What can weak lensing be used for?

[oldman]

Gravitational lensing (GL), first mooted in 1924, is now a technique being used by astronomers to detect dark matter. Lensing depends on the bending of light by the gravity of a mass concentration, such as a galaxy or galaxy cluster.

In glass, it is the spatial gradient of the optical path length that causes lensing. There is no lensing of light passing through a uniform parallel-sided glass slab.

With gravitation, I imagine that it is the spatial gradient of the integrated gravitational potential along the optical path that causes lensing. If I'm right, there can be no GL of light passing through a uniform distribution of dark matter (or energy, which also generates a gravitational potential).

Does this mean that the density of dark matter can only be measured from the distortion of galaxy shapes behind concentrated objects like galaxy clusters, and nowhere else? Might any estimate of the overall density of dark matter obtained from weak lensing measurements then be only a lower limit?

And how do we know that the dark matter recently detected is not in fact dark energy, or a mixture of the two? Or have I got it all wrong?

 

[Jorrie]

Background dark matter?

... Does this mean that the density of dark matter can only be measured from the distortion of galaxy shapes behind concentrated objects like galaxy clusters, and nowhere else? Might any estimate of the overall density of dark matter obtained from weak lensing measurements then be only a lower limit?

Hi Oldman, I'm also puzzled by this - according to the Wikipedia article: http://en.wikipedia.org/wiki/Weak_gravitational_lensing"
"By measuring the shapes and orientations of large numbers of distant galaxies, their orientations can be averaged to measure the shear of the lensing field in any region. This, in turn, can be used to reconstruct the mass distribution in the area: in particular, the background distribution of dark matter can be reconstructed. Since galaxies are intrinsically elliptical and the weak gravitational lensing signal is small, a very large number of galaxies must be used in these surveys.", specifically the emphasized phrase (emphasis by myself).

I can somewhat understand how the dark matter contribution to the total mass of the cluster is determined, but background dark matter? Science advisor help required!

Jorrie

 

[cesiumfrog]

Assume galaxies are oriented randomly and evenly in all directions. If viewed through a strong gravitational lens, these galaxies will all appear distorted to produce concentric rings around the center of the lens. If viewed through a weak gravitational lens, the galaxies will just appear to have orientations statistically tilted slightly to align less with the the center of the lens (producing very subtle rings). If astronomers observe this, they can infer the existence (and some properties) of such a weak gravitational lens. They can also separately attempt to directly observe the mass producing the lens (most likely a galaxy in the very middle of the distorted region), and infer properties of the visible mass there. If the two observations don't add up, they can conclude some mass is invisible to them. Without investigating wiki's references (as you ought, if you're that perplexed by its phrasing), I understand this is what they're calling the background distribution of dark matter. The approach is applicable not just to galactic dark matter distributions (unlike rotation curves), but also to dark matter throughout intergalactic space.