L. Samushia

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.

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.