
Researchers find a galaxy without dark matter - AKdeBerg
https://news.yale.edu/2018/03/28/researchers-find-galaxy-without-dark-matter
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raattgift
This is a couple of days old, and was discussed at length here:
[https://news.ycombinator.com/item?id=16701248](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...](https://tritonstation.wordpress.com/2018/04/04/the-dwarf-galaxy-
ngc1052-df2/)

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](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.).

