Gravitational Lensing with a Wine Glass

A Horseshoe Einstein Ring from Hubble | Image Credit: ESA/Hubble & NASA.

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. Sometimes along that long journey, the light rays pass through a particularly massive object - 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 on the right - taken by the Hubble Space Telescope (Image Credit: ESA/Hubble & NASA) - shows a particularly beautiful example, LRG 3-757. The “lens” is the bright red elliptical galaxy, LRG 3-757, at the centre of the image. Behind it is a much more distant blue galaxy that lies almost exactly behind LRG 3-757. We can still see this blue galaxy, however, because its light is bent around LRG 3-757 by the strong gravitational field. Since LRG 3-757 is very round and symmetric, the blue galaxy is distorted into a near-perfect “Einstein ring” - a truly remarkable phenomenon.

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 must 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.

A fun way to “simulate” a gravitational lens is with an optical lens. 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 when it moves through glass because it moves more slowly, whereas light bends around a gravitational lens because it must travel a longer distance through curved spacetime. Yet, it turns out that the equations are very similar. 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 blue “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!

::: Outreach | :::