Researchers find a galaxy without dark matter

https://news.yale.edu/2018/03/28/researchers-find-galaxy-without-dark-matter

by AKdeBerg • 8 years ago

2 1 8 years ago

news.yale.edu

raattgift• 8 years ago

This is a couple of days old, and was discussed at length here: https://news.ycombinator.com/item?id=16701248 (~ 280 comments).

Since then, however, McGauch (of modified gravity) put out a blog entry (dated yesterday): https://tritonstation.wordpress.com/2018/04/04/the-dwarf-gal...

I'd reword his second last sentence by adding a couple of words in two places: "MOND is one example of a class of bets that what we call dark matter is really the manifestation of gravitational physics beyond General Relativity"; he himself has catalogued several non-General-Relativity theories that resemble MOND especially in the non-relativistic limit (with Famaey, in their excellent and reasonably fair 2012 review https://arxiv.org/abs/1112.3960 section 7).

Virtually all of these are metric theories of gravity with extra degrees of freedom beyond those in General Relativity. Consequently, unless they want to be haunted by ghosts (which ruin otherwise well-posed Cauchy problems) the proponents are really making an aesthetic decision to label their new DoFs "modified gravity" maybe justifying the decision with some unusual coupling to the other fields or some argument that the field content isn't interpretable as particles even in principle. It is fairly straightforward to argue that they could make the opposite decision and label their new DoFs "dark matter" (and even particle Dark Matter on the assumption that their matter field is quantizable after all, at least in some effective limit) and the new physics would be identical.

There is of course still interesting physics to be discovered in the non-gravitational sector -- the Standard Model is (probably, hopefully) not the last word there for reasons beyond those motivated by galaxy-scale dynamics or cosmological results like the the BAO acoustic peaks in the cosmic microwave background (cf. sec 9.2 of Famaey & McGauch, although bear in mind that there is newer data since Fig 45.).

Researchers find a galaxy without dark matter

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