The faint gravitational-wave hum of the whole universe can't be detected
The old claim turned out to be wrong and was rejected by evidence.
TaughtLIGO can catch the sharp chirp of two black holes merging nearby, but the deep background hum (the overlapping ripples of countless supermassive black holes across all of cosmic history, stretching spacetime over years and decades) is far too slow and too faint for any instrument to pick out.
NowIn June 2023, the NANOGrav collaboration and three other pulsar-timing arrays announced compelling evidence for exactly that background: a sea of nanohertz gravitational waves, most likely from supermassive black-hole binaries, detected by watching pulsars scattered across the galaxy tick out of sync.
What actually happened
These waves are nothing like LIGO's. Their periods are years to decades, so no ground-based detector could ever span one. The trick was to use the galaxy itself as the instrument: millisecond pulsars are natural clocks so precise that a passing gravitational wave, by subtly stretching and squeezing the space between Earth and each pulsar, shifts when their pulses arrive by billionths of a second.
After 15 years of timing dozens of pulsars, NANOGrav found the tell-tale pattern (the 'Hellings–Downs curve' that a genuine gravitational-wave background should imprint on how pulsars correlate across the sky) and three other teams on three continents saw consistent signals.
The honest hedge, which the collaborations stress themselves: this is strong evidence, not yet a locked-down five-sigma detection, and the signal will keep firming up with more data. But the flat old statement, that the universe's low-frequency gravitational-wave background is beyond reach, no longer holds. A whole new window on the cosmos opened in 2023.
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Who was taught this
Still standard through 2023, so anyone who finished school between 1950 and 2023 learned the earlier version.