F. J. Castander

There's no tool to measure distances to faraway galaxies, but nature left a clue: after the Big Bang, sound waves created rhythmic clumps of matter. Today, this is noticeable as a slight preference for galaxies to be at a certain distance from each other — a built-in ruler.

A harbor lives by the tide schedule. Water rises and falls, and with it everything changes: the level at the dock, the composition of the sediment on the bottom, even the salinity in the far corners. A galaxy works similarly, except instead of water it has gas, and instead of the Moon, it has its own stars and neighboring clouds.