Cosmic duet: twin supermassive black holes near collision could send Earth-detectable waves within a century

Decades of radio observations reveal that the blazar Markarian 501 hosts two supermassive black holes in a tight binary, each hundreds of millions to billions of solar masses, orbiting about every 121 days at a separation of roughly 250–540 AU. The pair is expected to merge within less than 100 years, producing powerful gravitational waves that could be detected on Earth and offering new insights into extreme black-hole mergers.