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Dark Matter or Modified Gravity ?

  1. Jun 11, 2012 #1
    What are your personal opinions ??, Do you guys think gravity is just not fully understood that is why we cant explain certain events or DM is the probable answer to these discrepancies ?

    I personally dont think we need to create a somewhat imaginary matter (dark matter and dark energy) to explain what happens in some parts of space, i think we just dont really understand gravity enough and the current gravitational laws and theories are incomplete.

    What do you guys think ?
  2. jcsd
  3. Jun 11, 2012 #2


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    It's not really a matter of opinion at this point. The mounds of evidence in support of dark matter far outweigh any proposed alternative gravity theory. In short, hypothesizing dark matter allows you to solve a bunch of problems at once, whereas with modified gravity you have to hypothesize additional things as well (including dark matter!).
  4. Jun 11, 2012 #3
    Well, if it's not a matter of opinion, then why are there still people defending modified gravity theories? :)

    I think it's very simple: dark matter has to be directly observed before the modified gravity theories can be thrown into the bin. Very often one can be mislead by trying to evaluate probability in the ensemble of all possible physical theories as there is no objective measure there, only personal preferences. I would certainly agree that DM seems by far the more natural explanation, but then again, I don't know everything about nature yet...
  5. Jun 11, 2012 #4


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    Astronomy, Astrophysics, and Cosmology are not sciences built on personal opinions. As you must already know, we rely on the scientific method to arrive at our conclusions of how nature works. We ask a question, (as you have), research what’s already been learned, propose new hypotheses, and then test them. In these areas of science it’s difficult for laypersons to do the actual experiments so we rely on the professionals to devise and perform them. As for analyzing the data and reaching conclusions, most of us trust those same professional experts to perform those functions.

    So how does our personal opinion fit in? If someone doesn’t like dark matter or dark energy, or thinks they are not necessary, that is not sufficient reason to discard them. Proposing your alternative hypothesis would seem a logical step to take. Do you have some alternatives to suggest?
  6. Jun 11, 2012 #5
    Its not that I dont like DM, i have seen the scientific indirect evidence of it proposed existence. But i also came to the conclusion that our understanding of gravity is not complete, for example, Einstein's theory breaks apart in black holes and at the quantum level (i may be wrong). Im not saying he was wrong but rather his work is incomplete. Remember Newton, his theories worked for what was known at the time and it seemed absolutely correct not including Mercury's orbit until Einstein came about and changed that. At present times we have come to learn about quantum mechanics and Einstein's work may need some revision much how Newton's laws were revisited.

    I dont have any specific alternative theory but do you understand where im coming from ?, and i know just because we cant detect DM doesn't mean its not there but also take an objective stance towards the gravity problem.

    Like I had said before im not bashing on Einstein, but it seems to me that people are somewhat afraid to call Einstein out and challenge his ideas.
  7. Jun 11, 2012 #6


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    Surely Einstein's GR is not the full story as far as gravity is concerned; as you note, we have no quantum theory of gravity as of yet. However, the corrections from whatever QG theory you want to consider are almost certainly very small. QG is important roughly at the planck scale, [itex] l_p \sim 10^{-35} {\rm m} [/itex], while DM is a problem observed on roughly galactic length scales [itex] l_g \sim 10^{5} ly \sim 10^{21} {\rm m}[/itex]. That's over fifty orders of magnitude of discrepancy, so it's very difficult to believe that quantum corrections are the cause of the DM phenomenon.

    Instead, modified theories of gravity change its behavior in the low energy regime, precisely where the DM phenomenon is observed (see, for example, the most naive versions of MOND). These theories are a different game altogether, and as I said, a vanilla DM model seems to fit the data much better than any of these alternatives. That's not to say that they're wrong, or that work on them is completely useless, but simply that at present observations lend much more support to a DM-only model.
  8. Jun 11, 2012 #7


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    It isn't as people haven't tried to come up with such alternate theories of gravity, its that they can't be made to fit all the observational data.

    One observation in particular is the Bullet cluster. It is the collision between two galaxy clusters. During the collision, interactions between "normal" visible matter slows the matter down so that it separates slower than it came together. DM, on the other hand doesn't suffer from these interaction and sails on unaffected. IOW, the DM should separate itself from the visible matter.

    Now we cannot see DM directly, but with can see its gravity silhouette, by the gravitational lensing it causes. So what we would expect to see in such a situation is the visible matter and its gravity lensing and then a separate locus of gravitational lensing that has no visible matter associated with it, caused by the DM that has been "knocked loose" from the cluster in the collision.

