When you gaze into the abyss -- sometimes it shoots you back a cheeky grin. At least that's what this deep space picture taken by the Hubble space telescope seems to be doing -- but it's actually the result of something really cool.
In the picture, you can see two shining yellow "eyes" -- those are actually galaxies. There's also a bright white nose. And then smears of curving light indicate a smiling mouth and the lines of a partial circle around the whole group.
Those actually don't exist -- or at least not in the form that we see them in the photo.
In space, there is a lot of mass. Some of that mass we can see and measure, some we can't. But all that mass combined creates strong gravitational fields, particularly around large objects -- such as a galaxy cluster or a black hole. Now, we all know that gravity has an effect on physical matter; we see and feel it every second of our lives.
When gravitational fields get as big as they do in space, they also have a quantifiable effect on light as it travels across the vast distances -- as spacetime is bent by these fields (according to Einstein's general theory of relativity), so too is light, causing a lensing effect. This effect -- called gravitational lensing -- is something astronomers need to be aware of and correct for as they study the universe.
The strongest type of gravitational lensing is easy to spot, since it causes visible distortions -- such as those seen in the image above, where light is bent into a ring. These are called Einstein rings, and they only occur when the source of the original light, gravitational lens and observer are in exact alignment in a straight line.
Gravitational lensing is actually really interesting in its own right -- beyond something that needs to be corrected for. It can actually be used to help study dark matter. It is estimated that some 85 percent of the mass of the universe is made up of invisible, undefined dark matter. Although we can't see or measure this matter, we can observe its effect on the universe.