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

Alternative theory of gravity explains large structure formation without dark matter

  1. Dec 14, 2006 #1


    User Avatar

    When i read "Alternative" i immediately thought "crackpots!", but then I read on and found out they're on to interesting stuff.

    Last edited by a moderator: Dec 14, 2006
  2. jcsd
  3. Dec 14, 2006 #2
    There's also Conformal Gravity theory promoted by Philip D. Mannheim. This theory is suppose to explain dark energy and dark matter by suggesting that gravity gets weaker at great distances. It sounds something like MOND but based more on first principles than on curve fitting.


    He also has a video at the Perimeter Institute. I can provide a pointer to the video if there is interest. Lee Smolin appears in the lecture which tends to lend the idea some credibility.

    I have to wonder if the recent extented supernova data rules out any of these alternative theories.
    Last edited: Dec 14, 2006
  4. Dec 14, 2006 #3


    User Avatar
    Science Advisor

    Modified Gravity vs Dark Matter has been a long running debate. However recent observations of an interesting system of colliding clusters has strongly tipped the balance towards dark matter. http://cosmicvariance.com/2006/08/21/dark-matter-exists/" [Broken] explains it far better than I could.

    In general though, the problem with modified gravity in general is that the parameters describing the magnitude of the modification to GR/Newtonian gravity come out to be different every time you try and measure them. So you can could explain one galaxy rotation curve by a modified gravity model, but if you apply that model to a different galaxy the parameters need to be tweaked. The same goes for clusters and large scale cosmology. Of course Dark Matter dosn't have this problem as you would expect the amount of dark matter present in a galaxy halo to vary from system to system.

    The reason the dark matter is favored at present over modified gravity is that it fits a wide range of data with the same parameters. There are no modified gravity theories at present that can do better than dark matter with a single parameter set. This of course may change in the future!
    Last edited by a moderator: May 2, 2017
  5. Dec 14, 2006 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    There's also an apparent (to me) selectivity in the galaxies the MONDians seem to have chosen to model the rotation curves of (if that makes sense).

    On McGaugh's 'scorecard' webpage (don't have a link with me just now), some very well known spirals are missing - M31 and M81, for example. These are among the first to have had rotation curves measured, and among the best studied of spirals, period. How come no MONDian has chosen to analyse them?
  6. Dec 14, 2006 #5


    User Avatar

  7. Mar 18, 2007 #6


    User Avatar

    Experiment sets the ultimate test for Newton's laws

    A physicist in Australia has come up with an experiment that could potentially reveal a flaw in Newton's law of gravitation. If the flaw exists, it would be the first evidence in support of theories that explain the movement of galaxies without having to introduce "dark matter" (Phys. Rev. Lett. 98 101101).

    For the past 70 years or so, physicists have been bothered by a nagging question: why do the centres of galaxies rotate too fast for the amount of mass we can see through telescopes? The most popular answer is that most of the mass is hidden in large bands of "dark matter", a substance that is invisible because it doesn't interact strongly with light. If it exists, dark matter could account for 95% of the mass in galaxies, and would explain many other aspects of the universe.
    X marks the spot

    However, a lack of evidence for dark matter has led a small camp of physicists to promote an alternative answer: the gravitational force that holds galaxies together decays more gently with distance than presently estimated, meaning that Newton's law of gravitation is not quite as simple as an inverse-square relationship. The theory, which is known as modified Newtonian dynamics (MOND), proposes adding extra factors to Newton's 300-year-old equations so that the gravitational behaviour only alters at very low accelerations. Unfortunately the turmoil of gravitational forces produced in the galaxy means that such accelerations are hard to come by, leaving proponents of MOND with no easy way to test their theory.

    However, Alex Ignatiev from the Theoretical Physics Research Institute in Melbourne claims to have predicted instances on the Earth where most of these forces will cancel out. Ignatiev first considered how an object at rest in the centre-of-mass of our galaxy would appear to be accelerating when viewed from a laboratory on Earth. This involved listing all the major accelerations such as the Earth's rotation around the Sun and the Sun's orbit in our galaxy. He then looked for solutions where all of the accelerations add up to zero.

    The solutions indicated that, on either of the two annual equinoxes, there will be two places on the Earth's surface where the force cancellation occurs. For example, on the equinox of 22 September 2008, one will be in the far north of Greenland and the other will be on the opposite side of the world in Antarctica (see figure: "X marks the spot"). Ignatiev says that if a gravitational wave detector is set-up to monitor a static test object at one of these times and places, it might just be able to glimpse a tiny, 0.2 × 10-16 m deflection over a period of 0.5 ms – what he calls "SHLEM" (static high-latitude equinox modified inertia). If SHLEM is observed, it would be the first evidence in support of MOND.

    "Even if the result were negative it would be a very significant step forward, because an interesting theory would be ruled out," Ignatiev told Physics Web. "But if the predicted SHLEM effect were observed – well, we'd have to rewrite our most basic theories."

  8. Dec 11, 2009 #7
    Re: Alternative theory of gravity explains large structure formation without dark mat

    YOu got to be kidding me, the same parameters eh? Can you show ANY consistent relation to dark matter and visible matter? No there is not, dark matter is simply adding up visible mass, then filling in the balance with dark matter. Every single time new objects are discovered it changes the so called dark matter. So MOG needs far less tweaking, there is no purpose in applying different theories to ever more distant and less known galaxies and clusters because until we detect EVERY object in one galaxy we cannot make any leap over the simple "theory" term. No theory is above proof, even relativity with dark matter. Actually I would make the opposite argument.

    Whichever model works the best with the closest and more studied celestial groups with the least amount of variable integers would be the best. In other words we should put all our effort into explaining something we can see well enough to discover it all. I get so angry when people throw up, well this theory can't explain this cluster, or this galaxy, or this far flung corner of space we know next to nothing about. We are still discovering massive objects close to home that change our makeup of galaxies. Magnetars are my favorite example, here is a massive object we know very little to nothing about, new observations are placing more and more mass in our universe all the time. Once we have observed enough of these types of bodies, then we can possibly accurately approximate their distribution throughout our galaxy, or others.

    I mean lets start close to home, relativity cannot even explain how far our most distant probes have travelled without shoveling in undetectable energy sources and mass. They have had plenty of time to discover dark matter, its not going to happen relativity and dark matter are toast. Time to at least stop pretending we can understand and predict all the movements in the universe, there will be no magic equation until maybe we have colonized the entire galaxy and have detected every last object in our own backyard.
    Last edited by a moderator: May 4, 2017
  9. Dec 12, 2009 #8
    Re: Alternative theory of gravity explains large structure formation without dark mat

    Mond isn't close either, you have to use some kind of vector field to get close. Apply MOND to gravitational lensing and its dead in the water. MOND cannot account for gravitational lensing unless you add vector fields.
  10. Dec 14, 2009 #9
    Re: Alternative theory of gravity explains large structure formation without dark mat

    No it's not. There are specific properties that dark matter has to have for this to work. Just to name one, the dark matter has to be cold.

    No it doesn't. All of the new objects that have been discovered are baryonic (i.e. made of ordinary matter) so we find something that isn't baryonic, it's totally irrelevant to dark matter.

    Just because it's closer doesn't mean it's better studied.

    I don't think you quite understand the nature of the problem. Suppose you have lots of hidden ordinary matter in the universe, you then have a big problem because this increases the density of the early universe, and the denser the universe is, the more deuterium gets burned off. Also, you calculate the distribution of galaxies, and if the dark matter was baryonic, then the galaxy distributions would be less "fluffy."

    If it turns out that there is a lot more ordinary matter than think is there, then we really have some major problems. So finding lots of unknown objects made of ordinary matter actually causes the dark matter problem to get worse, not better.

    Also, what you are saying in general makes no sense. I know more about the moon and the sun than I do about Rio de Janeiro, since I can see the moon and the sun, and I've never been to Rio. Similarly, we have a *lot* of information from the early universe, but we don't have that much information about the Oort cloud or planets around Alpha Centauri.
  11. Dec 14, 2009 #10

    Vanadium 50

    User Avatar
    Staff Emeritus
    Science Advisor
    Education Advisor

    Re: Alternative theory of gravity explains large structure formation without dark mat

    There are plenty of reasons to dislike MOND, but this is not one of them. The N in MOND stands for "Newtonian", and MOND does little to constrain the family of relativistic extensions. So the proper question to ask is whether the observed lensing is consistent with a relativistic extension of MOND or not, and this is precisely the question asked by Mortlock & Turner (2001, PASA, 18, 189), who concluded that it was.
  12. Dec 14, 2009 #11
    Re: Alternative theory of gravity explains large structure formation without dark mat

    I'm sorry but there has been many papers published since 2001 completely discrediting MOND's lensing explanations. NOt unless your talking of the TeVeS brand. Just Google MOND and lensing and look at the current info. I feel like I"m the only person here that watches the cosmic news lol. The evidence is completely overwhelming, the only way MOND is going to have any progress is under the relativistic generalization of TeVeS. Or by publishing some papers explaining the lensing data held against it. TeVeS has been able to account for lensing effects, however hasn't done the CMB yet.

    You know I want to say that there is no complete theory. One day we may be able to predict all movements and everything in existence but that will never mean we can understand what forces do the moving, nor does it really matter because we will always continue searching for a better universe. We may one day find out the universe is in a locker, but you know what we would do then? Start making theories on whats down the hall....
  13. Dec 15, 2009 #12
    Re: Alternative theory of gravity explains large structure formation without dark mat

    Without a model (observational data,then model with some principle, then check) otherwise is pure data fit.
    but at the extent of available data only a new model can solve the puzzle.
    open your mind, find it, discuss it, be prepared to put our actual convictions to test.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook