Gravitational Lensing

In one form or another, we're all used to how gravity behaves - it alters the paths of massive objects.  If it weren't for gravity, a ball you throw should go in a straight line forever - the pull of gravity causes its path to bend  toward the ground.

Lensing Schematic - CMUAccording to Einstein's theory of general relativity, the "pull" of gravity is actually caused by the bending of space.  A large mass (like the Earth) distorts the fabric of spacetime in its vicinity, and this distortion alters the path of other objects.  One can imagine this by thinking of spacetime as similar to a big rubber sheet - every object makes a dent in the sheet (bigger objects make deeper dents), and the paths of marbles rolling on the sheet are affected by the presence of these dents.

One consequence of Einstein's work is that gravity should even bend the path of a beam of light (or x-rays, or radio waves, or any other electromagnetic radiation) in a particular way.  This means that a big, massive object can distort the image of a distant light source in a manner similar to an ordinary magnifying glass.  This effect is known as gravitational lensing, and it is one of the hottest fields of study in modern cosmology.  A beautiful example is shown in the Hubble Space Telescope picture below.

GravLens - NASA HSTCosmologists look for places in which a massive, relatively nearby cluster of galaxies sits almost exactly between the Earth and a bright, distant galaxies and quasars.  By studying how the images of these distant objects are distorted by the gravity of the big cluster, we can calculate how massive the cluster is. 

Once we've used this method to estimate how massive the cluster is, we can compare this answer to the amount of visible matter.  By using the details of the image distortion, we can also get a rough "map" of the distribution of the dark matter in the galaxy cluster.  Cosmologists have concluded that galaxy clusters (like galaxies themselves) are immersed within enormous clouds of dark matter that outweigh the stars by a factor of ~10.

For more details, see pages by Joanne Cohn and Pete Newbury.

Jeff Filippini, UC Berkeley Cosmology Group (August 2005)
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