How do scientists detect gravitational waves from across the universe?

The answer

LIGO and Virgo detect gravitational waves using laser interferometry, measuring length changes of just 10⁻¹⁹ meters - one ten-thousandth the width of a proton - across 4-kilometer arms. Since the first detection in September 2015 (GW150914, two merging black holes), over 90 gravitational wave events have been cataloged through three observing runs. In 2023, pulsar timing arrays including NANOGrav announced evidence for a gravitational wave background - a constant hum of spacetime ripples likely from supermassive black hole mergers across cosmic history. LIGO's fourth observing run (O4), which began in May 2023, features improved sensitivity detecting events up to 160 megaparsecs away for neutron star mergers.