluster can be considered a point mass. But the images used to draw the gravitational contours and X-Ray contours show the impact of the individual components (DM, dust, stellar). Again, I agree totally that the dust will bend light more than the stellar material, but I don't agree that it is able to focus it, which is what would be required to see the effects in a far off place like Earth. $\endgroup$ – user32023 Oct 10 '15 at 16:14
  • $\begingroup$ @DonaldRoyAirey Do you understand what a curved space means? It means that the geodesics that the light has to follow are curved, not straight lines. The geodesics are defined as a solution to the gravitational equations for the masses in the cluster. Generally the larger the masses the larger the space curvature/distortion, which the photons have to follow. $\endgroup$ – anna v Oct 10 '15 at 16:47

90% of the baryonic matter is in the form of gas. The majority of the gravitating matter, that is causing the strong lensing effects, is dark and appears to be associated/clumped within the two clusters and around their galaxies.

As is completely explained in the papers you have been referred to in previous questions (e.g. Paraficz et al. 2012), the gas actually has a fairly mild effect on the lensing signatures, but it needs to be included in the modelling because the gas is offset from the dark matter. The intra-cluster gas is modelled using X-ray observations and included in the gravitational modelling along with the visible matter in the galaxies and two clumps of dark matter associated with the galaxy clusters. The latter are dominant (along with the dark matter halos around the galaxies). According to this paper the gas makes up about 9% of the gravitating matter and provides only "an external shear to the strong lensing model".

  • $\begingroup$ The MOND people say that their modification of gravity works just as well at bending light as Dark Matter. The ΛCDM people then say, well, if that's the case, why doesn't the 90% of the mass in the gas cloud bend the light even more. To which the MOND people say "...". I don't follow this argument, however. If the gas doesn't act to focus the light (within the scale of the weak lensing contour maps), then how would we measure the bending? What I'm saying is, the gas could be bending the light more, but in a lot of different directions. $\endgroup$ – user32023 Oct 10 '15 at 16:41
  • $\begingroup$ @DonaldRoyAirey Now you start talking about MOND? You will have to read papers about MOND to ask how that deals with gravitational lensing. As I understand it, even the MOND proponents concede there must be an awful lot of "dark matter". Your observation that the gas doesn't seem related to the visible lensing is precisely the point. $\endgroup$ – ProfRob Oct 10 '15 at 17:28
  • $\begingroup$ Yeah. I just got through reading the stuff on TeVeS and it's probably more ad hoc than Dark Matter. I don't even think Miligrom holds it in high regard. Anyway, I was paraphrasing from a paper by Miligrom: arxiv.org/pdf/1508.04001v1.pdf and I projected the TeVeS stuff. $\endgroup$ – user32023 Oct 10 '15 at 20:50
  • $\begingroup$ I've re-edited the question to try and remove the ΛCDM or MOND model issues. $\endgroup$ – user32023 Oct 10 '15 at 20:58

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