The title sounds really fancy and complicated, but gravitational lensing is really an awesome recent discovery and is pretty simple to understand. Think about how a magnifying glass works. You hold it over a book, and the light bouncing off the book goes through the glass and makes the writing appear bigger. The magnification is due to light being bent by the curved magnifying glass . Well it turns out the gravity from large objects can act just like a lens!
The picture above is a combination of a Hubble (visible light) and Spitzer (infrared light) image that shows an example of gravitational lensing. That really bright fuzzy spot in the center is actually a cluster of hundreds of galaxies. Together, the gravity holding them together acts as one giant lens or magnifying glass. The wispy lines around the outside of the cluster are actually images of different galaxies, far behind the galaxy cluster, whose light has been bent and magnified (they have also been stretched out a bit too). So these galaxies (the book pages) are technically hiding behind the big galaxy cluster (the magnifying class) and normally we wouldn't be able to see them. But the cluster has acted like a lens by bending the light around the outside of the cluster, and magnifying it so that these hidden galaxies are visible. It's like nature has provided us with a super massive and super powerful telescope! The galaxies in this specific image are actually 13.5 billion years old! They are currently the oldest know objects in outer space.
Image credit: NASA, ESA, CRAL, LAM, STScI
