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Gravitational lensing and Dark matter

  1. Oct 25, 2007 #1
    Does any one know if gravitational lensing confirms the existance of Dark matter?
  2. jcsd
  3. Oct 25, 2007 #2

    Chris Hillman

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    This is a FAQ

    It would be better to say "supports" the existence of dark matter. See for example Ned Wright's Cosmology FAQ and his Cosmology Tutorial:

    I have suggested that a friendly mentor move this to the Cosmology forum at PF, since it seems to me that your question concerns mainstream cosmology more than it concerns speculations regarding the future of particle physics. That is, you are asking about phenomenological evidence for something we provisionally call "dark matter", not about possible particle physics explanations of what this stuff might be.

    [EDIT: It was moved to "Astrophysics", a reasonable compromise.]
    Last edited: Oct 25, 2007
  4. Oct 25, 2007 #3
    Does any one know if gravitational lensing supports the existence of dark matter? Where can I read about this? Are there any examples of this? I looked up the link that you posted Chris but this did not show me any answers to my question.

    Would gravitational lensing be different for a givin galaxy if Dark Matter was not present?
  5. Oct 25, 2007 #4

    Chris Hillman

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    Some more links for you

    As I already said, yes, it does.

    A short answer: yes, that is the point.

    A better answer: we know how to compute the lensing we would expect if "the mass-energy we understand and already knew about" (mostly stars, but also stuff like interstellar dust clouds) accounted for the mass-energy associated with a given visible galaxy. But the lensing we observe (when a foreground and background galaxy are almost aligned wrt our solar system) clearly suggests that--- surprise!--- there is quite a bit of additional mass-energy. This is phenomenologically identified with "dark matter", but so far no-one knows what this might be, just that it seems to be out there, in abundance. Specifically, we have some idea of how it tends to be distributed wrt the mass-energy we understand: it is associated with visible galaxies but seems to extend beyond the stuff we already knew about, forming a "halo" of invisible "dark matter" around the ordinary matter in each galaxy.

    [EDIT: As cristo pointed out in Post #5, this analysis assumes that our default theory of gravitation, gtr, is good for galactic scales. There is really no reason to think this is not true--- gtr has been tested many many times in many ways, and has passed every test so far--- hence the general acceptance of the conclusion that "dark matter" must in fact exist.]

    The answer to your question was very widely publicized not long ago by the mainstream media, so I am somewhat puzzled why you have experienced difficulties in obtaining information. The pages by Ned Wright I mentioned do answer your question, but I guess you didn't want to look for the answer. So try instead this article by Phil Plait (of BAUT fame)
    or this blog post by Sean Carroll (of gtr textbook fame)

    You can find many more links at http://en.wikipedia.org/wiki/Dark_matter;
    I must warn as always that Wikipedia itself cannot be safely used as an information resource unless you can verify whatever a given version of some article claims by consulting a reliable source, such as a (conventional) encyclopedia. In this case, I happen to know that Plait and Carroll are reliable sources, so the articles I cited should be acceptable for a school project or whatever.
    Last edited: Oct 25, 2007
  6. Oct 25, 2007 #5


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    As Chris has already mentioned, (weak) gravitational lensing does indeed support the fact that dark matter is out there in the universe. (Note that lensing will never confirm the existence, though, since it relies on our model of gravity being completely correct: the same model that requires the existence of dark matter). As to where you can read specifically on gravitational lensing; well, there are obviously many, many sites on the internet containing such information. A quick google search gave me this http://astro.berkeley.edu/~jcohn/lens.html, but try searching for yourself for more. As a rule of thumb, a website ending .edu (or .ac.uk) are university webpages, and so are more likely to provide correct information than .com or other websites. (Obviously, there are lots of exceptions, but this is a general guide).

    Of course there are other ways in which one may detect dark matter particles. One such (newish) method is through so called "direct detection," which basically says that if the dark matter consists of some unknown particles, then the earth will be moving through these particles as it moves through the dark matter halo of the galaxy. Thus, one would expect to be able to measure the recoils of other particles due to these dark particles.

    That is just for one possible explanation of dark matter particle. There are many more and, of course, there are people who modify gravity so as not to need dark matter, but they are not for discussion in this topic, since I've already gone a way off topic already!
    Last edited: Oct 25, 2007
  7. Oct 25, 2007 #6

    Thanks Chris and Christo I knew you guys would have the answers. I am not a college student working on a project I am just curious about this interesting subject of Dark Matter. Thanks for the Info I will check the links you provided.
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