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Questioning dark matter and Reinventing Gravity

  1. Jan 1, 2009 #1
    I have started reading "Reinventing Gravity" by John W. Moffat. He basicly presents the concept that, to my best understanding, Einstein may have been incorrect in his calculation on gravity because the stars are moving faster then they should be according Einstein's theory of gravity and that in order to explain this physicists came up with the idea of dark matter. According to Professor Moffat, if dark matter cannot be found and proven then we must rework the entire theory of gravity.

    So if dark matter really can't be proven, what does this mean to the world of physics?

    Has anyone read this book? if so what are your thoughts
     
  2. jcsd
  3. Jan 1, 2009 #2
    Hi Panic....

    Seems like you understood correctly,,,observations seem to show that the outer reaches of sprial galaxies move at a more steady velocity than expected...suggesting extra unseen mass is present,,or maybe variations from Einstein/Newton gravitational theory appear beyond certain angular velocities...

    Since Vera Rubin popularized apparant variations from GR in spiral galaxies a preliminary consensus has apparently formed that dark matter makes a better explanation than errors in Einstein's gravitational theory. (Historically, many such consensus have turned out WRONG! Like a steady state universe, for example...brought to an end by Hubble's observations.)

    We'll see which turns out to be true. It currently appears that dark energy is the cosmological constant causing the accelerating expansion of the universe so there is a LOT that is not well understood. So now it looks like all the matter and energy we thought we understood makes up only about 5% of the universe....hardly the status expected!!!
     
  4. Jan 1, 2009 #3
    It's unlikely we will fly off the earth anytime soon due to any change in gravitational laws!!
     
  5. Jan 1, 2009 #4

    i personally find the idea of dark energy so counter-intuitive and unlikely that i must adhere to some modification of GR as a more elegant solution to the observed motion.

    where would dark energy come from? - not from any known type of chemical or nuclear reaction, or it would be detectable from the energy balance equations for those reactions...

    seems like it would be evenly dispersed throughout the universe, (yes?), rather negating any type of localized effect, hmm? doesnt interact with anything? mucho confusiato...
     
  6. Jan 1, 2009 #5
    "where would dark energy come from?" could be similar to asking "where did mass come from?". Not sure that can be answered with any degree of certainty.


    I would have thought dark energy wouldn't be evenly dispersed at all in much the same way that heat energy or magnetic energy is not evenly distributed.

    One could imagine that more than likely it would not be static and could interact quite dramatically with local conditions by modifying space-time in much the same way as mass does.
     
  7. Jan 1, 2009 #6
    This is the "cosmological constant" envisioned around 1915 by Einstein....the energy density is supposedly constant...even as cosmic expansion occurs!!!...likely related to vacuum energy.....you can also read about inflationary cosmology and see how the constant energy density there fueled initial expansion....all theory, of course, but we have some insights...
     
  8. Jan 2, 2009 #7
    When I said what does this mean for the "world of physics" I was meaning how will the scientific commuity react to a change in our understanding of gravity laws and how much of an impact will this have on scientific study.
     
  9. Jan 2, 2009 #8

    ZapperZ

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    Aren't you confusing "dark matter" with "dark energy"?

    They are really not the same thing, and in fact, work in the opposite direction to each other.

    And everyone who responded so far seems to not care about the confusion. So what are you people responding to?

    Zz.
     
  10. Jan 4, 2009 #9

    atyy

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    It depends on the cosmologist(tau), where tau is the proper time along his/her/its worldline. Some are sometimes careless about stating the limits of science. But others would agree with you: "However, general relativity is simply a theory of physics, and it must always be tested against observation. ...... The book is certainly not closed on new theories. If the dark matter particle is detected, much of the motivation to look at ideas like these will disappear. But if dark matter searches show that the required particles are not there, then scientist will take these theories much more seriously." Schutz, http://books.google.com.sg/books?id=jR3S1ValtgYC&printsec=frontcover#PPA404,M1
     
  11. Jan 4, 2009 #10
    WMAP has showed that dark matter is 23%
    http://map.gsfc.nasa.gov/universe/uni_matter.html
     
  12. Apr 21, 2009 #11
    According to Einstein's gravity.

    Moffat and plenty of other physicists, theoretical and experimental, are trying to remove fudge factors and "unknowables" from physics in an attempt to simplify it. Moffat's MOG theory is very interesting, and I highly recommend the book to anyone interested. He also goes through a relatively comprehensive analysis of 'competing' theories and explains the inconsistencies in them while proving MOG does not break down the same way.

