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Introduction to Gravitational Lensing

Gravitational lensing is the distortion of light emitted by distant galaxies as it travels through the universe.  Fundamentally a prediction of Einstein’s theory of general relativity, gravitational lensing has become a cornerstone of modern astrophysics and a major means of testing the parameters that determine the fate and history of the universe.

Einstein’s theory of general relativity explains that the force we feel as gravity is really the curvature of space and time by massive objects.  The predictions of general relativity are now very well tested and several modern technologies, including Global Position Satellite (GPS) systems, fundamentally use general relativity’s predictions.

In the picture provided by Einstein, massive objects bend or warp space-time so that they alter the path of any entity that passes them.  The effect is not unlike a heavy ball placed in the center of a trampoline.  If we think of the trampoline is “space,” then small objects rolling along the surface of the trampoline will be deflected by the “bowl” created by the heavy ball.  In this way, the sun creates a distortion in space that causes the earth and other planets to remain in orbit.

One prediction that distinguishes general relativity from previous notions of gravity is that this warping of space-time influences light as well as massive objects.  In the picture shown to the right, we see two light rays deflected by the warping of space caused by the planet.  Measuring the angular spread between those two light rays can be used to determine properties of the planet, including the planet’s mass.  Carefully making these observations and analyzing the data lies at the heart of gravitational lensing.

Gravitational lensing by clusters of galaxies leads to a wide range of observations.  Large galaxy clusters can contain 50 to 100 galaxies, each with 100 billion stars.  The total mass of these super clusters is around 1015 times the mass of our sun.   With this much mass, severe distortions of light can occur.  In the HST image in the left box below, the elongated red and white arcs are the stretching out of ordinary sized objects by the yellow blobs (the cluster) in a process called weak gravitational lensing.  In the HST image on the right, we can see multiple copies of the same bluish galaxy – a process called strong gravitational lensing.  There is only one bluish galaxy, but the gravity of the yellow cluster members has so distorted the path of light rays traveling in this region that we see multiple copies of the same background galaxy.



Hubble Space Telescope image of Abel 2216 Hubble Space Tel. image of 0024+1654