Cosmic Distortions & Gravitational Lensing

An Einstein Ring, LRG 3-757 was discovered in 2007 in data from the Sloan Digital Sky Survey (SDSS), the image shown above is a follow-up observation taken by the Hubble Space Telescope. Image credit APOD

An Einstein Ring, LRG 3-757 was discovered in 2007 in data from the Sloan Digital Sky Survey (SDSS), the image shown above is a follow-up observation taken by the Hubble Space Telescope. Image credit APOD

Cosmic rings are a dazzling gift provided by the laws of physics.

Just as the lenses in a pair of glasses can refract (bend) light to allow us to see better, large celestial objects can do the same. It’s like having a telescope built by nature. It occurs when light from distant background stars, galaxies, quasars or supernovas are distorted into rings or even duplicate images by interacting with the curvature of space-time. This is just another example why science is so fricken awesome!

Einstein acknowledged gravitational lensing in his general theory of relativity, in which he proposed that because space-time is curved by gravity, so too should light if it passes near a massive object. The first test of his claim came during the solar eclipse of 1919. Since the sun is the most massive object in our solar system it seemed a worthy place to find distortions due to gravity. Arthur Eddington photographed the sun while being eclipsed by the moon. The moon acted as a filter, blocking most of the sun’s light allowing him to view images of the universe behind the sun.

Astonishingly, he saw light shining in the background that should have been obscured by our star, instead the light was being lensed by the gravity of the sun. These stars weren’t just in the background; they were actually behind the sun.  It was the proof needed to get Einstein the Nobel Prize for physics in 1921. If it weren’t for the reality of space-time being malleable, we wouldn’t see the beauty of gravitational lensing.

THE PHYSICS BEHIND IT

The lensing effect acts as a magnifying glass allowing a viewer to see objects that lie behind a larger foreground structure. Let’s use a distant galaxy as an example. The light from that star is traveling through space in all directions. While on its journey if it happens to pass near a massive object, such as a foreground galaxy or group of galaxies, the light will follow the gravitational curve of the massive object(s). This will distort the light from distant objects.

In the words of Neil deGrasse Tyson, “some lenses are like fun house mirrors. They can take the background galaxy and flip it left, right, and upside down, distort it and make the image appear multiple times” (Star Talk Radio). If the background object is directly behind the massive galaxy we will see the background object in different locations with the same magnitude. The background image may appear as a halo, or a ring around a bright object; this is called an Einstein Ring. (Top photo).

If the background light is slightly askew, meaning that the background object’s center doesn’t match up with the center of the foreground object, it will cause the light to smeared out in the form of an arc, or we can see a duplicate image, one with more magnitude than the other(s).  This is because it takes some of the light a longer amount of time to travel from it’s original source to the viewer. As the light bends its way around the far side of the foreground object it essentially is taking the long route and reaches us later. Meanwhile, the light traveled farther from the massive object took the fast lane, this light will appear with more magnitude because of the off-center natures of the lens.

Watch the short clip to see how different types of foreground objects will produce different lensing outcomes. Galaxies produce different types of images than say, a black hole would.

Gravitational lensing comes in three types strong lensing, weak lensing, and microlensing.Strong lensing is what we have described until now. These are categorized by obvious light distortions. This occurs most often when massive objects such as galaxy clusters are used as the lens. In weak lensing shear (the stretching of light) and convergence (the light’s magnitude) are still present, but not in a drastic way. Scientists can use lensing to measure dark matter in large-scale structures within the depths of our universe. With gravitational microlensing there is no smearing of light or multiple images seen; rather the objects’s magnitude will change over time, it may appear brighter than stars relatively close, despite the object being great distances away.

SIMULATING GRAVITATIONAL LENSING AT HOME

 Have you ever stared through the bottom of a wine glass? Not because you’ve had too much to drink, but to watch the light twirl around the base. This is actually a great gravitational lensing simulation that you can do at home. Watch the video to see how you can create the illusion.

 

 

 

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