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What Is It?
Einstein's theory of general relativity predicts gravitational lensing as one of the phenomenon that occur in the Universe. This effect is best described by imagining an observer on Earth looking into the night sky. An object such as a star, located at a distance in the Universe will normally appear at its usual position in the night sky to the observer. Now imagine a massive celestial object such as another star or galaxy that passes in between the distant star and the observer. The light coming from the distant star will not travel in a straight line.
The gravity of the celestial object will cause the light to be bent at a certain angle, making the object appear at a different position to the observer, sometimes creating multiple images and at other times magnifying the object. This effect of lensing by the celestial object is called gravitational lensing. According to Einstein's theory, the light coming from the star gets deflected due to the intervening matter, which will curve the fabric of space and time.
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Why Do Multiple Images Appear?
The maximum amount of bending occurs closest to the gravitational lens, whereas the bending is at its minimum furthest from the lens. If the observer is in a straight line with the light source, the image will appear to be a ring around the gravitational lens. However, if the observer is not directly aligned with the light source, the image appears as an arc segment. This is known as an Einstein ring. With even larger lenses, such as groups of galaxies, the bending of light is even more pronounced, creating scattered images of the light source, in turn resulting in multiple images.
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Types of Gravitational Lensing
There are three main types of lensing that occur in the Universe. These are briefly described below:
1. Strong Lensing - This type of lensing causes light to be bent to the extreme. The gravitational lens is usually created by very massive objects, such as clusters of galaxies and the light source is close to the lens. As explained earlier, this will cause multiple arc segments to appear, and the images will also vary with time. Strong lensing is useful in studying massive objects such as clusters and determining their properties.
2. Weak Lensing - In weak lensing, the lens source is generally not very massive. The light source is, however, stretched and magnified. Multiple images do not form in this type of lensing. Galaxy surveys, and the distribution of background dark matter can be studied using weak lensing.
3. Microlensing - In microlensing their is little or no distortion of the source. However, the light from the source changes with time and the object appears brighter. This technique is especially useful to study objects that are very faint or dim and can be magnified using microlensing.
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Finding Lenses and MACHO's
Finding gravitational lenses is not easy. In the past, most were discovered accidentally. Using the Very Large Array Telescope (VLA) in New Mexico, scientists have been able to discover 22 additional lenses. These discoveries have been significant since they have allowed us to better understand the Universe.
One of the important discoveries still awaiting scientists is that of a type of dark matter called MACHOs (Massive Compact Halo Objects). MACHOs have been postulated to exist and are thought to be very massive but invisible. Using MACHOs as gravitational lenses, scientists could one day discover their existence, as MACHOs passing in front of a source would cause the source to become brighter for many days or possibly even weeks.
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Image Credits: Wikimedia Commons/MichaelSachs (http://commons.wikimedia.org/wiki/File:Gravitational-lensing-angles.png)/