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 detector at the Laser Interferometer Gravitational-Wave Observatory (LIGO) in Hanford, Washington. Then, traveling at the speed of light, this wrinkle in space-time passed through LIGO’s second detector in Livingston, Louisiana, just a fraction of a second later.

The results were published Thursday in the journal Physical Review Letters.

Cosmic Forces

Gravity is the weakest among nature’s four fundamental forces. So only extreme cosmic events like supernovas, neutron stars and merging black holes can make detectable gravitational waves. The waves are so weak that they’d warp the distance between Earth and sun by just the width of a hydrogen atom. But as these waves pass through LIGO’s twin detectors, its enormous lasers can pick up on the truly tiny stretches and squeezes of space-time. You can think of it like a seismometer for measuring mini quakes in the cosmos’ gravitational fabric.

When LIGO gets a hit, the gravitational wave makes a characteristic signal that scientists’ call a “chirp” because of the sound it makes once translated into a format human ears can hear.

This was the third such detection since Albert Einstein first predicted gravitational waves a century ago as part of his general theory of relativity, or theory of gravity. Taken together, these observations form the first samples of a black hole census with far-reaching implications.

Before colliding, the binary black holes spotted earlier this year weighed in at 19 and 31 times our sun’s mass. After merging, the pair created a single black hole 49 times more massive than the sun. Einstein’s equations tell us that energy and mass are interchangeable. And so the missing solar mass worth of energy was radiated out across the universe as gravitational waves.

And with this detection, scientists for the first time think the two black holes might have been spinning in opposite directions. That could reveal clues about the lives of the stars that formed them. It’s possible that the two stars lived in a dense stellar cluster.

Before LIGO, astronomers didn’t know that so-called solar mass black holes, which form when stars die, could reach such extreme sizes.

This census can also help explain an enduring mystery in astronomy. Scientists have seen supermassive black holes that dominate entire galaxies, as well as small black holes that form after stars die. We even now know about so-called intermediate mass black holes weighing as much as thousands of suns. But how do these all form? Do many small black holes combine intro larger and larger behemoths? LIGO is just starting to piece together this puzzle.