Our sun was still dim. Waves crashed on martian beaches. Life was emerging on Earth.
That’s when the ghosts of two dead stars — black holes dozens of times more massive than our sun — merged in a far-off corner of the universe. In their final moments, these binary black holes were circling each other hundreds of times per second, as each one spun at 10 times that rate.
The rumbles of distant thunder from that collision reached Earth on Jan. 4 of this year, passing through the d