Before 2015, gravitational waves remained unconfirmed despite theoretical predictions and orbital decay observations of binary pulsars consistent with their existence. On September 14, 2015, twin LIGO detectors measured periodic changes in four-kilometer laser arm lengths that signaled gravitational waves from an inspiraling and merging pair of black holes. That detection inaugurated gravitational-wave astronomy, a new observational channel sensitive to spacetime ripples rather than electromagnetic radiation. By 2025, LIGO in its fourth observing run increased known black hole merger detections from about 90 to roughly 300. The rapid progress faces potential abrupt termination unless radical measures are taken to sustain operations.
Then, on September 14, 2015, all of that changed. The twin LIGO (Laser Interferometer Gravitational-wave Observatory) detectors both saw small, periodic changes in the length of their four-kilometer-long laser arms, which heralded the arrival of gravitational waves from two black holes that had recently inspiraled and merged from across the Universe. With that one event, the era of gravitational wave astronomy was born: a wholly new type of astronomy that didn't use telescopes, but rather was sensitive to the ripples in spacetime
Now in 2025, after years of observing runs punctuated by a steady series upgrades between them, LIGO - in the midst of its fourth observing run - has just more than tripled the number of known black hole mergers, going from around 90 at the end of the third run to around 300 today. Here's how far we've come in the last decade, and how, if we don't take radical action, this era might come to a sudden, unceremonious end.
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