LIGO Hanford has done it again.
Gravitational waves — ripples through space and time — have been detected by the Laser Interferometer Gravitational-Wave Observatory north of Richland and its twin in Livingston, La., for the second time in human history.
The National Science Foundation had been working toward the detection of a gravitational wave for 40 years when the LIGO observatories first succeeded in September.
Scientists announced Wednesday that a second gravitational wave had been discovered a little more than three months later on the evening of Christmas Day. The findings are published in Physical Review Letters.
Both times the waves were created as two black holes orbited around each other, gradually getting closer until they collided to form a single, massive black hole.
A portion of a combined black holes’ mass is converted to energy, which is emitted as a final, strong burst of gravitational waves.
The first gravitational wave detection was so pronounced that it could be picked out easily in the data generated at LIGO.
“This was more like what we were expecting to see,” said Fred Raab, head of LIGO Hanford.
It is a promising start to mapping the populations of black holes in our universe.
Gabriela Gonzalez, LIGO Scientific Collaboration spokeswoman
The event was more subtle than the first observation months earlier.
The September gravitational waves were created by black holes that were about 29 and 36 times the mass of the sun before they collided. The gravitational waves were stronger than those seen in December, causing the energy to radiate more quickly.
The gravitational waves detected in December were created by black holes about 14 and 8 times the mass of the sun.
Scientists say the smaller mass was significant. Because the black holes were lighter, the energy was in a band that could be observed by the twin LIGOs for almost a full second, while the first event was in the band for just two-tenths of a second.
The detected signal came from the last 27 orbits of the black holes before their merger.
“It is a promising start to mapping the populations of black holes in our universe,” said Gabriela Gonzalez, LIGO Scientific Collaboration spokesperson and professor of physics and astronomy at Louisiana State University.
In both cases, the events that created the gravitational waves occurred more than 1 billion years ago. The first black hole merger observed may have happened 1.3 billion years ago and the second merger may have been 1.4 billion years ago, but there’s enough room for error in the estimates that it’s difficult to say conclusively that one was older, Raab said.
1.4 billion years since gravitational waves were created by merging black holes
LIGO completed its first operating run in mid-January after five years of overhauling and upgrading the twin observatories. An initial run from 2002-10 made no detections.
The improvements to create what is called “Advanced LIGO” gave the observatories the capability to be 10 times more sensitive.
On an engineering run to test equipment four days before the official start of observations in 2015, the first detection of gravitational waves was made Sept. 14.
Elated scientists decided to extend the observing run a month longer than planned.
“The dedication of our staff supporting this decision paid off,” Raab said.
He had spent Christmas Day in the LIGO Hanford control room and had just finished dinner at home when he got a call.
A rapid search algorithm being used to scan data flagged possible gravitational waves three minutes after the waves had passed through the observatory, and scientists were notified.
The gravitational waves had actually shown up 1.1 millisecond earlier at the LIGO observatory in Louisiana and then at Hanford as they traveled north.
Having two observatories helps confirm the detection and locate the event in the sky.
LIGO detects gravitational waves by looking for barely detectable changes caused by the waves in the length in 2.5 mile long vacuum tubes.
LIGO Hanford has two vacuum tubes that extend for 2.5 miles across the Hanford shrub steppe at right angles. At the end of each, a mirror is suspended on fine wires.
A high-power laser beam is split to go down each tube, bouncing off the mirrors at each end. If the beam is undisturbed, it will bounce back and recombine perfectly.
But a gravity wave pulsing through the Earth stretches objects lengthwise and causes them to compress sideways. A circle would become an ellipse.
At LIGO, one arm would become longer and the other shorter. The laser beams would no longer recombine as expected if the waves were strong enough for the change to be detected at what’s the most precise measuring device ever built.
In the case of the December event, a change in the two arms was detected that was equivalent to detecting a fraction of an atomic diameter in the distance from the Earth to the Sun, Gonzalez said in a press conference at a meeting of the American Astronomical Society.
The tiny movement from a gravitational wave has to be sorted out from movement caused by other things, such as vibrations caused by ocean waves on the Pacific Coast.
But if a sufficiently strong gravity wave passes through Earth, both LIGOs, separated by 1,900 miles, should show near identical data.
All data from the initial run of the advanced LIGOs have now been analyzed for mergers of black holes with the total mass of 100 times or less the mass of the sun, Raab said.
But data evaluation continues looking for other events, such as gravitational waves from supernovas.
We already can start to piece together how many binary black holes there are in the universe and how often we might see them.
Fred Raab, head of LIGO Hanford
The initial detection of gravitational waves from black holes colliding confirmed a major prediction of Albert Einstein’s 1915 general theory of relativity.
It also opened up a new way to study the universe and the nature of gravity.
When the two LIGOs start up for their second run with advanced equipment, the focus will again be on black holes.
“We already can start to piece together how many binary black holes there are in the universe and how often we might see them,” Raab said.
Two observations in the initial run of Advanced LIGO exceeded expectations, said France A. Córdova, director of the National Science Foundation.
By looking for the abundance of binary black holes relative to their mass and other information, clues might be revealed about how binary black holes formed initially and how the universe evolved.
“All we know is black holes occur in binaries, but we don’t know why,” Raab said. “Every future detection will shed more light on that.”
The twin LIGOS will be restarted this fall for a possible six-month operating run. Further improvements in detector sensitivity, including more powerful lasers, are expected to allow LIGO to reach as much as 1.5 to 2 times more of the volume of the universe.
In the latter part of the operating run, the two LIGO observatories in the United States will be joined by Virgo, a European interferometer based in Italy. Using three interferometers will provide a better location in the sky of the signals.