Question about dark matter

  • Thread starter TheDonk
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What I've heard is that galaxies are spinning weirdly. They are spinning on the outer regions too fast. Also that the velocity of the outer stuff is 10 times greater than the gravitational force of our galaxy itself. So we say that there is this dark matter that we can't see at any wave lengths that makes up for 90% of a galaxy's mass. This accounts for the galaxy staying in one peice but how does this account for the outer regions moving as fast as they are? If dark matter is effected by gravity then it would clump into the middle of the galaxy like everything else and you would still get a very similar ratio of gravity throughout the galaxy that you would if it didn't exist.

Could it be that dark matter doesn't clump into the middle and is evenly distributed? Would that even help explain the speed of the outer stuff either? Plus this seems wrong if it is effected by gravity unless there was another force doing some repelling so that the density of the darkmatter was very low (if that makes sense).

I hope my rambling makes sense, and I know some of what I've said is assuming a little bit, but... if you need me to clarify, I'd be happy to.
 

Answers and Replies

  • #2
mathman
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The outer regions moving fast is the primary evidence for the existence of the dark (non-baryonic) matter. If it wasn't there, the outer regions would be flying off into space.

As far as clumping, the main reason is the lack of interaction. Dark matter particles, being essentially non-interacting except for gravity, will oscillate freely through the galaxy. Gravity speeds up these particles going toward the galactic centers. Passing through they slow down on the way out. It is like a pendulum in a vacuum - there is nothing to slow it down.
 
  • #3
Nereid
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  • #4
some believe that dark matter in galaxies could be super-massive black holes in the center. This would explain how stars on the outside are rotating so fast. There was an excellent program on the Discovery Science Channel not too long ago.
 
  • #5
wolram
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dark matter is the ultimate get out clause, if some theory
does not work, then throw in the x factor that makes it
work. thats what therorists are good at.
 
  • #6
mathman
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wolram is too cynical

Although it is true that theorists try different ideas in an attempt to explain various phenomena, they all recognize that only when there is some experimental or observational evidence that agrees with theoretical prediction, it is simply educated guessing.

There are several different ideas about what is non-baryonic matter, but the black hole at the center of the galaxy is not one of them. There appears to be evidence that non-baryonic matter extends outside the domain of visible matter (stars).
 
  • #7
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There are candidates for dark matter with strange names like Wimpzillas (very massive WIMPs, an allegory to the monster Godzilla), neutralinos or Q-balls. Who can tell.
Actually, the concordance model, the model accepted by mainstream cosmologists, depends on dark matter, and the concordance model fits very well the data. I think that MOND is a dead theory
 
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  • #8
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Wolfram is not so cynical...

The Dark Matter is not like other successful theories of physics. For a start, it's an effect you can only measure one way, meaning that the invisible mass is posited because of a gravitational theory. The way it is measured for is also by gravitational inference.

The independent confirmational results everybody (who believes inthe DM theory) wants to see are not around - DM detectors have not produced a single confirmational result.
 
  • #9
Nereid
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Welcome to Physics Forums Seyfert.

AFAIK, there are at least four independent sets of observations pointing to dark matter:
- rotational curves of galaxies
- gravitational lensing of background galaxies and quasars
- temperature and pressure distributions of intra-cluster gas, from X-ray observations (especially Chandra and XMM)
- the CMB, especially as observed by WMAP.

Matter which interacts with baryonic matter principally via gravitation is proposed as the explanation of these observations; with the exception of the CMB, the distribution of mass - in galaxy halos, and in clusters - derived from the observations should be the same.

Further, the total dark matter in the universe - derived from fitting cosmology models to observations of the CMB - should be consistent with the sum of that inferred from galaxy and cluster observations.

Does anyone know if there are significant inconsistencies?

Unlike for the rest of astronomy, especially now that the solar neutrino anomaly has been explained, where we can tie observations of things in the sky to stuff we can touch and feel here on Earth, we don't have any bits of dark matter in our labs.

Hmm, where does that leave black holes and gravitational waves then?
 
  • #10
wolram
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The Dark Matter is not like other successful theories of physics. For a start, it's an effect you can only measure one way, meaning that the invisible mass is posited because of a gravitational theory. The way it is measured for is also by gravitational inference.
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true, and this is why im skeptical, we are looking for
an unknown with an unknown, dark matter, gravity.
 
  • #11
Nereid
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Originally posted by wolram
The Dark Matter is not like other successful theories of physics. For a start, it's an effect you can only measure one way, meaning that the invisible mass is posited because of a gravitational theory. The way it is measured for is also by gravitational inference.
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true, and this is why im skeptical, we are looking for
an unknown with an unknown, dark matter, gravity.
Isn't this also an accurate characterisation of black holes and gravitational waves?

AFAIK, neutrinos were in the same boat (so to speak) from the time they were first hypothesised until they were observed, nearly 30 years. Then there was another period of over 40 years before a fundamental anomaly regarding their nature was resolved.

Wouldn't it be more accurate to say that the current status of dark matter is that it's a 'work in progress'? and as such illustrates well the provisional nature of *all* science?

Let a thousand alternative ideas to explain galaxy rotation, cluster temperature and mass, galaxy and cluster lenses, and WMAP observations blossom! All that I would ask of those proposing such alternatives is:
1) match the observations at least as consistently as the dark matter theory does
2) show seamless consistency with existing physics theory, in the domains of relevance

BTW, MOND does pretty well on #1, but fails on #2 (as the proponents freely admit).

[EDIT: periods re neutrino discoveries corrected]
 
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  • #12
wolram
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Let a thousand alternative ideas to explain galaxy rotation, cluster temperature and mass, galaxy and cluster lenses, and WMAP observations blossom! All that I would ask of those proposing such alternatives is:
1) match the observations at least as consistently as the dark matter theory does
2) show seamless consistency with existing physics theory, in the domains of relevance
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well said, i think your right in every respect, i am not
"in the box", when i disagree about the existence of dark
matter, its just an intuition with no foundation.
i also think the theory of spacetime is wrong and therfore
gravity waves will not be detected, again with no foundation,
but the only ones that will ever prove a doubting thomas
like me wrong are the mainstream scientists.
 
  • #13
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Thanks, Mathman, your first post helped me the most because the clumping thing was really getting me.

Also i don't think a supermassive black hole in the centre of the galaxy would have any difference on the speed the outside is rotating compared to the inside. Mainly because you get the clumping thing happening like you do with regular matter.
 

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