    This is exactly what we see in the Bullet cluster.

    This can't be explained by a modified theory of gravity alone unless it incorporates DM.
  9. Jun 12, 2012 #8

    Lot's of people have come up with specific alternative theories. There are two problems, dark matter and dark energy. Dark *matter* is thought not to be the result of alternative gravity, because what you do is to measure the "lumpiness" of the universe, and we've only be able to get the right amount of "lumpiness" with dark matter.

    The other thing is that with gravitational lensing, we've been able to "map" dark matter.

    Now *dark energy*. The field is wide open since modified gravity is as good as any other of the ideas.

    Here is one paper that reviews them.


    Note that it has over 600 citations.

    Here is another review paper with 277 citations.....


    Yup. One problem with popular science presentations is that they just mention the final result. They don't mention the hundreds of alternatives that were tried but didn't work.

    The way that most alternative gravity models work is that the are f(R) models. If gravity worked very different at solar system distances, we'd know about it fast. So you come up with a model that works very much like GR at short distances, but are different at large distances.

    That's because popular accounts of science aren't very good at explaining the process of science. It's not that people worship Einstein, but rather that for dark matter we have good reasons for thinking that it's not modified gravity.

    Now for dark energy.......
  10. Jun 12, 2012 #9


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    If you need some amount of non-baryonic dark matter to make MOND work, the fundamental motivation for MOND is fatally wounded.
  11. Jun 12, 2012 #10
    Why? Why should DM behave any differently under gravitation than normal matter? This is a huge weakness for DM: it's a "just so" type of theory that behaves just the way you want it to behave so that everything works out right. If the galaxy is spinning too fast, no problem: there's more DM (that we can't see). If the galaxy's spinning too slow, no problem: there's less DM (that we can't see). The whole thing verges far too close to untestability.

    Not true. Bullet Cluster type effects can be explained by several modified gravity theories. See Brownstein & Moffat 2006, and Angus et. al. 2006.
  12. Jun 12, 2012 #11


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    DM does not interact electromagnetically. It does not "collide" with itself or normal matter, as such collisions are electromagnetic interactions.
    The collisions that occur with normal matter also results in loss of kinetic energy which is radiated away as EM radiation. DM does not interact with or emit EM radiation, so this method of shedding energy is not available.

    Thus normal matter is subject to both gravitational interaction and electromagnetic interaction, while DM is only subject to gravitational interaction. Ergo, DM would be expected to lose less KE in the collision of cluster.
  13. Jun 12, 2012 #12
    Back to your opening question. I for my part think a modified understanding of gravitation or the dynamics of the universe is needed. In my view neither modified gravity=MOND or DM seems really up to it. MOND is modifying the math to match observations (not adding insight to why) and LCDM adds mass to fit the math. Really not the same level of deep understanding on fundamental interactions like in GR. The quantum approach like loop gravity is not that ecxiting either and are as far as I know trying to reach the same level of explanation as GR which means we are still left with unknown/nondetected particles or unexplained mathematics. My hunch is that we need a theory with a more global approach incorporating what we now know on the overall structure of the universe. But thats just speculation and guessing of course;)

    Recently there has been an interesting thread in PF about missing DM in our own galaxy. which among others mentions Kroupa et al who challenges the LCDM-model from a MOND-perspective and claims that LCDM is ruled out by recent findings.

    I dont have enough postings so I cant link to the thread in PF or to their (Kroupas) blog (They discuss amongst other things the bullet cluster). The name of the thread is:

    Serious Blow to Dark Matter Theories?

    And the blog is called the-dark-matter-crisis on SciLogs.eu.
  14. Jun 12, 2012 #13


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    The MOND explanation for the bullet cluster is ... somewhat lacking. See
    "Modifying Gravity: You Can't Always Get What You Want" by Starkman http://arxiv.org/abs/1201.1697.
  15. Jun 12, 2012 #14


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  16. Jun 12, 2012 #15
    I'll post it, because it's dynamite. Here's the blog.

    And here's the discussed paper, which rules out a spherical DM halo in the Milky Way at the 4[itex]\sigma[/itex] confidence level, using dynamical observations of bright stars above and below our position on the galactic plane.

    So I guess we're looking for DM that only attracts in one direction?
  17. Jun 13, 2012 #16
    That's the easy part. The hard part is making the numbers work out.

    I think that they are overreaching their conclusions. The big piece of evidence for LCDM is baryon acoustic oscillations, and modified gravity has not been able to reproduce that without introducing some dark matter.

