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Existance of Dark matter

  1. Jan 2, 2009 #1
    From everything I have read so far, we only know of Dark matter by means of gravitational inconsistencies. My question is this: Why is it so outside of the box to suggest that our understanding of gravitation itself may be faulty? Why are we inventing mysterious particles that are distributed across the cosmos but don't interact with anything? It sounds extremely fishy every time I hear it, and I am surprised that there isn't more discussion surrounding the issue.
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  3. Jan 2, 2009 #2


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    there's been discussion of modified laws of gravity going on for 10-20 years

    countless researchers, couldn't begin to list them all, but some leaders are Moti Milgrom and John Moffat, and Jacob Bekenstein.

    What do you think are the main kinds of evidence for DM, like galaxy rotation curves? I can't tell exactly what you think unless you spell out the types of evidence you are aware of. The are several, but a big clincher was mapping clouds of DM and showing that they could move independently of clouds of ordinary matter. We see their density contours by lensing---how they distort the background.

    My impression is modified gravity advocates became a lot quieter starting 2006-2007. They lost much of their support, and have been publishing less. I think they basically couldn't fit the new data that was coming in---especially imaging. Many of the papers on the topic being published now are by critics, challenging the modified gravity ideas.

    A lot of work went into making up modifications of GR to explain observed effects without resorting to DM. But since 2006 we hear less about it because the proposed modifications don't work convincingly.

    I had great hopes for modified gravity myself until 2006 or so. I thought it might be able to replace the need for DM.
    Last edited: Jan 3, 2009
  4. Jan 3, 2009 #3
    I believe that people have waited for Gravity Probe B final results since 2006!
  5. Jan 3, 2009 #4


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    I am not aware of any significant impact, of GPB's results, expected for astrophysical/cosmological DM (the topic of this thread).

    For sure, had the GPB results been inconsistent with GR, at the >3σ level, there may have been some interesting implications for DM; however the then main 'new theory of gravity' contender (for astrophysical DM) - MOND and its relativistic descendants - had no dog in GPB's race, did they?
  6. Jan 3, 2009 #5
    Marcus: thank you for that answer.
    I don't study astrophysics so I cannot go into that much detail. I read a lot of popular literature articles on DM though, and never is it really discussed that one potential explanation for the discrepancy is that our laws of gravity are faulty themselves.

    I wish that modified gravity theory would work out. Perhaps the reason that gravity is such an odd man out in the standard model is because we don't understand it in the first place. I wonder -- Could it be that with one stroke someone could come up with an equation for gravity that would both, eliminate the need for DM and also yield the Theory of Everything as a corollary?
  7. Jan 3, 2009 #6


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    quite a few people have had that dream and although it looks increasingly unlikely that things will turn out as you say regarding DM
    there remains a greater chance that something like that could turn out regarding dark energy.

    DE is very different. DM behaves like matter particles. It collects into clouds and its gravity seems to help clusters of galaxies form where it concentrates in regions of high DM density. By constrast, DE behaves as if it cannot drift around and collect in higher concentration patches, it is always just a constant uniform density of 0.63 nanojoules per cubic meter, no more no less.

    You could say that DE behaves less like some type of matter. At least in my view it behaves more like how a correction to the law of gravity might look. I'm not saying it IS but if you had said the same thing you did, but about DE....if you had said:

    "Could it be that with one stroke someone could come up with an equation for gravity that would both, eliminate the need for DE and also yield the Theory of Everything as a corollary?"

    Then I would have to grant that wacky as it sounds, there might still be a sense in which that could happen.

    DE you could say is another name for a small positive cosmo constant Lambda, and there are some approaches to quantum gravity that actually require a small positive Lambda, won't work without it. Can't say what that means or if it means anything--they could be wrong or right. Just a hint that better understanding of gravity, quantum gravity, could give some insight about the reality underlying Lambda (a.k.a. DE).
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