A long time ago, in a galaxy 130 million light-years away from Earth, two dead stars called neutrons merged into a kilonova and set off a spectacular explosion that rippled through the universe. In August 2017, the energy from this event finally reached Earth, with 70 observatories from around the world detecting multiple wavelengths of light in the form of a gamma ray burst.
While the visuals were spectacular, it's what the scientists heard that will forever change the way we study the universe. For the first time, two new observatories, the Laser Interferometer Gravitational-Wave Observatory (LIGO) and the Virgo Interferometer, detected gravitational waves emanating from the kilonova.
"This is the first time the cosmos has provided for us what I would call a talking movie," David Reitze, executive director of the LIGO laboratory, said at a news conference on Oct. 16. "We've moved from the era of silent movies to talking movies. In this case, the audio soundtrack comes from the [gravitational wave] 'chirp' of the neutron stars … as they are orbiting together and colliding. And the video is basically the light that we see after the collision."
The discovery is huge because it confirms long-standing theories and simulations, the first of which was posited by Albert Einstein in 1916, in physics, mathematics and astronomy. It also ushers in new fields of gravitational wave astronomy and something called multi-messenger astronomy.
"Not only do we see a burst of gravity waves — a wave of gravity traveling through space at the speed of light — but we also see various forms of light emissions," University of Illinois physics and astronomy professor Stuart Shapiro shared with the News-Gazette. "We see infrared radiation, we see radio waves, gamma rays and X-rays, among other forms of light. And they coincided and they're all coming from the same source. This is remarkable."
A lucky break
Astronomers have known about several binary neutron star systems for years, but it was unclear when an actual collision in their orbits might take place. And while simulations were performed on what might happen in the aftermath of a merger, it was only recently that both the LIGO and VIRGO observatories were able to simultaneously capture the gravitational waves alongside the dozens of other installations detecting the light.
"This thing exploded 130 million years ago," Maria Drout, of the Carnegie Observatories, told National Geographic. "But if it had happened a month later, we wouldn’t have been able to see it at all. The detectors would have been turned off, and it would have been behind the sun."
Wave of the future
So how do gravitational waves stand to help us in our quest to better understand the universe? The immediate advantage is that astronomers have a new tool for detecting new details about violent events in the cosmos. In the future, as the nascent field of gravitational wave astronomy expands, it may also open the door to finally answering what the universe is made of. As EarthSky points out, only 5 percent of the universe is ordinary matter, with the remainder made up of mysterious dark matter and energy.
"This gives us a better idea of the content and history of the universe," Shapiro added. "We have models that show how from a nearly homogenous soup of radiation and gas and dark matter our universe formed clumps of galaxies and then stars, and we believe we understand how those stars evolved through nuclear burning and died to give rise to neutron stars and black holes. And this now confirms further the way in which the entire process has taken place."