Gravitational Lensing

      Gravitational lensing takes place when the light from a distant star or galaxy passes by a less distant object on its way to an observer (in our case, on its way to Earth). The result is that, rather than seeing one relatively clear image of the distant object, the observer sees multiple distorted images of that object. The effect is somewhat looking through a glass of liquid at a distant light.

The Astronomy Picture of the Day has three nice images of gravitational lensing:
A quasar lensed by a galaxy.
Several galaxies lensed by a dense cluster of galaxies.
A galaxy lensed by a cluster of galaxies.
Take a look at them.

      The significance of gravitational lensing to the Big Bang theory is that it provides a way to document which of two galaxies is farther from us - the lensed one must be further than the lensing one. Big Bang theory would predict that the one farther away would be moving away from us faster. Observation of red shifts confirms that prediction and thus adds support to the Big Bang theory.

      Of less interest to us, but of interest to the history of science, is gravitational lensing's role in supporting Einstein's Theory of Relativity. Einstein predicted that massive objects could bend light (a radical idea in the early 1900s). The observation of gravitational lensing later in that century (as in the images above) was confirmation of the prediction from Einstein's theory and thus strong support for the theory.

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