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mutleyeng
2009-Feb-16, 08:03 PM
hey all,
Now the extent of my understanding of this stuff is that we "know" it exists for two reasons.
first, it has to exist to fit our idea of the universe.
second, that we can see its gravitational effect through bending of light.
ok, so my probably really stupid question is...how do you know where light is being bent by dark matter, and when it may be by black holes, or even stacks of black holes?

drhex
2009-Feb-17, 12:18 PM
I think we believe dark matter exists mainly because galaxies could not maintain their shape at the speed they are rotating if there weren't more mass around. (and also by the way galaxies orbit each other).

Ufonaut99
2009-Feb-17, 01:18 PM
hey all,
Now the extent of my understanding of this stuff is that we "know" it exists for two reasons.
first, it has to exist to fit our idea of the universe.

Actually, we know it exists through observations - originally by Fritz Zwicky (although it wasn't really appreciated then), and then it hit big time with the measurements of galaxy rotation by Vera Rubin.

I've sometimes had it said to me that it's a fudge required by the BB. Nothing could be further from the truth - the observations of BM and DE came from way off left field, and all cosmologies had to integrate them into their models and predictions.



second, that we can see its gravitational effect through bending of light.
ok, so my probably really stupid question is...how do you know where light is being bent by dark matter, and when it may be by black holes, or even stacks of black holes?
As drhex says, from galactic rotations, Dark Matter is inferred to be a wide halo surrounding galaxies (and also holding galactic groups together) - whereas black holes reside in the centre of the galaxies.

antoniseb
2009-Feb-17, 02:02 PM
...how do you know where light is being bent by dark matter, and when it may be by black holes, or even stacks of black holes?

We are looking at dark matter in part through an effect called weak lensing, in which the gradient of the dark matter we are looking through bends or realigns the light from background galaxies in a systematic way. We see that the location of the gravitational mass is much more distributed than a stack of black holes would be.