On September 14, 2015 at 5:51am ET, the Laser Interferometer Gravitational-Wave Observatory (LIGO) detected ripples in the fabric of spacetime. One hundred years after Albert Einstein predicted the existence of gravitational waves, a ‘chirping‘ sound was discovered. Nadia Drake explains at National Geographic:

Two colliding black holes, one with 36 times the mass of the sun, and the other with 29, emitted those gravitational waves as they spiralled into one another and eventually collided.

From roughly 1.3 billion light-years away, these waves spread like ripples in the cosmic pond and washed over Earth on September 14, causing a minuscule but measurable change in the distance between four sets of mirrors—two in Louisiana, and two in Washington state.

In the last second before the black holes merged, they released 50 times more energy than all the stars in all the galaxies in the universe were releasing, combined.

“It’s the first time the universe has spoken to us in gravitational waves,” said David Reitze of Caltech during a press conference announcing the discovery on February 11.

To scientists monitoring that mirror-based experiment at the Laser Interferometer Gravitational-wave Observatory (LIGO), the signal received on Earth carried the characteristic “chirp” predicted to accompany the death and unification of two black holes.

“We can hear gravitational waves, we can hear the universe,” said Gabriela Gonzalez of Louisiana State University. “We are not only going to be seeing the universe, we are going to be listening to it.”

From CalTech, this is LIGO & The First Observation of Gravitational Waves.

Plus one more beautifully-made explanation from The New York Times: