It is rare in astronomy to have a purely physics-based technique for studying the distant universe Rooted in General Relativity, the image distortion and time delay of light from distant objects caused by foreground gravitational lenses offers such a window on the universe Using only combinations of measured redshifts, angles, and arrival times of source intensity fluctuations, lensing observations can probe the mass distribution of the lens, the rate of expansion of the universe (the Hubble constant), the acceleration of expansion (dark energy), and the total amount of matter in the universe. The HST has made and will continue to make uniquecontributions to this new window on the universe.
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