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Dark matter question

  1. Nov 7, 2008 #1
    Guys - I need some help here. I am just an interested layman, so i dont have deep understanding of the math involved in gravitational theory, but I remain quite confused regarding the concept of dark matter and dark energy.

    I understand that this is a result of large-scale gravitational observations, wherein large bodies of mass (galaxies, clusters, etc) do not behave exactly as predicted by GR, but i have great trouble imagining that there is some magical entity of mass/energy that is completely undetectable.

    from my limited understanding, it appears that a slight modification of GR, for example an adjustment of the gravitational constant, might be an alternate explanation of the observations - is that correct, or can such adjustment not truly explain what we are seeing?

    if not, how do you reconcile the idea that dark matter and dark energy exist without being detectable, and in apparent conflict with the very successful standard model?

  2. jcsd
  3. Nov 7, 2008 #2


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    You mean Modified Newtonian dynamics?


    I don't know what you mean that dark matter would be undetectable, and that DM and DE is "in conflict" with SM. That the SM is not the "theory of everything" since it don't incorporate gravity at all, so SM have never made any claim to explain all physics.. hence no conflict.

    There are many "particle"-theories of DM, such as SUSY and so on, where some of these particles may be discovered at the LHC experiment at CERN.
  4. Nov 7, 2008 #3


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    It's a good point.
    Newton's laws worked perfectly except for a small problem with the orbit of mercury but that was enough to throw them out and replace them with GR. Presumably anybody that tried to explain it by adding extra unseen moons or invisible material would have been laughed at.
    But when the new theory doesn't fit observational evidence then instead of saying GR is wrong a whole new class of matter is invented to prop up the theory. The main basis for this is that GR is just such a neat theory that for it to be wrong would by unfair!

    Fixing GR to explain galaxy rotation without adding the extra dark mass would be very ugly, adding lots of new terms that only worked over certain distances. Saying that a theory is wrong because it isn't elegant isn't exactly the scientific method you get taught in schools - but it does seem to work!

    Space is rather big and empty - it could be filled with quite a lot of stuff without us being able to see it. Even if that stuff was cold dark matter (ie normal gas, dust, rocks, and failed stars) rather than exotic forms of matter.
    Somebody worked out how many beer cans you could put in space (/m3) and still not detect them - but I can't find the reference.
    Last edited: Nov 7, 2008
  5. Nov 7, 2008 #4
    We are all confused!!!
    I'm with mgb on this so far....either our theory of gravity has a flaw which appears beyond accelerations of magnitude c^2/R in some elliptical galaxies OR there appears to be something we now call dark matter...but I don't think dark energy is required to address the issue of constant rotational galactic velocities. Dark matter is not a perfect solution: it appears to work in about 80 of 100 surveyed elliptical galaxies.

    Dark energy arises in a different context, in terms of the cosmological constant and the our accelerating expansion of the universe. Einstein originally accounted for a cosmological constant in GR because he needed something to force the universe to stasis since everybody thought it was static at the time. (Even he did not have the imagination to believe his own "expansionary" mathematical results.) When Hubble observed the universe WAS actually expanding, that was confusing enough,. but then it was found it was an accelerating expansion as well...and nobody knows why...But how acceleration is powered, how the cosmological constant may be dark energy, and how these may have flowed from early phase transitions during inflation early in the life of the universe, nobody knows...

    As noted, gravity has nothing to do with the standard model...it covers, in a glued together fashion, qantum theory,field theory and the three other forces...gravity remains outside, unique, and awaits "unification" with the standard model forces...sloppy, likely incomplete, but workable most of the time.

    "We know much, we understand little"
  6. Nov 13, 2008 #5


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    There's a new educational video on "The Mystery of Dark Matter" put out by Perimeter Institute for Theoretical Physics. Its free to view. It may help with your questions. Its used to explain the existence of dark matter and discusses why its still a "Mystery". good luck.

    www.perimeterexplorations.ca[/url] if that doesnt work go to their website at [url]www.perimeterinstitute.ca[/URL]
    Last edited by a moderator: Apr 23, 2017
  7. Nov 18, 2008 #6
    2000 years ago, electromagnetic radiation (Radio/Microwaves/UV) would have been completely undetectable, yet they existed. We just haven't figured out a way to not only detect, but manipulate dark matter/dark energy. I firmly believe in dark matter/energy, and can't wait for it to be proven that it exists. How could it not? It explains so much.
  8. Nov 19, 2008 #7
    irish - i am a little surprised at your response. from the perimeter site:
    "The rest of the universe is made of unseen material that does not emit, reflect, or absorb any type of electromagnetic radiation."

    this, to me, indicates matter which is not made up of the particles in the standard model, ie, no electrons, etc. i find that aspect to be so unlikely, that the entire concept of dark matter becomes deeply suspect.

    my real question here was - is there a modification of GR that might explain our observational results, rather than inventing a new kind of mass/matter?
  9. Nov 19, 2008 #8

    Neutrinos are unseen and do not emit, reflect, or absorb any type of electromagnetic radiation. And yet they exist in enormous numbers. The only reason that we can detect them at all is that they happen to (a) feel the weak nuclear force, and (b) are unbelievably numerous.

    You're being prejudicially electromagnetocentric. All that would be required for a particle to not "emit, reflect, or absorb any type of electromagnetic radiation" would be for it to contain no electric charges. Why are you so certain that every particle in existence must contain electrical charge--- so certain that you are willing to dismiss any such possibility out of hand?

    On the contrary, we know of many particles that do not feel all four forces. Neutrinos for one. Electrons and other leptons for others. It would not be terribly surprising if there existed another particle that did not interact with EM (we already have one entire family of such particles). If such a particle exists, it would not be surprising that it has not yet been detected (how would you do so?).
  10. Nov 19, 2008 #9

    But today most physicsts believe gravity formulations work....and that dark matter is a better explanation...I don't think anybody really knows...

    In the past "most physicsts" have usually been wrong...and pioneers like Heisenberg, Einstein, Maxwell, Hawking, led the way to new understanding...."most physicsts" do not lead....
  11. Nov 19, 2008 #10
    I don't think GR needs to be modified to explain the existence of dark matter. The building up of dark matter on the leading edge of a unit of mass as it accelerates through empty space is what eventually causes diminishing returns. Mass approaches infinity as it gathers more dark matter as it accelerates closer and closer to the speed of light. Einstein knew this 100 years ago even though the name dark matter had not yet been applied.
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