[URL='https://www.physicsforums.com/insights/author/urs-schreiber/']Urs Schreiber[/URL] said:
Similarly, MOND-like modifications of the laws of gravity are further constrained by the new data, see
here:
- Jose María Ezquiaga, Miguel Zumalacárregui, "Dark Energy after GW170817" (arXiv:1710.05901)
- Jeremy Sakstein, Bhuvnesh Jain, "Implications of the Neutron Star Merger GW170817 for Cosmological Scalar-Tensor Theories" (arXiv:1710.05893)
- Sibel Boran, Shantanu Desai, Emre Kahya, Richard Woodard, "GW170817 Falsifies Dark Matter Emulators" (arXiv:1710.06168)
Of course MOND faces bigger problems already,
- Scott Dodelson, "The Real Problem with MOND", Int. J. Mod. Phys. D, 20, 2749 (2011). (arXiv:1112.1320)
but still.
Dodelson (2011) is really just attacking a straw man. Everyone has known since the beginning that MOND-proper is a toy model and needs to be generalized to be relativistic and doesn't capture cluster phenomena. And, it was generalized with TeVeS and a similar approach was made with Moffat's MOG theory (that work for clusters and cosmology). Apparently Boran (2017) is a blow to those relativistic approaches that are more general and not pure toy models.
Ezquiaga (2017) and Sakstein (2017) are not primarily going after MOND-like modifications. They are instead addressing a different group of gravity modifications usually pushed by GR theorists (e.g. some f(R) theories of gravity) designed only to deal with dark energy and not with dark matter - almost the opposite of what MOND-like gravity modification theories do, MOND-like gravity modification theories often don't address dark energy phenomena at all. Ezquiaga argues that gravity doesn't propagate at the speed of light in TeVeS, but I'm skeptical of that claim (he relies on
another paper for this throw away statement in his conclusion) and it is certainly a theory specific argument and not a generalized modified gravity argument. Ezquiaga (2017) also makes clear that some modified gravity theories do make the cut:
Motivated by these results, we identify the theories that avoid this constraint and thus can still be used to explain DE (see a summary in Fig. 3). Within Horndeski’s theory, the simplest models such as quintessence/kessence, Kinetic Gravity Braiding or Brans-Dicke/f(R) are the ones surviving. Beyond Horndeski theory, viable gravities can be obtained in two ways. One can apply a derivative-dependent conformal transformation to those Horndeski models with cg = 1, since it does not affect their causal structure. An example of this is the derivative conformal transformation of GR. Alternatively, one can implement a disformal transformation, which does alter the GWs light-cone, designed to precisely compensate the original anomalous speed of the theory. Specific combinations of Horndeski and GLPV Lagrangians are representatives of this class.
In more general grounds, the bounds on cg severely restrict the kinetic term of gravity to be canonical (of the Einstein-Hilbert form), up to field redefinitions that preserve the causal structure. This requirement provides a strong selection criteria for viable modified theories of gravity, applicable also to theories other than scalartensor gravity. Massive gravity [21], bigravity [55] and multi-gravity [56] all fall in the safe category as long as matter couples minimally to one of the metrics. Note that in the cases with more than one dynamical metric(s), tensor perturbations of the auxiliary, uncoupled metrics will in general travel at a different speed. In minimally coupled scenarios this effect is only detectable by graviton oscillations with the physical metric [57].
Boran (2017) does place significant limits on the parameter space of MOND-like theories that use gravity modification to explain phenomena attributed to dark matter. But I'll defer further commenting on that paper as I haven't had a chance to really dig into it yet. FWIW, at first glance it looks to me like the case of Boran (2017) is probably overstated, but I'm willing to keep an open mind for now.
Certainly, nothing in Boran (2017) in any way impairs the approach taken in the following series of papers that involve a massless boson as a force carrier:
* A. Deur, "
A possible explanation for dark matter and dark energy consistent with the Standard Model of particle physics and General Relativity" (2017).
* A. Deur, "
Self-interacting scalar fields in their strong regime" (November 17, 2016).
* Alexandre Deur, "
A correlation between the amount of dark matter in elliptical galaxies and their shape" (July 28, 2014).
Incidentally, I don't agree that Deur's approach is actually consistent with classical GR as currently formulated, although the tweak that he makes in coming up with his own regime that handles rotation curves, cluster data, elliptical galaxies and cosmology tests, at least at a back of napkin level of precision, are very subtle and very principled. In both results and theoretical motivation it is probably the best of the current gravitational explanations of dark matter phenomena, although it has been ill developed as the author has had to devote most of his work to his day job in QCD and doesn't have the funding, support or following necessary to really kick the tires of this approach.
The other point to recognize is that dark matter particle theories are in very deep trouble in ways which this data point doesn't address. Truly collisionless dark matter is all but ruled out, and the parameter space of self-interacting dark matter theories is also highly constrained.
See, e.g., Lin Wang, Da-Ming Chen, Ran Li "
The total density profile of DM halos fitted from strong lensing" (July 31, 2017); Paolo Salucci and Nicola Turini, "
Evidences for Collisional Dark Matter In Galaxies?" (July 4, 2017).
