Earlier this year, researchers from several pulsar timing array (PTA) experiments around the world published evidence for a gravitational-wave background—an all-sky “hum” of gravitational waves at nanohertz frequencies. The waves, which have frequencies that are nearly 10 orders of magnitude lower than the waves detected at ground-based interferometers like the Laser Interferometer Gravitational-Wave Observatory, may originate from binary supermassive black holes that have orbital periods of years to decades.
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