|Mar6-05, 07:52 PM||#1|
Virgo problem for MOND
Does VIRGOHI21 Pose a Problem for MOND?
Scott Funkhouser, Occidental College, Los Angeles
"If the inferred parameters of the recently discovered dark galaxy VIRGOHI21 are verified then thedynamics of the body may represent a counter-example to the accelerations predicted by MOND."
my comment: I think several people on this forum have already been discussing this, and it has be in the media IIRC
So far this the first journal article type preprint I've seen on arxiv about it,
so I'm flagging it in case it can be of use to anyone.
as far as MOND, it aint over till it's over and it aint over yet. Has Ohwilleke commented on this Virgo dark galaxy thing?
|Mar7-05, 02:19 AM||#2|
As you note, MOND has at least as many problems as the CDM solution. I resist tossing away the GR model of gravity just because it does not appear to answer every question. GR has withstood every rigorous test devised to date. I'm not against empirical solutions - they are often useful and lead to new science. But MOND still has a lot more hills to climb before that horse earns the right to graze atop the mountain.
|Mar7-05, 11:28 AM||#3|
It has been discussed in some other threads. See here: http://www.physicsforums.com/showthread.php?t=64698 and here: http://physicsforums.com/showthread.php?t=60229 (moderated thread).
Virgo might actually do a lot to confirm MOND rather than disprove it. MOND predicts that there will be large apparent "dark matter" effects in low surface brightness galaxies. http://arxiv.org/abs/astro-ph/9805120 Naturally, then, one would expect that a galaxy with surface brightness so low that it approaches vanishing would have a huge quantity of apparent "dark matter", in the MOND case flowing from the fact that a diffuse hydrogen gas cloud over a large expanse of area should have very weak gravitational fields, which is where MOND effects are most significant.
In contrast, it is not at all clear that lambda CDM theories would have a priori predicted 1000-1 dark matter ratios in such circumstances.
The one page article cited in the OP here closes with a line that pretty much makes it irrelevant. In layman's terms it says, "take what I just said with a big spoonful of salt because my data sucks".
|Mar8-05, 09:36 PM||#4|
Virgo problem for MOND
I hope we keep it current.
your thread had slipped my mind when i was wondering where to post this item.
It looks like Jim Graber was referring to the same "dark galaxy" or galaxy-size dark cloud, when he posted this:
|Mar9-05, 01:56 AM||#5|
Food for thought from this discussion:
MOND is an interesting enough idea that a number of people have tried to develop it into a sensible relativistic theory that doesn't obviously fail in one way or another. There was a nice argument a year and a half ago by Soussa and Woodard showing that you couldn't do this in a purely metric formalism without getting the wrong results for gravitational lensing (http://www.arxiv.org/abs/astro-ph/0307358). By adding an extra vector and an extra scalar, Bekenstein may have managed to get around this constraint, though at the expense of an extraordinarily complicated model.
An immediate worry is that by introducing a unit vector field, Bekenstein
is going to get spontaneous Lorentz invariance violation. There has been
some work on similar, although not identical, theories in a very different
context -- searching for ways to further test Lorentz invariance -- by
Jacobson and Mattingly. There, the presence of a unit vector field causes
a number of potentially undesirable results. For example, it can easily
screw up binary pulsar orbital decay (you get new radiative modes), and
can lead to a variety of Solar System problems. Bekenstein says that the
post-Newtonian parameters related to preferred frame effects haven't yet
been computed in his model, and I think it's likely that when they are,
they will at least require some very fine tuning of coupling constants to
get consistency with observation.
Beyond that, though, I would read Bekenstein's papers as a demonstration
of how hard it is to get a phenomenologically viable version of MOND. Note,
for example, that his action contains an arbitrary function F that has to
be carefully chosen, and looks very peculiar (look at eqn. (5.12) of the
preprint http://www.arxiv.org/abs/astro-ph/0412652), as well as very peculiar kinetic terms for the scalar field in the action. All in all, it's a nice demonstration of why one might prefer dark matter.
|Mar11-05, 01:55 PM||#6|
This is an abstract of a 2002 paper, which is a bit old, but it really lays out the case for MOND well, so I'm posting it here for future reference:
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