Description: Objects with mass can bend the path followed by light that passes nearby their gravitational influence. This effect, predicted by Einstein's theory of general relativity, was first observed during the solar eclipse of 1919 when the bending of starlight from several stars near the Sun was measured. Gravitational lensing is most apparent for very massive objects such as galaxies or galaxy clusters. If observers on Earth are looking at a distant object whose light is thus bent (by "the lens"), the object will appear distorted. This distortion always implies light (de)magnification, typically allowing us to better see otherwise faint background objects. When the lens has enough mass concentrated in a small angular area, multiple images of the same background object are produced, each having its light reaching the observer at different times. Measuring these so-called "time delays" is one of the best ways to determine the value of the Hubble Constant on extragalactic scales. Multiple images from a lens, on the other hand, allow us to precisely determine its mass (using modelling); this is a very useful method for finding the mass of galaxies and, especially, galaxy clusters.

Related Terms:

• Dark Matter
• Galaxy
• Galaxy Cluster
• Supermassive Black Hole
• Hubble Parameter

Term and definition status: This term and its definition have been approved by a research astronomer and a teacher

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