Through the cosmic looking-glass

Tuesday, January 18, 2011
by Nikki Staab and Diane Boudreau

Looking deep into space is like experiencing the universe in a house of mirrors where everything is distorted. Of course there are no mirrors in space. The distortion is actually caused by gravity.

You know gravity as the force that keeps your feet planted on the ground. Albert Einstein showed that gravity also causes light to bend. The effect is normally extremely small. But when light passes close to a very massive object such as a massive galaxy, a galaxy cluster or a supermassive black hole, the bending of the light rays becomes more noticeable. When light from a distant object is distorted by a objects in the foreground, scientists call it “gravitational lensing.”

Astronomers have started to apply this concept in a new way to determine the number of very distant galaxies and to measure dark matter in the universe. A new study shows that the tool may be even more necessary than originally thought when looking at distant galaxies.

When light from a very distant object passes a galaxy much closer to us, it can take a detour around the foreground object. As light bends around the object it gets magnified. This “gravitational lens” acts like a natural telescope. It provides a larger and brighter view of the distant galaxy. But the view is distorted.

A very massive object distorts the view of faint objects beyond it so much that the distant images are smeared into arcs around the foreground object. According to Rogier Windhorst, an ASU astronomer, this effect is kind of like looking through a glass Coke bottle at a light on a balcony. The light gets distorted as it passes through the bottle.

Windhorst and other scientists say that gravitational lensing probably distorted images of distant galaxies recently taken with a camera on the Hubble Space Telescope. However, a new space telescope that is being developed, called the James Webb Space Telescope (JWST), will be able to handle these distortions.

When you look back to when the universe was young, you are seeing objects that are millions or billions of years old and very far away. The older and farther away the object, the more distorted it will appear when it reaches our eyes.

“The very distant universe is like a house of mirrors that you visit at the state fair – there may be fewer direct lines-of-sight to a very distant object, and their images may reach us more often by a gravitationally-bent path. What you see is not what you’ve got!’’ says Windhorst.

Future studies will need to account for a significant gravitational lensing bias. The JWST could help us make sense out of the distant universe. It will have exquisite resolution and sensitivity at longer wavelengths to disentangle the distant objects from the foreground.

Read more about Hubble Space Telescope images of distant galaxies in "Telescope takes pictures of the past."


Rogier Windhorst is a co-author of a recent paper on gravitational lensing in the journal Nature. Read the full study.