Imagine a taut rope: a sharp tug sends a wave racing along it. That's how spacetime behaves when distant cataclysms spawn gravitational waves, predicted by Einstein. Earth and the Moon are like two beads on that rope. The wave nudges them almost imperceptibly, and laser beams bouncing off reflectors on the Moon can detect a change in distance dozens of times finer than a grain of sand.

This method bridges the gap between LIGO, which catches fast oscillations from merging black holes and neutron stars, and observations of pulsars — cosmic beacons that sense slow ripples. In this way, we'll eavesdrop on the low-frequency hum from inflation — the ballooning of the infant Universe — clarify the nature of dark matter and dark energy, refine the universe's expansion, and catch echoes of the cosmic microwave background — the light emitted when the cosmos became transparent.