Once again, it appears Einstein was on to something.

Last Thursday, a team of researchers announced that they heard the sound of two black holes colliding a billion light-years away. The scientists recorded the sound, which sounds like a faint chirp.

This sound, according to physicists, is the first direct evidence of gravitational waves, the existence of which Einstein predicted one hundred years ago.

“We are all over the moon and back,” Gabriela Gonzàlez, a spokeswoman for the LIGO Scientific Collaboration from Louisiana State University, told the New York Times. “Einstein would be very happy, I think.”

The discovery of gravitational waves confirms Einstein’s 1915 general theory of relativity. According to this theory, a pair of black holes orbiting each other lose energy by emitting gravitational waves. The loss in energy causes them to orbit closer and closer to each other, a process that can take billions of years, though it rapidly increases in the final minutes. In the final fraction of a second, the two black holes collide at half the speed of light to form one massive black hole. A portion of the black hole’s combined mass is converted to energy, which is emitted as gravitational waves.

The waves themselves affect spacetime as they radiate outward, compressing space in one direction and stretching it out in another.

On Sept. 14, 2015, these gravitational waves reached earth, and the Laser Interferometer Gravitational-Wave Observatory (LIGO) picked them up.

In a press release, the LIGO Scientific Collaboration (the official title of the research group) describes LIGO as “two identical detectors carefully constructed to detect incredibly tiny vibrations from passing gravitational waves.” LIGO’s antennas have perpendicular arms that stretch 2.5 miles, forming giant “L”s on the landscape. The arms are essentially large concrete and steel vacuum chambers with mirrors hanging by glass threads hang at the end of each arm.

This construction protects the lasers that the LIGO uses, which allows them to detect changes as small as one ten-thousandth the diameter of a proton. Scientists cannot even observe this change under the most powerful microscopes.

One detector is in Louisiana, the other in Washington. Scientists from the California Institute of Technology and MIT, with funding from the National Science Foundation, collaborated on the development of this project, which is 40 years in the making.

Dr. Kip Thorne from the California Institute of Technology, and one of the leading researchers of the project, told the New York Times: “Until now, we scientists have only seen warped spacetime when it’s calm. It’s as though we had only seen the ocean’s surface on a calm day but had never seen it roiled in a storm, with crashing waves.”

MIT astrophysicist Nergis Mavalvala told Reuters, “We are really witnessing the opening of a new tool for doing astronomy. We have turned on a new sense. We have been able to see and now we will be able to hear as well.”

Gravitational waves provide a new avenue through which scientists can study the universe and the objects in it.

All prior knowledge of the universe comes from electromagnetic waves such as radio waves, visible and infrared light, X-rays and gamma rays. These waves encounter interference as they travel through the cosmos, providing only distorted information.

Gravitational waves do not experience this distortion and can provide information about phenomena such as black holes, which do not emit electromagnetic waves.

With such an important discovery, scientists are ecstatic.

“The discovery of gravitational waves is, I think, the most important breakthrough in modern science,” Szabolcs Marka, the leader of the Columbia Experimental Gravity Group, told CNN.

Carlos Lousto, an astrophysicist at Rochester Institute of Technology, agreed.

“This is the holy grail of science,” he told Reuters.