    I wouldn't be too surprised if you had a situation in which LCDM gets stripped away from galaxies. It also wouldn't surprised me if CDM were unstable so it goes poof after X billion years. Maybe the dark matter is turning into dark energy.

    Lots of things are possible.

    I'll take it as given that LCDM has problems at galactic scales, but then we have to find some explanation why it works so well at predicting acoustic peaks.

    It's interesting. But to really challenge CDM, you have to find some alternative explanation for the cosmological evidence for dark matter.
  18. Jun 13, 2012 #17
    We are looking for something that doesn't appear at galactic scales, but produces baryon oscillations.
  19. Jun 13, 2012 #18


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    As Chronos pointed out, the errors in this paper have ben very clearly pointed out by Bovy and Tremaine, who obtained the expected dark matter density when doing the analysis correctly.
  20. Jun 13, 2012 #19
    Also, the blog talks about the Karachentsev as if it were some new and shocking development, whereas we've know for the last 20 years that the amount of luminous matter is a lot lower than the amount that is predicted by LCDM.

    The fact that there are lots of inconsistencies between LCDM and observations at the galactic scale is widely known and not particularly alarming. LCDM was not designed for modelling the formation of galaxies at cluster scales, and trying to get it to fit with observations of galactic clusters is an area of active research.

    But the reason that people are trying to make the data fit LCDM rather than giving up is that LCDM gives excellent predictions about CMB radiation distribution and large scale galaxy correlations. For people to seriously question the LCDM framework, you need to challenge those bits of data.
  21. Jun 13, 2012 #20
    And normal matter is ruled out because ... ?
  22. Jun 13, 2012 #21


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    Normal matter is ruled out because Big-Bang Nucleosynthesis measurements, CMB radiation measurements and measurements of galaxy clustering all place tight constraints on the amount of baryonic matter, and there just isn't nearly enough. Look at the attached graph, which I lifted from this paper.

    All of these measurements require a lot more matter than can be accounted for by baryons. Also note that these are three completely different measurements. If we had something fundamentally wrong, we would expect them to not even overlap. Yet they do overlap in a small region. The agreement of the Lambda-CDM model with multiple different types of measurements is one of the reasons why most cosmologists believe it. No alternate gravity model even comes close to explaining all of these measurements at once. Maybe someone will come up with one, but nobody has yet, despite a lot of effort.

    Attached Files:

    • CDM.pdf
      File size:
      39.3 KB
    Last edited: Jun 13, 2012
  23. Jun 14, 2012 #22
    So, starting with the assumption that GR gravity is correct, you have proven GR gravity is correct. Got it.

    Or a different physics, which apparently was not tested by these authors.

    Let's put it this way: IF GR gravity is correct, THEN you need DM. That's obviously true and non-controversial. But don't pretend you've proven anything by saying that.
  24. Jun 14, 2012 #23


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    You've proven that a model DOES exist which explains the observations. This is a highly non-trivial statement. It's easy to say that a different physics could explain all of the observations without DM. However, as I and others have said, nobody has been able to come up with such a different physics. To sum up:
    (1) GR + dark matter explains a huge body of different observations remarkably well.
    (2) No other model exists which even comes close.
    (3) Most physicists therefore accept this model and are searching for direct evidence of dark matter.
  25. Jun 14, 2012 #24
    No. Starting with the assumption GR is correct, we've shown that our observations are not in contradiction with GR by modifying our model for what interacts gravitationally. GR is extremely well tested; not a single observation so far has gone against it. So I doubt we're going to toss it out the window for silly spinning galaxies that can be explained by matter that doesn't interact electromagnetically.

    Because, again, we know this dark matter doesn't interact electromagnetically, as we can't see it.
  26. Jun 14, 2012 #25
    Right. You've shown that by imagining that there's something there that we can't detect, we can "save the phenomena" (as Ptolemy would say). Sorta like an epicycle. It works, no doubt about it. But is it true?

    GR gravity is extremely well tested at solar system scales and below. At larger scales it's hardly been tested at all, and when it has been, DM must always be invoked to save the phenomena. This is like saying the Earth can't be round because it looks flat locally.

    Not true. Neither the Pioneer anomaly nor the flyby anomaly are explained by classical GR gravity.

    Those silly spinning galaxies make up most of the Universe. Meanwhile, all attempts at actually detecting DM have failed, and LHC experiments are showing smaller and smaller mass regions in which the putative DM particles can hide out. What happens when those regions converge down to zero?

    Neutrinos don't interact electromagnetically either, but we detect them anyway. And we still should be able to detect DM experimentally. And we haven't.
    Last edited: Jun 14, 2012
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