E. Gaztañaga

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.

The early universe was a hot soup of particles and light. In this soup, sound waves, fluctuations in density, spread outward from clumps of matter like ripples on a pond. When the universe cooled, these waves froze, leaving an imprint in the distribution of galaxies: they are slightly more abundant at a certain distance from each other.

In the early universe, when it was just a few hundred thousand years old, matter had not yet gathered into stars and galaxies; it was a hot soup of particles and light. Sound waves rippled through this soup — compressions and rarefactions, like ripples from a stone thrown into water.

In Chile's Atacama Desert stands a telescope that doesn't look at stars, but at the oldest light in the Universe — the cosmic microwave background. This light has been traveling to us for nearly 14 billion years, ever since the Universe became transparent. On its way, it passed through clouds of hot gas around galaxies.

If you toss a handful of sand into calm water, the ripples spread out evenly. Likewise, matter after the Big Bang should have flown out equally in all directions. But in reality, the galaxy map shows a slight ripple — here a few more, there a few less. This ripple is the imprint of those first moments, when space had not yet become perfectly smooth.

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.