In article <monsq7$j5d$
1...@speranza.aioe.org>, Martin Brown
<|||newspam|||@
nezumi.demon.co.uk> writes:
> Thanks. That link is very interesting. I had thought that MOND was now
> also ruled out of the game now but that page seems to say not.
MOND was developed to explain flat rotation curves of galaxies without
the need for dark matter. Of course it still works for this. There is
only one adjustable parameter (which seems to have a "natural" value),
and over time many other phenomena could be explained with the same
value of the adjustable parameter. This has not changed either. This
original MOND was not really a theory, just an empirical fit.
Certain theories based on MOND have been ruled out.
There is now much evidence for dark matter apart from flat rotation
curves of spiral galaxies. This cannot be explained by MOND, at least
not by the empirical MOND or a small extrapolation of it. However, it
could of course be the case that the MOND effect is real but that there
is also dark matter. There is no reason this could not be the case.
Psychologically, it might not be desirable to supporters of MOND who
don't like dark matter, but the universe is not obliged to conform to
our tastes. It might not be desirable to supporters of dark matter, but
the simplest explanation is not always the best. For example, it turned
out that neutrinos have mass, and are thus non-baryonic dark matter, but
the masses are too small to allow neutrinos to make up all non-baryonic
dark matter, so we already have a case where the simplest explanation
didn't turn out to be right.
Of course, one should prefer the simplest explanation unless a more
complicated one works substantially better (that a more complicated one
could work somewhat better is not sufficient). The question is whether,
in the case of spiral galaxies, MOND or DM is the better explanation.
Note that MOND doesn't give just approximately flat rotation curves,
which DM simulations can also give, but predict many details. It is not
obvious that these simply "fall out" of DM, and at least some claims
that they do is not convincing.
Personally, I am a bit sceptical of MOND, at least until some underlying
theory predicts something like it, but on the other hand my impression
is that the supporters of MOND in general know more about astronomy and
astrophysics than many of the detractors, and many "refutations" of MOND
are quite superficial and show that the detractors haven't investigated
MOND in detail.
Note that Oxford University professor James Binney, who literally wrote
the book on galactic dynamics, has been a MOND supporter for a while
now. MOND is not some fringe or crackpot theory, but an alternative
hypothesis which has not yet been ruled out.