The universe may be a quantum foam Henrik Sorensen/Getty

The expansion of the universe is accelerating, and we’re not sure why. It may have to do with quantum fluctuations that make every point in space constantly grow and shrink, producing a roiling foam that, on the whole, is always expanding.

This is a complex solution to the “vacuum catastrophe”, which concerns one of the most hotly-contested numbers in physics, the cosmological constant. The constant describes the energy stored in the vacuum of space and most theories predict a value that is up to 120 orders of magnitude higher than …