In September 2015, news that gravitational waves were detected after two black holes collided made a splash in the scientific community.
Ninety-nine years before, Albert Einstein theorized that if the fabric of the cosmos was really a four-dimensional mesh of space and time as described in his theory of general relativity, then any object with mass can warp space-time, creating what we call gravity.
If two very massive objects, such as black holes collided, then they would cause a ripple in the fabric of space time dramatic enough for astronomers to detect in the form of gravitational waves.
The 2015 discovery by the Large Interferometer Gravitational-Wave Observatory (LIGO) was touted as a new way to do astronomy. Until that point, the only way astronomers could gather information about the universe was by analyzing light collected with telescopes.
Now, however, astronomers announced that they have seen a cosmic event in both gravitational waves and, more significantly, light.
On Aug. 17, a collision of two neutron stars, the extremely dense corpses of stars more massive than our sun but not quite massive enough to become black holes, collided.
The event was seen by gravitational wave detectors, including the two LIGO sites in Washington and Louisiana and the Virgo interferometer in Italy, as well about 70 observatories around the world searching the skies across the spectrum of light.
“It’s a new way to look at the universe, combining two different methods,” said David Sand, University of Arizona assistant professor of astronomy.
Sand is also the principal investigator of the Distance Less Than 40 Megaparsec (DLT40) survey, one of the teams running an observatory that followed up the gravitational wave detection alert.
The event has sent the astronomical community into a frenzy.
About 50 papers alone were published on Monday, Sand said. “Yeah, people are going bananas.”
This discovery not only generated new knowledge, but confirmed many theories. And, typical of science, many more questions arose.
Luck favors
the prepared
A long time ago, in a galaxy far, far away, two neutron stars slowly inched toward each other in their orbital dance.
As time passed, they spun faster and faster as they moved closer and closer. In their last seconds, they neared the speed of light and finally collided with unimaginable energy, sending ripples through space and time.
Traveling at the speed of light, both the gravitational waves and the light produced from that collision reached Earth 130 million years later, on Aug. 17, 2017.
Sand’s DLT40 survey images 500 galaxies a night, hunting for supernovas. In addition to its normal supernova survey, the team, and many others around the world, also has a deal with LIGO to follow up any gravitational wave signals it might detect.
That night, Stefano Valenti, project scientist for the DLT40 survey, received a LIGO alert to find the event hidden in a large region of the sky. Valenti instructed the robotic telescope to point the “souped-up” 16-inch optical telescope called PROMT in Chile low in the sky, along the horizon, Sand said.
Sand was watching “Rogue One: A Star Wars Story” on the treadmill at the gym the night he got a message saying, “Hey, there’s something there.”
“And I’m like, ‘OK whatever, it’s probably an asteroid,’” Sand remembers saying. “Get another image. We did and it was still there. I was freaking out.”
The DLT40 team was the second to observe the event, but the third to post its observation, because team members couldn’t get hold of the graduate student who was usually tasked with posting their observations to the LIGO bulletin.
“We were like, ‘We need to post this now’ and he was the only one with a password. He was out driving somewhere with his wife,” Sand said, laughing, as he remembered the tension of the moment. “We’re like, ‘Dude! This is important!”
“(We) started to see the evolution of the object was different,” Valenti said. “It was evolving much faster than other objects,” growing dimmer faster than supernovas do. “This was the most exciting thing in my career.”
A multifaceted discovery
The event has ushered in a slew of discoveries and new questions.
Among them, and the most fundamental, is the new form of multimessenger astronomy that arose from this event, gravitational waves plus light.
This event will also teach astronomers much more about mysterious neutron stars.
One of such solved mysteries includes the confirmation that roughly half of the elements heavier than iron, including gold and platinum, are generated in the explosion debris called the Kilonova. The other half of heavy elements are formed in supernova explosions.
Kilonovas were also observed for the first time in this event. Kilonovas are like the middle-children of the Nova family, between Supernovas — the brightest and longest-lived and Novas — the dimmer and shorter-lived.
Sand’s team is focusing its research on refining the expansion rate of the universe, which is known to be increasing with time.
The team is on a Nature paper.
“It’s not competitive yet with other techniques, but once we have a couple dozen of these things, it will be,” Sand said.
So what can be next for gravitational wave astronomy? Astronomers want to get more detectors online. In the next few years, detectors will be added in Japan and India.
In 2015, LIGO was just two detectors in the United States. With the addition of the Virgo interferometer, the triangulation of gravitational wave sources has gotten better. But the more detectors, the more precisely the event source can be pinned down.
And the more powerful these detectors get, the further out they can “see.” If the distance at which these events can be detected is doubled in all directions, astronomers won’t see double the number of events — they will actually be able to see eight times as many.
“It’s all about the long game,” Sand said.
“There are deep implications that you can’t get from one event but over a decade, building up the numbers and statistics. ... it gets at more fundamental issues.”
