More Dark Matter And Modified Gravity Limitations

* Direct dark matter detection experiments are starting to impose meaningful limitations on dark matter particles sub-GeV masses for particle that interact via the weak force but not the other two Standard Model forces. 

Constraints are meaningful down to about 10 MeV. This is heavier than an up quark, a down quark, an electron, or any kind neutrino. But, for example, these results pretty convincingly disfavor weakly interacting dark matter particles with masses comparable to a muon (105.658 MeV), a strange quark (92-104 MeV) (which in any case is never observed outside of a hadron), or the lightest hadron (which is the neutral pion (134.977 MeV)), for which cross-sections of interaction more than 0.01 pb (10^-38 cm^2) are ruled out.

This progress was made possible by looking for interactions between electrons and dark matter, rather than atomic nuclei and dark matter.

In a related development the Standard Model prediction for the weak charge of the proton (which is relevant to the cross-section of interaction between particles that interact via the weak force and protons such as hypothetical WIMPs), and the experimentally measured value, confirm each other and are much lower than one would naively expect. This confirmation also functions as yet another of many meaningful global constraint on beyond the Standard Model physics near the electroweak scale. The measurement "can constrain new weakly coupled particles with masses up to a few TeV, or even 50 TeV particles if they are strongly coupled to matter (g*～4π)." But, the strongest constraints on deviations from the Standard Model prediction for the weak charge of the proton actually come mostly from earlier experiments rather than the most recent one.

These new direct dark matter detection doesn't impact "warm dark matter" scenarios with typical dark mark particle masses in the keV range or completely sterile dark matter candidates with no non-gravitational interactions. But, it is yet another nail in the coffin of the WIMP paradigm preferred in supersymmetry based dark matter scenarios.

* New direct dark matter search results from the Xenon1T experiment will be announced on Monday, May 28, 2018. In all likelihood, the results will exclude additional dark matter parameter space, because the rumor mill would be buzzing by now if the results would involve an actual claimed direction detection of dark matter.

* A credible argument has been made to slightly relax on of the empirical constraints on primordial black hole dark matter, by opening up more free parameters in the models considered, but only modestly.

* New empirical constraints have been placed on self-interacting sterile neutrinos.

* A new way to empirically test dark matter proposals by looking at the character of "stellar streams" around galaxies has been proposed. The theoretically predicted properties of stellar streams under various dark matter hypotheses have been determined, so now astronomers can can compare what is observed to these predictions, something that will be possible to observed with "next generation telescopes", so this is something today's graduate students and currently practicing astrophysicists will see in the medium term future. In general, heavier dark matter particles imply more clumping, more collisions, more gaps in the stream, and a concentration of stars closer to the center of the streams than lighter dark matter particles.

So far as I know, comparable predictions have not been made for theories like MOND in this system which is well within its domain of applicability but very much complicated by external field effects in these systems (which when present, tend to cause the system to behave closer to a Newtonian gravity model with no dark matter).

* A recent review article compiles limitations on the parameter space of simplified dark matter models from a variety of different kinds of evidence including both astronomy evidence and particle collider based limitations, and also conveniently and clearly explains the leading simplified dark matter models. If you click through to read the body text of any of the links in this post, you should do so for this study as it has a number of informative parameter space exclusion charts for a wide array of simplified dark matter models - almost all of which have large shaded excluded regions - that aren't suitable for including in this post as each one requires considerable explanation.

* Massive gravity is also pretty much dead due to new experimental constraints, after being revived due to a loophole that overcame earlier problems identified with it.

Massive gravity is interesting because it makes careful analysis of graviton self-interactions absolutely necessary in a way that is often glossed over in massless graviton theories, which is useful because while gravitons lack mass, they do not lack mass-energy. But, the predictions of massive gravity theories are wrong even in the limit as graviton mass approaches zero. 

* The gravitational wave event GW170817 constraints relativistic generalizations of MOND, for example, ruling out the first and most elegant of those (TeVeS), but does not rue out all relativistic generalizations of MOND.

Post-Script: QCD formulated in GR Terms

While not exactly on the point, one interesting recent paper that doesn't naturally fit in any neat box expresses the quantum chromodynamics a.k.a. QCD a.k.a. the Standard Model description of the strong force in an equation form closely analogous to the equations of general relativity. Usually, quantum gravity researchers go in the opposite direction and try to find formulas analogous to those of the other Standard Model forces for general relativity.