    As far as dark matter, Moffat's MOG theory adds an additional vector field to gravity theory (which may, or may not be conceptually equivalent to dark energy or the Cosmological Constant). In doing so, the theory is able to predict the perihelion advance of Mercury all the way through the seemingly anomalous orbital speed of matter toward the outside of galaxies and super galactic structures. This eliminates the need for dark matter, which at this point produces a more elegant, predictive theory that contains no free variables.

    Dark energy is not eliminated necessarily, although it arises as a base concept of the theory and is considered a part of gravity rather than a mysterious property of spacetime.

    Also, toward the end of the book (most of it was written chronologically - so the theory evolves over its course), Moffat makes some interesting claims that remove singularities from black holes and even universe at t=0.

    Very interesting book and his arguments for MOG are a good read.

    Dark matter is generally considered to exist in a halo around massive structures, such as galaxies and galactic clusters, or in other mathematically convenient regions in cases like the bullet cluster. When explaining the behavior toward the outer edges of those structures, it's dark matter, not energy that is usually invoked.

    Dark energy is often considered the force responsible for the acceleration in the expansion of the universe. Moffat et al often refer to it as vacuum energy under the postulation that it is stronger in a vacuum.

    Although after reading Reinventing Gravity, it might be possible to confuse the two anyway, since dark energy is more-or-less included in the theory and dark matter is made irrelevant because the theory accomplishes the same thing. Basically, dark energy as a part of gravity is kind of the reason we don't need dark matter under MOG, to simplify it (probably too much).
     
    Last edited: Apr 21, 2009
  13. Apr 22, 2009 #12

    JesseM

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    SLOSifl, does Moffat address observations which seem to show dark matter having an atypical distribution relative to the matter it's associated with, like this observation where the two dark matter clouds associated with visible gas clouds seem to have overshot the visible clouds when they collided, or this observation where a cloud of gas is flattened in a way that suggests an egg-shaped dark matter halo?

    (also, does your username have anything to do with the old show 'Sifl and Olly'?)
     
  14. Apr 22, 2009 #13
    I would say it's no more counter-intuitive than the "action at a distance" of attractive gravity.

    To me, a repulsive force seems less "spooky" than the attractive force of gravity.

    Either way, GR won't really require modification, it allowed for the possibility of "dark energy" from the beginning.
     
  15. Apr 22, 2009 #14
    Yes, those observations of dark matter distribution is one of the key situations where Moffat was able to apply his existing theory and see results that matched exactly. I'll go grab the book and edit in an excerpt in a minute.

    edit: Dammit, I loaned the book to a friend last night. Either way, the distribution of "dark matter" in and around the larger structures and the large plasma clouds are explained by MOG. The bullet cluster and the other, more recently discovered one...which is what I needed the book for...are both in agreement with Moffat's STVG/MOG.
    Yep!
     
    Last edited: Apr 22, 2009
  16. Apr 23, 2009 #15

    JesseM

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    Interesting, thanks. I wonder if other physicists who lean towards the more "mainstream" perspective on gravity have discussed Moffat's analysis, and if so whether they've concluded it's basically sound (meaning that they agree his theory can accomodate these results in a natural way, not that they are convinced the theory is actually the most plausible one) or whether they have criticized it.
    Nice, I was a huge fan of that show, glad to see someone else remembers it!
     
  17. Apr 24, 2009 #16
    I'm not saying Moffat is unequivocally correct, but he does spend a lot of page-time explaining why other theories fail and how MOG can pass those same tests. STVG contains no free variables and scales extremely well, which adds a simplicity and elegance - and more importantly an innate ability to predict results which is lacking in standard dark matter theories. It's compelling to read, and is very contemporary, so it hasn't even had a chance to become commonly accepted.

    Worth a read, or at least a look (wikipedia STVG to get some info). MOG is very clean, and makes dark matter irrelevant, which is good because it's almost inherent that we can't detect it. Newtonian gravity works on earthly scales, and Einstein's gravity works on solar systemic scales, so a theory that reduces to *both* as you step down is nice.

    I think it's more relevant to follow a predictive and experimentally verifiable line of thought than to obfuscate a theory to the point it's almost required that we never can test it. String theory and m-theory fall in to a really bothersome void of verification.

    Moffat makes some really bold claims, but if he's right, then MOG/STVG is a more natural evolution of Einstein's gravity than anything else out there right now. The book itself isn't nearly as accessible as something like A Brief History of Time, but it spends a ton of time dealing with the history of gravity theory and builds on it incrementally in an intuitive manner, which any extension of gravity should.
     
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