Rewriting cosmology?: Evidence of MOND.

Originally posted by DeepThought
However my mechanism was correct, they do get trapped in the centre of the system.

Yes, they do, but it takes a long time, much longer than for ordinary matter, because non-gravitational interaction is a much more effective way for radiating potential energy.
So the dark matter that went into the sun, is a lower proportion of the dark matter in the region than the proportion of ordinary matter that went in.
Similarly for the big hole(s) at the galactic centre.
This is also why the dark matter halo is bigger than the visible galaxy and also why it is not as strongly disk shaped - if at all.

Originally posted by twhitehead
Yes, they do, but it takes a long time, much longer than for ordinary matter, because non-gravitational interaction is a much more effective way for radiating potential energy.
So the dark matter that went into the sun, is a lower proportion of the dark matter in the region than the proportion of ordinary matter that went in.
Similarly for the big hole( ...[text shortened]... lo is bigger than the visible galaxy and also why it is not as strongly disk shaped - if at all.

This is also why the dark matter halo is bigger than the visible galaxy...

That isn't exactly what the guy in your reference is saying:
http://cdms.berkeley.edu/Education/DMpages/FAQ/question25.html

The bulk of dark matter needs to be inside the galaxy (i.e. with an orbital radius less than the radius of the visible galaxy) to affect the orbit rate.

For dark matter trapped in the sun see the last paragraph of:
http://cdms.berkeley.edu/Education/DMpages/FAQ/question12.txt

Apparently they are estimating that the central black hole gets about 10% of its mass from absorbing dark matter:
http://cdms.berkeley.edu/Education/DMpages/FAQ/question35.html

Edit: also what this entry is talking about is a brane world version of dark matter:
http://cdms.berkeley.edu/Education/DMpages/FAQ/question23.txt

Originally posted by DeepThought
The bulk of dark matter needs to be inside the galaxy (i.e. with an orbital radius less than the radius of the visible galaxy) to affect the orbit rate.

Yes, it would have a density profile similar to the visible matter with highest density in the centre and less further out, but overall it stretches out further and is more spherical.

Apparently they are estimating that the central black hole gets about 10% of its mass from absorbing dark matter:
Which ties in nicely with what I said. Dark matter makes up 65% of visible matter but only 10% of the black hole. This means that visible matter collapses into the black hole faster than dark matter does.

Originally posted by humy
seems to me then that both dark matter theory and MOND have their own problems and it is currently difficult to rationally give favour to one over the other. I wonder if they could both be wrong!?

Or perhaps they could both be right…?

EDIT: I see this possibility has already been broached.

I have a dim view of dark matter. It's tossed about like it's established fact in many books and documentaries (including the latest Cosmos series), when in fact it's just a wild-assed guess with nothing solid to back it up. Like string theory, it's nearly unfalsifiable: if experiment doesn't bear out a prediction, just twiddle the knobs on a few free parameters to explain away the failure. MOND, and other alternative ideas, deserve at least as much attention as dark matter.

Originally posted by Soothfast
I have a dim view of dark matter. It's tossed about like it's established fact in many books and documentaries (including the latest Cosmos series), when in fact it's just a wild-assed guess with nothing solid to back it up. Like string theory, it's nearly unfalsifiable: if experiment doesn't bear out a prediction, just twiddle the knobs on a few free p ...[text shortened]... failure. MOND, and other alternative ideas, deserve at least as much attention as dark matter.

That observations do not match known laws of physics on the observable matter, is an established fact. Supposing the existence of some as yet unobserved matter is a lot more than a 'wild-assed guess'. It fits extremely well with observations in many different ways. Now it is entirely possible that MOND will also fit the observations - mathematics has a way of doing that, where two different underlying effects give the same result. But at the present time, it is known that dark matter in the form of WIMPs fits the observations, but MOND has not yet been shown to do so.
I disagree however that it is unfalsifiable. I think that with some careful calculations it should be possible to distinguish between dark matter and and given MOND.

Originally posted by Soothfast
Or perhaps they could both be right…?

EDIT: I see this possibility has already been broached.

I have a dim view of dark matter. It's tossed about like it's established fact in many books and documentaries (including the latest Cosmos series), when in fact it's just a wild-assed guess with nothing solid to back it up. Like string theory, it' ...[text shortened]... failure. MOND, and other alternative ideas, deserve at least as much attention as dark matter.

A modified theory of gravity as well as a fifth force should be distinguishable from dark matter. With MOND, one would expect visible matter only to be attracted to other visible matter. Whereas with dark matter theories one would expect visible matter to sometimes be attracted to something unseen. A good, and as far as I know the only, example of this is the Bullet Cluster, where the (alleged) dark matter has been separated from the visible stuff.

The real problem with the dark matter hypothesis is that it has turned into a paradigm theory too quickly, so that whenever there is a new observation they immediately start working out where the dark matter could be rather than thinking about what else could be causing the effect that they've seen.