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

Dark matter/energy and Gravity

  1. Jan 20, 2010 #1
    Do you think dark matter and dark energy actually exist or have we just got the theory of gravity completely wrong but just havent realised it yet?
  2. jcsd
  3. Jan 21, 2010 #2


    User Avatar
    Science Advisor
    Gold Member

    For starters, dark energy and dark matter are unrelated. Their only commonality is the word 'dark'.
  4. Jan 21, 2010 #3
    yh thats apparent to me too, but you've misunderstood, the question is about the relation of dark matter and energy with gravity, in that concept all 3 are related, and just for extra theres a good chance they are directly related, they both have "problems" with gravity, dark matter because galaxies are spinning too fast and would fly apart(gravity) and dark energy because the universe's expansion is accelerating, which again is a gravitational problem hence the cosmological constant which physicists are now interested in using again, so they seem to have something in common, please read the question more carefully next time, now if you've have something interesting to say on this topic please do so, with all due respect, i havent got time for time wasters
  5. Jan 21, 2010 #4
    As of this moment, yes, but I could change my mind tomorrow.

    Something that you need to realize that there are probably hundreds of papers that have proposed alternative theories of gravity, and dozens of different alternative models, and no one has quite come up with anything that is as simple and elegant as assuming dark matter. That's not to saw that no one will. Someone could come up with something tomorrow. and if they do I'll change my mind on this.
  6. Jan 21, 2010 #5
    Also it's unlikely that we've gotten gravity *completely* wrong. The models of gravity seem to more or less work most of the time. One of the difficulties in coming up with a gravitational explanation for dark matter or dark energy is that your new model has to come up with the same predictions as the old models when you don't deal with situations where there is dark matter or dark energy.

    So the general approach that people have taken is to start with standard gravity and then add a "correction" that only becomes big when you have dark matter or dark energy. No one really has gotten this to work well.

    What makes it even more interesting is that it's not either/or. You could have dark matter, dark energy, *and* non-standard gravity.
  7. Jan 21, 2010 #6
    From what I see our current theory of gravity seems to very limited, it doesn't work on the quantum scale and now apparently when you extend it to sizes bigger than a few galaxies it doesn't seem to work either, in my own opinion i think its a bit far fetched just adding an unknown force to compromise something we don't know about, its a bit like the ether concept, we're trying to explain a new phenomenon using old ideas, gravity is probably a lot stranger than we all imagined or even can imagine
  8. Jan 21, 2010 #7
    It does work well at the quantum scale until you get to really, really, really small scale (Planck's length).

    As far as large scales, we don't know. It could be that our models of gravity work just fine for galaxies. The fact that are models aren't completely broken makes the problem much harder. If you have something that doesn't work at all, you can invent anything you want. If it doesn't you can;t/

    No one is trying to explain anything right now. Most alternative models of gravity are what people call "phenomenological models". What you do is to just assume that gravity acts in a certain way and then see if you can get the observations you see. You don't try to think about *why* gravity acts in the weird way.

    The trouble is that even at that level, people haven't been able to get anything that works. If you add dark energy, then everything seems to work.
  9. Jan 21, 2010 #8
    If you are dealing with something unknown, you will get nowhere by trying to figure out what it is. You make progress by figuring out what it isn't.
  10. Jan 21, 2010 #9


    User Avatar
    Science Advisor
    Gold Member

    I dont get it. Dark matter and gravity are clearly connected. Dark energy is not even in the picture,
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook