Why Are Dark Matter And Modified Gravity In Such Conflict? (Synopsis)

“All we know so far is what doesn’t work.” -Richard Feynman

The past month has seen a slew of papers out highlighting the tension between modified gravity and dark matter. Both recognize the same puzzles and problems with the Universe, and both ideas recognize that either one could be valid. In fact, if you look at the two greatest “crises” in gravity in the 19th century, it’s arguable that dark matter (Neptune) solved one, the Uranus problem, while modifying gravity (with Einstein’s general relativity) solved the other.

The X-ray (pink) and overall matter (blue) maps of various colliding galaxy clusters show a clear separation between normal matter and dark matter. Image credit: X-ray: NASA/CXC/Ecole Polytechnique Federale de Lausanne, Switzerland/D.Harvey & NASA/CXC/Durham Univ/R.Massey; Optical & Lensing Map: NASA, ESA, D. Harvey (Ecole Polytechnique Federale de Lausanne, Switzerland) and R. Massey (Durham University, UK).
The X-ray (pink) and overall matter (blue) maps of various colliding galaxy clusters show a clear separation between normal matter and dark matter. Image credit: X-ray: NASA/CXC/Ecole Polytechnique Federale de Lausanne, Switzerland/D.Harvey & NASA/CXC/Durham Univ/R.Massey; Optical & Lensing Map: NASA, ESA, D. Harvey (Ecole Polytechnique Federale de Lausanne, Switzerland) and R. Massey (Durham University, UK).

Now in the 21st century, we have a whole Universe to explain, and while dark matter is definitely the leading theory, the idea of modifying gravity isn’t crazy. Moreover, it has a success that dark matter can’t match: on galaxy-scales and below.

The details of the small-scale structure predicted by dark matter do not match with what we observe. The hope of the dark matter camp is that improved simulations and models will come to reproduce them accurately and robustly. Image credit: NASA, ESA, and T. Brown and J. Tumlinson (STScI).
The details of the small-scale structure predicted by dark matter do not match with what we observe. The hope of the dark matter camp is that improved simulations and models will come to reproduce them accurately and robustly. Image credit: NASA, ESA, and T. Brown and J. Tumlinson (STScI).

In the end, it will take a big step forward for a true victor to emerge, but here’s where the science stands right now.

8 thoughts on “Why Are Dark Matter And Modified Gravity In Such Conflict? (Synopsis)”

  1. I find it strange and amusing that each camp is emotionally invested in the controversy, hoping that their idea is right. Why?The real excitement is discovering the scientific truth, whatever it may turn out to be.

  2. @ Ted

    In the scope of humanity, yes, progress and scientific truth is what matters. But from the individual or team standpoint… no one wants to be remembered as the ones who were wrong. So it’s hardly strange that the people involved, are involved personally as well.

  3. The modified gravity people need to explain why they think there are no other quanta in nature beyond those of the standard model of particle physics. If one grants the possibility that there are additional types of quanta then there is no need to be opposed to dark matter’s existence.

  4. I was wondering though if there actually is a plausible orbit for Vulcan under Newtonian mechanics that is capable of reproducing the precession in the orbit of Mercury, which we now understand to be caused by relativistic effects.

    There is a big difference in the parallel though. The gravitational field around Mercury is fairly strong as it is the closest planet to the sun, so if there is a modification to the theory of gravity, its effects would be most visible where the gravitational fields were strongest.

    It is a certainty that general relativity, in spite of all its successes, is not the last word on gravitation. The fact that the theory has singularities and that it is fundamentally incompatible with quantum mechanics is a clear sign. However, such deviations would more likely manifest in extremely strong gravitational fields, e.g. neutron stars, black holes, or in the very early universe. We’d not expect a quantum theory of gravity to show effects in objects on the scale of galaxies, where even Newtonian gravity should work well, else we’d also be seeing the same effects doing odd things to the orbits of the planets in our solar system.

  5. Louis,

    You actually have it a bit backwards. The MOND folks are perfectly correct to assume that so long as all observations can be accounted for without anything other than the Standard Model, we should not postulate any new entities outside of that model. MOND was originally developed to explain the observed galactic rotation curves, which could not be explained by GR. It is very successful at this, even more so than the dark matter models. If that were the only observation out there, then it would be perfectly correct to NOT assume anything outside the Standard Model.

    Of course, that’s not the only observation. All other cosmological observations have not been successfully explained by MOND. MOND cannot account for the CMB power spectrum or the primeval elemental abundances, for instance. Dark matter does a much better job of explaining these observations. That is why dark matter is needed, not just because we should not assume that the Standard Model is all there is. In fact, it is just the opposite. Without observational evidence to the contrary, we SHOULD assume that the Standard Model is all there is.

  6. “Why?The real excitement is discovering the scientific truth, whatever it may turn out to be.”

    Well, they’re BOTH trying to discover the scientific truth, whatever it may turn out to be, but you sit there sniggering at them for doing so.

    If you have the scientifically valid and correct theory, you don’t get to the stage of proving it without being really invested in that theory being the true one. But if they do that, you will laugh at them and insinuate that they’re doing it all wrong (heck, even ARE wrong).

    Why? All you seem to want is to feel superior to those doing the work without having to do the effort. What REALLY matters is doing the work to find out.

  7. I wonder if anyone has considered that reality might be some combination of both theories. Maybe gravitation does have to be modified, but the remaining differences at the cosmological scale require some lower portion of dark matter.

  8. Yes, lots. However, DM *could* explain everything on its own, with some specific density functions being given, but MOND can’t. Not without giving every galaxy a different set of physical laws, which would have to be specially begged in in such a way as not to change the emission spectra of atoms.

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