## Monday, January 08, 2007

### Dark matter: a 3D map

The picture above shows clumping of the dark matter. The long left-right direction corresponds to the redshift i.e. the distance from the Earth i.e. time. The left face of the picture shows what was happening 3.5 billion years ago while the right side of the picture captures the situation 6.5 billion years ago.

The remaining two dimensions of the picture are the usual angular dimensions on the sky. The picture doesn't show the whole sky but only two degrees or so (the apparent diameter of the Moon times sixteen) in a certain direction.

The image was created by Cosmic Evolution Survey - "COSMOS" - using the Hubble telescope. You can see that the dark matter was more uniform in the past (right side of the picture) than it was more recently (left side of the picture). The validity of Einstein's equations was assumed and the stress-energy tensor was "directly" derived from the Hubble images by analyzing the deformation of images induced by gravitational lensing caused by these clumps of dark matter. The relevant regime of the lensing here is "weak lensing". It's not enough to create multiple images but it creates distortions and amplifications whose precise analysis allows one to determine the strength of the "lens".

The visible baryonic matter seems to be made out of cherries concentrated in the same regions as dark matter - one could say that the visible matter is another level of clumping. See also

Via Gene Day.