Occam's Machete

Gravitational lensing has been used to show the effects of massive objects between the observer and some distant observed objects.  It is easy to understand how a massive object will bend light and some rather striking images show the effect.  What is interesting is that gravitational lensing can actually be used to map dark matter in the universe.  If you know what an object should look like you know when its light has been bent causing the image to be distorted even when the light is bent by invisible dark matter. 

As with many ideas that seems complex the trick is rather simple.  Look first at this image of the galaxy cluster Abell 2218.  Here you can see that the shape of the distant galaxies have been distorted by the large galaxy at the center of the picture.  Our eye picks out the distortion easily because we are used to seeing elliptical galaxies, not the bent slivers in this image.  It turns out that the amount of the curve can be used to measure the magnitude of the lensing and therefore the magnitude of the gravitational effect.  In this image the gravitational effect is from a visible object but if you imagine the same picture with no central bright blob you would still notice the distorted curved galactic ellipses.  And that is exactly how gravitational lensing is used to measure dark matter. 

There are problems with this type of distortion measure though since dark matter is highly diffuse.  Any photon moving through the center of a diffuse cloud of dark matter will move straight through it since the effect of the dark matter is to tug equally in all directions.  But when a photon moves near a diffuse cloud of dark matter it will tug more in the direction of the center of mass, so the photon bends in that direction.  By measuring the very slight curve in what should be a nice regular ellipse astronomers can calculate a line to the the center of mass of the cloud of dark matter.  By measuring the curvature of several galaxies they can triangulate to find the location of the dark matter. To get more precise they need to subtract the lensing caused by visible matter but you get the idea.

By putting together many observations astronomers are getting a better idea of the large scale structure of dark matter in the universe.  Here is one map of dark matter structures.  And here is link to a new study that describes the process in much greater detail (though I can't find an image to go with it.)