One possible way to discuss the reversal of perspective could be to show how it is just one among several optical effects of the curvature of spacetime. In fact, when in the late 1990s two separate teams of astronomers discovered that the universe’s expansion is now accelerating—the “dark energy” business that earned three astronomers this year’s Nobel Prize for Physics—they did so while studying anomalies in the optical effects of curvature.

But for this post, I want to avoid talking about curvature and instead I want to keep things as basic as possible. As you’ll see, the cosmic lensing follows from a few simple facts.