I don't know what happened to my response.
Retrying

On Feb 25, 9:56 pm, "Jonathan Thornburg -- remove -animal to reply"
wrote:
wrote:
MONDhas been quite successful in fitting rotational speed data for
the galaxies. It has been tested on over 120 galaxies and there have
been no failures. Please seehttp://www.astro.umd.edu/~ssm/mond/.

However, there also seem to be serious problems in trying to useMOND
as an explanation for galaxy rotation curves. See, for example,
http://fr.arxiv.org/abs/hep-ph/0105083
whose abstract reads:
Recently, several interesting proposals were made modifying the
law of gravity on large scales, within a sensible relativistic
formulation. This allows a precise formulation of the idea that such
a modification might account for galaxy rotation curves, instead
of the usual interpretation of these curves as evidence for dark
matter. We here summarize several observational constraints which
any such modification must satisfy, and which we believe make more
challenging any interpretation of galaxy rotation curves in terms
of new gravitational physics.

I understands the points 1, 2, and 4, and MOND works with them, as
the paper says. I don't understand the 3rd point it seems that it is
putting forward a theory that the potential in the disk plane will be
non-newtonian but perpendicular to it it will be newtonian, then uses
several observations to prove it. I don't know why is that. It should
have done the opposite. It also doesn't talk about MOND or any
other theory in that perspective, if they match the observations.


http://fr.arxiv.org/abs/astro-ph/0104435 is another critique, arguing
that trying to extendMONDto explain larger-than-galaxy phenomena
(particularly cosmology) runs into serious problems.

Well actually I have no problems with Dark Matter. We don't
really know the whole universe and there could be curiosities
that are too strange. We also know that MOND is not a
good theory. It is just a simple equation, which works very
well. I would say too well at the Galaxy level.

Even TeVeS is not a good theory. It is workable, but not
very good. It is ugly and ad-hoc. Which means that it will
not survive for very long.

What MOND does say that for most of the galaxies there
exists very little if any Dark Matter. It provides fits for galaxies
that no other theory, GR+DM or otherwise can match.

Also till date there have been no failures of MOND. I define
failure as in a body of mass that shows less gravitation
than MOND predicts. If there is more gravitation than MOND
predicts, it could be because MOND is wrong or because
there is some type of dark matter. But existence of DM
is not fatal for MOND. Unless you get very religious about
it, you can always admit that we don't know everything
about the universe and DM may exist.

The fact that it works as well as it does means that MOND
phenomenology is a fact at Galaxy levels. Which means
that no theory can be accepted unless it contains MOND
as an explanation. MOND cannot be ignored after fitting
120 galaxies which is probabilistically impossible if Dark
Matter could be distributed randomly.

The only way GR+DM can explain MOND by saying that
during structure formation DM and BM (baryonic mass)
where created so as to give MOND phenomenology. And
that the structures have diverged in a way so as to keep
MOND phenomenology at the galaxy scale.

Now this poses an obvious fine tuning problem, but we
may be able to live with it, if we can come up with a
possible structure theory that could explain MOND,
ie fit the observations based on the BM only, as
MOND does.

Currently the best CDM fits to the Rotation curves of
galaxies require two free parameters per galaxy.
This is way too inferior. MOND does much better with
a single universal parameter.

Now I will attempt to prove that there can be no
DM structure formation theory that can explain MOND.
For this I will use the excellent fits at
http://www.astro.umd.edu/~ssm/mond/n1560.gif
and other similar fits at
http://www.astro.umd.edu/~ssm/mond/mdlg.gif
and at http://www.astro.umd.edu/~ssm/mond/mdlg.gif_2

Now if you see these fits you will notice that most of
these galaxies have a lot of deviation from standard GR.
That is they require a lot of DM. Actually much much
more DM than there is BM in them.

You will also notice that MOND is also predicting the
wiggles in the rotation curve. This must mean that
the wiggles must also be present in the DM profile.

I believe that you will also agree that even if the
DM and BM evolved together, so as to maintain the
MOND phenomenology, it is still a tall order to
assume (and an even bigger fine tuning problem)
that they will evolve exactly the same. There will be
some deviations.

I also believe that at the time of structure formation
the rotation curves of the structure would be very
smooth, and the wiggles would occurr over
time due to irregularities getting magnified with time.

This means that the Wiggles in an otherwise smooth
curve would be due to the irregularities in the BM and
DM components. I would expect much less deviations
in the DM structure than the BM structure, because
DM does not interact much.

This will mean that the wiggles will be mostly due to
BM but will be quite a bit damped. ie the wiggle in
the BM structure will have quite a bit less effect on
the rotation curve. So that MOND will predict a much
bigger wiggle than in the rotation curve.

But these curves show that the wiggles are fit quite
well by the MOND theory. The magnitude of the
wiggles are also nearly the same. This would mean
very little DM if it exists. This is quite contrary to
the GR expectation.

Obviously if DM is not in these galaxies then GR
does not work well at these scales. And if GR does
not work at these levels, then we can't be sure of
its implication on Clusters, and cosmological scales.

We need a better theory. TeVeS may be a better
theory than GR, but it still needs more DM for
larger structures. It is ad-hoc and quite ugly, but
may be better to work with till we get a really good
theory.

I hope that my logic makes sense. Of course everything
depends on the fits at Stacy's site to be correct.
Which I believe they should be, because nobody refutes
them.

regards,
-anandsr