Late in the 20th century, scientists discovered something amazing: gravity doesn’t just suck, it also blows. This knowledge comes to us by looking at distant supernovae and determining how fast they are receding from us. It turns out that the rate at which objects are receding from us is accelerating. The Universe isn’t just expanding; it is expanding faster each day.

General relativity can cope with that—sort of—by adding a cosmological constant. This constant turns up naturally from the math, but neither the math nor the physics tells us what its value should be. Explaining what this constant might mean physically also turns out to be a bit tricky. There are many models, but the big obstacle to most of them is that they don’t just add a constant term to general relativity. Instead, most of them have additional physical consequences.