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Author Topic: What is the evidence for the existence of dark matter?  (Read 10031 times)

Offline Murchie85

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What is the evidence for the existence of dark matter?
« Reply #25 on: 03/05/2010 10:02:36 »
Hmm I am not so sure I agree that having say a 80% -20% or 70%-30% distribution would effect the deviations so much that they would not be observable. Dark matter is still unknown and the sloan and wmap rely on detecting the effects of gravity from dark matter, in fact all matter has a relationship with gravity.

What I am getting at is until we have conclusive evidence that DM is completely made of exotic matter lets not rule it out. The universe is big, very big and the data obtained by the satelights rely on such small variations or tiny clues that I would not at all be suprised that if some of the current estimates turn out to be wrong.
 

Offline PhysBang

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What is the evidence for the existence of dark matter?
« Reply #26 on: 03/05/2010 16:59:20 »
Hmm I am not so sure I agree that having say a 80% -20% or 70%-30% distribution would effect the deviations so much that they would not be observable. Dark matter is still unknown and the sloan and wmap rely on detecting the effects of gravity from dark matter, in fact all matter has a relationship with gravity.
It makes a huge difference, because WMAP and SDSS depend not simply on the relationship between matter and gravity, but also on the relationship between light and baryons. Because of the tight relationship between baryons and photons in the early universe, the universe as we see it today simply could not exist unless there is a substantial non-baryonic component to the matter density of the universe. The distribution of ordinary matter in the early universe was too uniform to grow into the galaxies that we see today unless there were pockets of dark matter that acted essentially as seeds of structure. (The alternative is to throw out almost everything that we know about large-scale structure formation, and nobody seems willing to do that.)
 

Offline Farsight

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What is the evidence for the existence of dark matter?
« Reply #27 on: 04/05/2010 16:18:16 »
Can I point out that a non-uniform energy distribution causes gravity. Matter only causes gravity because of its energy content. This is important, because space has its vacuum energy or energy density, and this has a mass-equivalence by virtue of E=mc˛. Should the energy density vary, it's going to cause gravity. 

Check out Einstein's Leyden Address where he talks about a gravitational field, saying space "is neither homogeneous nor isotropic, compelling us to describe its state by ten functions (the gravitation potentials gμν)". Also check out The Foundation of the General Theory of Relativity where on page 185 of Doc 30, he says "the energy of the gravitational field shall act gravitatively in the same way as any other kind of energy".

This is saying that a gravitational field is non-homogeneous space, where the energy density is non-uniform, causing an extra gravity component. But we don't expect to find dark-matter particles in a gravitational field. Hence we shouldn't be surprised when we can't find dark matter particles at large. The cause might simply be inhomogeneous space. After all, space is dark, and it has that mass equivalence. The evidence for gravitational anomalies is rock solid, but dark matter is merely one hypothesis that attempts to explain it, and there is no actual evidence for dark matter per se. Large-scale structure formation does not support the case for dark matter in the form of WIMPs or any other particles, merely for variations in energy density, whatever the underlying cause. 
« Last Edit: 04/05/2010 16:22:31 by Farsight »
 

Offline saspinski

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Re: What is the evidence for the existence of dark matter?
« Reply #28 on: 05/08/2014 02:03:11 »


1. Rotation curves of galaxies: We can look at how fast a galaxy is rotating and use that to calculate how much matter is in the galaxy (and roughly its distribution). We can also look at how much light is coming from the galaxy and use that to tell how much matter is there (and roughly its distribution). These two do not match up.



In a rotation curve of stars like http://astro.berkeley.edu/~mwhite/darkmatter/rotcurve.html [nofollow],
star speeds are about 150 km/s. That means they will travel 4,7 x 1011km or 0,000016 kparsec after 100 years. If we suppose they travel in a circle around the galaxy center, that result in 0,00009 degrees of arc for a radius of 10 kpc as shown in the graphic.

My doubt is: astronomers have instruments precise enough to say that after some years of monitoring, the stars are orbiting the galaxy center? Instead the galaxy could be expanding. In that case there is no contradiction between the graphic and the gravitation theory. Stars distant to the center are travelling faster than expected to be orbiting, but they would be just escaping from the orbit and that expansion is too small to be detected in a reasonably scale of time.
 

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Re: What is the evidence for the existence of dark matter?
« Reply #28 on: 05/08/2014 02:03:11 »

 

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