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outreach [2016/01/21 08:47]
jr0018
outreach [2016/01/21 08:48] (current)
jr0018
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 You can also find some outreach materials and teaching aids on this page. Please feel free to use these, but if you do we would be grateful if you could acknowledge the astrophysics group at the University of Surrey and to link to this webpage. You can also find some outreach materials and teaching aids on this page. Please feel free to use these, but if you do we would be grateful if you could acknowledge the astrophysics group at the University of Surrey and to link to this webpage.
  
-----+::: [[:​outreach:​lensing|Gravitational Lensing]] | :::
  
-==== Gravitational Lensing ==== 
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-{{ ::​250px-a_horseshoe_einstein_ring_from_hubble.jpg?​350|}} 
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-Light from bright distance galaxies has to travel a very long distance to get to us here on Earth, in some cases billions of light years. Along that long journey, the whole Universe expands, making it take even longer to reach us. Sometimes this light has to travel through a particularly massive object to get to us - a massive galaxy, or a collection of many galaxies called a "​galaxy cluster"​. If this intervening object is massive enough, its gravity severely distorts the spacetime around it bending the path of the light rays. The effect is very similar to how an optical lens - like your glasses - bends light and for this reason, we call it "​gravitational lensing"​. The image above - taken by the Hubble Space Telescope - shows a particularly beautiful example, LRG 3-757. The "​lens"​ is the bright red elliptical galaxy at the centre of the image. Behind it is a much more distant blue galaxy that lies almost exactly behind the lensing galaxy. Yet we can still see it because its light is bent around the lensing galaxy by its strong gravitational field. Because the lens is almost perfectly round, the situation is highly symmetric. The light can travel around the lensing galaxy from any angle and the result is a near-perfect blue "​Einstein ring". A truly remarkable phenomenon. ​ 
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-Such gravitational lenses are fascinating because they inform us about the mass contained within the lensing galaxy. Using computer models to reconstruct the lens, we find that there much be far more mass than is visible in stars and gas alone. Understanding the nature of this "dark matter"​ is a key challenge for physics and astronomy over the next decade. Lenses are also exciting because they probe the expansion history of the whole Universe. This allows us to probe not only the mass distribution in the lens, but also our cosmological model understanding more deeply where we came from and where we will go next. 
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-A fun way to "​simulate"​ a gravitational lens is with an optical lens (see the image above). Glass lenses also bend light rays and so can produce effects very similar to gravitational lenses in the Universe. The key difference is that light bends in glass because it moves more slowly, whereas light bends around a gravitational lens because it must travel a longer distance through curved space. Yet, it turns out that the equations are very similar indeed. In fact, you can build a pretty good model of the lens LRG 3-757 using nothing more than an empty wine glass! Draw a bright round circle on a white piece of paper. This represents the distant "​source"​ galaxy. The base of the wine glass represents the lensing galaxy LRG 3-757. Now, line up your circle behind the base of the wine glass. If you look through the base, the circle will appear distorted. If it is a long way from the stem of the glass it will distort only slightly, appearing elliptical. This is called "weak lensing"​. As you bring the circle closer to the base, it will become kidney-bean or banana shaped. This is called "​flexion"​. When the circle lies right behind the base of the wine glass, you will see it distort into a near perfect ring - just like the "​Einstein ring" in LRG 3-757!