These "MOND" gravity theories purport to explain observations better than the theory of dark matter, but they don't. They explain some things and leave gaping holes elsewhere. On the whole they are hugely inferior theories in terms of matching observational evidence. Their only advantage is that they lack the existence of massive, weakly interacting particles, which a lot of people seem to have strong objections to despite the fact that several types of such particles are already known to exist.
Throughout the entire history of the development of the theory of dark matter the bias has always been against the idea of true dark matter, unseen particles that make up most of the mass of the Universe. But at every single step all of the other possibilities have been eliminated as realistic explanations of the evidence. The cold WIMP dark matter theory is the only one that explains the structure of the cosmic microwave background radiation, the large scale structure of the Universe, and the structure and behavior of galaxies and galaxy clusters. At this point the theory of dark matter is remarkably well hemmed in, it would be absolutely shocking to an extreme degree if it did not truly exist in the form we think it does.
There's additionally a very elegant mechanism behind the formation of dark matter. It seems most likely to be composed of super-symmetric particles. Such particles would only be able to form in extreme, high-energy conditions. Precisely the sort of conditions that existed in the early era of the Universe immediately after the Big Bang. So the early Universe would have been creating huge quantities of such particles, which would then not interact much due to their weak interactivity. It wouldn't be until the Universe had expanded enough and cooled down enough to halt their production. This would neatly explain why so much of the mass of the Universe is in the form of dark matter rather than conventional matter.
> These "MOND" gravity theories purport to explain observations better than the theory of dark matter, but they don't.
MOND consistently fits the rotational velocity profile of galaxies better than dark matter. This isn't too surprising as MOND originated as an ad-hoc formula derived from observations of said rotational profiles, however MOND works very well over a very wide range of galactic profiles. The mass deficit observed in tidal dwarf galaxies is also quite strong evidence against just dark matter. I don't think anybody is seriously claiming that MOND is a complete theory, though of course there have been attempts by theorists at creating complete models, but they've failed to explain large-scale structural observation and the cosmic microwave background. This just means these attempts at creating a complete theory are probably wrong, not that there isn't something funky going on with galaxies that isn't completely accounted for by dark matter.
> They explain some things and leave gaping holes elsewhere.
This is a false dichotomy. MOND is not incompatible with dark matter. This "us vs them" mentality is really quite harmful. A combination of MOND and dark matter matches most observations very well, far better than dark matter in isolation.
> Their only advantage is that they lack the existence of massive, weakly interacting particles, which a lot of people seem to have strong objections to despite the fact that several types of such particles are already known to exist.
While I'm sure there are quite a few people for whom this is the case, you shouldn't dismiss the possiblity there are people who take MOND seriously because it legitimately fits the data better in some circumstances.
Well, Newtonian physics can be readily derived from General Relativity as a sensible approximation. General relativity has a sensible backing in Hamiltonian mechanics.
The relativistic version of MOND, TEVES, is known to be unstable and its action is a complete mess. Moreover, since the action is attempting to add a scalar field and a vector field (which in relativistic theory is just a matter field) it is exactly proposing the existence of an unexplained matter field: dark matter.
I imagine that in a combined mond+dark matter regime, one would need a lot less of the dark matter flying around... Any sense of how the ratio of dark matter to regular matter plays out amongst people working on hybrid theories?
> A combination of MOND and dark matter matches most observations very well, far better than dark matter in isolation.
Isn't that unsurprising that a theory with more degrees of freedom can fit the data better? It's hard to say if this is interesting without knowing if it fits data significantly enough better to make up for the extra parameters. Dark matter (well, Lambda-CDM) is appealing for being able to fit a wide range of observations with a fairly simple model that's entirely compatible with GR.
These "MOND" gravity theories purport to explain observations better than the theory of dark matter, but they don't. They explain some things and leave gaping holes elsewhere. On the whole they are hugely inferior theories in terms of matching observational evidence. Their only advantage is that they lack the existence of massive, weakly interacting particles, which a lot of people seem to have strong objections to despite the fact that several types of such particles are already known to exist.
Throughout the entire history of the development of the theory of dark matter the bias has always been against the idea of true dark matter, unseen particles that make up most of the mass of the Universe. But at every single step all of the other possibilities have been eliminated as realistic explanations of the evidence. The cold WIMP dark matter theory is the only one that explains the structure of the cosmic microwave background radiation, the large scale structure of the Universe, and the structure and behavior of galaxies and galaxy clusters. At this point the theory of dark matter is remarkably well hemmed in, it would be absolutely shocking to an extreme degree if it did not truly exist in the form we think it does.
There's additionally a very elegant mechanism behind the formation of dark matter. It seems most likely to be composed of super-symmetric particles. Such particles would only be able to form in extreme, high-energy conditions. Precisely the sort of conditions that existed in the early era of the Universe immediately after the Big Bang. So the early Universe would have been creating huge quantities of such particles, which would then not interact much due to their weak interactivity. It wouldn't be until the Universe had expanded enough and cooled down enough to halt their production. This would neatly explain why so much of the mass of the Universe is in the form of dark matter rather than conventional matter.