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Robh
01-10-2009, 11:02 AM
Following on from the New Scientist article* on the dearth of satellite galaxies about our Milky Way (25 instead of the expected thousands based on the Dark Matter hypothesis) and the ABC Science article** on the unexplained speed of stars orbiting satellite galaxies**, we have a new problem ...
www.newscientist.com/article/dn17892-galaxy-study-hints-at-cracks-in-dark-matter-theories.html?DCMP=OTC-rss&nsref=online-news
Is MOND gaining weight?

If you missed the articles, here are the threads ...
For *
http://www.iceinspace.com.au/forum/showthread.php?t=49292
For **
http://www.iceinspace.com.au/forum/showthread.php?t=49744

Regards, Rob

bmitchell82
01-10-2009, 11:34 AM
a very interesting topic indeed...

sjastro
03-10-2009, 07:46 AM
Particle physicists have a greater problem with dark matter than astronomers.

There doesn't appear to be a particle in the Standard Model that describes dark matter.

While modifying gravity is the "easiest" option, attempts to modify gravity in the past to explain observation has always ended in failure.

Steven

AlexN
03-10-2009, 05:21 PM
The problem is if you disregard dark matter, you're then required to disregard the notion of dark energy...

The issue then, is that you have the universe, which expands at a relatively steady rate, however this rate indicates that there is more matter in the universe than we can understand or observe.

Under Friedmann's model of the universe, there are three possible models you can extrapolate..

1. the universe is expanding sufficiently slowly that the gravitational attraction between between the different galaxies causes the expansion to slow down and eventually stop. The gravitational attraction will then cause the universe to begin to contract.

2. the universe is expanding at such a rate that it will never become static, although gravitational attraction between dense bodies will cause the expansion to slow a little..

3. The universe is expanding at a rate only just fast enough to avoid collapse, the speed at which galaxies are moving apart gets smaller and smaller but never reaches a point universal contraction...

Model 1 would require the galaxy to be significantly more dense with particles, galaxies, etc. There would simply need to be much more matter in the universe for the current rate of expansion to be stopped.

Models 2 and 3 would require significantly less matter than we know to be in the universe...

Alternatively, as Steven mentioned, you can "modify" gravity in order to compensate for the lack of matter. After all gravity is one of the weakest forces in the universe.. However this introduces different problems, As steven said.

There needs to be something else out there that exerts gravitational force in order for the universe to stay together at its current rate. Back in the old days Einstein had the idea of a substance called "ether" that was present everywhere, even in the cold vacuum of space... Now, this was used in the early days of relativity... Perhaps he was on the right track, although he never considered the fact that Ether could have a mass, and there for, an element of gravitational attraction...

Great topic when you're looking for food for thought.

xelasnave
04-10-2009, 03:20 PM
Dark matter is only supportable if you accept there is a force of attraction... there is no such force in my view and relying upon it to explain our Universe will leave us always in the dark... everything is dark they cant understand... dark matter dark energy ...
The cold dark matter model will be proved wrong... (which makes cern a worry)
Trying to explain gravity via a notion generated from a mind exercise of a man in a lift in space and trying to relate the forces experienced by a human to me is no way to observe nature and take any understanding from the experience.
Now when doing the mind exercise one were to consider just what is in space that could take us closer to an understading of the physical realities and the nonsence humans have introduced to explain some really simple things.
Go to where there is more nothing than anywhere else... the voids..and ask what is there?..if you answer nothing be happy with the lift notion to ex-plain it all...if you can however appreciate all that passes by and where it came from and the environment such will create I feel one will get a better view than a notion that in nothing there is in fact nothing...for there is a great deal in nothing so much that it may well influence much more than the current views science permit us to do.
The notion there is no aether is laughable all the MM experiment showed was no relationship between the aether and light transmission...the gravity B probe will show fram dragging which if one thinks about it is the physical measuring of a distortion in the aether and its context within a space mapped by a space time geometric grid. Space is the aether I suppose and it would be foolish to say that space is "nothing"
alex

Paddy
05-10-2009, 02:53 PM
I think that we are increasingly in the dark and I think this is good!

allan gould
05-10-2009, 07:47 PM
Alex
The whole dark matter/dark energy senario falls flat on its face because the first opening premise that they use is that the universe is isotropic ie homogeneous in all directions. This first principle they state is wrong. All the galaxy surveys show it at all levels. There are clumps and voids. The first assumption therefore is wrong and the rest doesn't fit either. There is no particle in the standard model that fits dark matter either. There was a paper in New Scientist that showed that the lack of luminosity of type 1 supernova was due to dust absorbtion because of the inherent clumping of galaxies. Ill have to find it.
Id go for Mond before I went for dark energy and dark matter - its all theory with no observational data. If you look at the real data with the errors inherent in their measurements you would throw it out.
Just my opinion.

AlexN
05-10-2009, 08:20 PM
Interesting Allan... I'd love to have a read of that article you mentioned...

Thanks for your post, I'm always happy to hear other views and opinions on these topics... as I said before, great food for thought.

g__day
06-10-2009, 09:35 AM
A few comments:

I don't know that we've ruled out s-Neutronies (spelling?) as dark, heavy counter-parts - where oh where is LHC to rule these things in or out.

I'd agree the Universe at sufficiently large scales is both isotropic and homogeneous isn't a well supported fact - they are two huge simplifying assumptions to make Einstein's rather hairy equations solvable at all. If the Universe is fractal at all levels this is clearly not true (and evidence continues to mount for this). Too even if we are situated in an anaomolus point of the Universe (i.e. being in a very low density region) than accelerated local expansion is explainable without any dark energy needing to exist.

MOND is appearing to fit better and better with observable evidence - now we need a framework describing why (rather than how) it works. I think we need more data before our sciences can evolve much further - the existing and preposed models need to be held to account and either ruled in or out.

Nesti
07-10-2009, 08:54 PM
Dark Matter and Dark Energy are not directly related.

See lesson 2 or 3 to see a proper explanation on how dark energy works.
http://www.youtube.com/watch?v=s8UrYIZhm60
http://www.youtube.com/watch?v=s8UrYIZhm60

Enchilada
08-10-2009, 03:31 AM
Yeah. But who has 01hr 47m 37s to listen to all of it?

As an established dark matter and dark energy skeptic, so what is the point of seeing it? (or have you changed your tune on the matter too?)