The LIGO observatory near Richland has detected gravitational waves, or ripples through space and time, for a third time.
“We are really moving from novelty to to a new observational science, a new astronomy of gravitational waves,” said David Shoemaker, of the Massachusetts Institute of Technology.
All three times the gravitational waves were generated by the collision of black holes to form a larger black hole.
The Laser Interferometer Gravitational-wave Observatory at Hanford and its twin observatory in Louisiana made scientific history by detecting gravitational waves for the first time on Sept. 14, 2015, confirming Albert Einstein’s theory of relativity. A second observation came three months later on Christmas Day.
The most recent detection was made on Jan. 4, LIGO officials announced Thursday after analyzing data from both observatories. Findings are being published in Physical Review Letters.
“The event was a lot like our first detection except that the black holes were another two times farther away, some 3 billion light years away,” said Shoemaker, the elected spokesman for the LIGO Scientific Collaboration, a body of more than 1,000 international scientists.
The first two black hole mergers detected were 1.3 and 1.4 billion light years away.
They may have played an important role in our early universe. And we are starting to get a glimpse into how they behaved.
Laura Cadonati, of the Georgia Institute of Technology
The mass of the most newly discovered black hole formed by the mergers is about 49 times the size of Earth’s sun, a size in between the earlier detections. The first black hole was the size of 62 suns and the second was the size of 21 suns.
“Before our discoveries we did not know for sure that black holes existed,” said Laura Cadonati, of the Georgia Institute of Technology and the LIGO Scientific Collaboration deputy spokeswoman.
“We know now that they do exist,” she said. “They may have played an important role in our early universe. And we are starting to get a glimpse into how they behaved.”
The latest detection has scientists intrigued by the spin of the two black holes before they collided.
The black holes were spinning individually as they orbited around each other, gradually getting closer until they collided to for a single, massive black hole. In this case at least one of the black holes does not appear to have been spinning in the same direction as the orbit. Scientists call it not being aligned.
“This is the first time that we have evidence that the black holes may not be aligned, giving us just a tiny hint that binary black holes may form in dense stellar clusters,” said Bangalore Sathyaprakash of Pennsylvania State University and Cardiff University, one of the editors of the new published paper.
Under one model to explain how binary pairs of black holes are formed, it is proposed that they are born together when each star in a pair of stars explodes. Because the stars would have been spinning in alignment, the black holes continue spinning in alignment.
At full sensitivity we could expect to have an order of one per day or one per week of these types of events.
Laura Cadonati, of the Georgia Institute of Technology
In the other model, supported by the most recent gravitational wave detection, the black holes form separately and then pair up within crowded stellar clusters and may not be spinning in the same direction as their orbital motion.
“We have found a new tile to put in the puzzle of understanding the formation mechanism,” Cadonati said.
The first detection of black holes was made after a five-year upgrade to the Hanford and Louisiana LIGOs to make them 10 times more sensitive in a project called Advanced LIGO.
The second operating run of Advanced LIGO started on Nov. 30, little more than a month before the most recent detection was made. The operating run is expected to continue through August.
Gravitational waves can be generated by other violent events in space, including the mergers of neutron stars.
Michael Landry, head of LIGO Hanford, is confident that a binary neutron star merger will be detected, providing information about extreme states of nuclear matter, he said. It’s just a matter of whether it is discovered as this operating run continues or in a future run.
Work will continue to increase the sensitivity of the LIGO observatories and the volume of space they search, increasing the chances of detection of more gravitational waves and gravitational waves from different sources.
“At full sensitivity we could expect to have an order of one per day or one per week of these types of events,” Cadonati said.
These are the most powerful astronomical events witnessed by human beings.
Michael Landry, head of LIGO Hanford
Normally people think of space as being nothing, as having no properties at all, said Landry. But when gravitational waves pass through, they expand or contract space. As space is stretched in one direction it contracts in the other. A circle would become an ellipse.
During the most recent observation, the mass of two suns was converted into gravitational waves that distorted the shape of space.
“These are the most powerful astronomical events witnessed by human beings,” Landry said.
At LIGO Hanford, two vacuum tubes extend 2.5 miles each at right angles across the shrub steppe landscape. As the gravitation waves passed through Earth, they would have lengthened one of the tubes and shortened the other.
A change in the distance of the tubes is detected with a laser beam. The beam is split with half going down each of the tubes to bounce off mirrors suspended at the ends. If the mirrors change distance, the beam will not perfectly combine as its two parts bounce off the mirrors and return.
To verify a detection, data is collected at both the Louisiana and the Hanford LIGOs. Sometime this summer, the two LIGO observatories in the United States are expected to be joined by a third observatory, Virgo, a European interferometer based in Italy, after upgrades are completed on it.
Before the start of the current operating run, the power of the LIGO Hanford laser was doubled. However, the sensitivity increase was only modest because of issues caused by the increased power.
Before the start of the next operating run, the third for Advanced LIGO, more improvements are planned. They include upgrading mirrors, work to eliminate the scattering of laser light as it reflects off objects such as the beam tubes, and improving computerized controls.
“These upgrades will improve our reach into space,” Landry said. “We’re looking forward to making additional binary black hole detections in that run, but also the possibility of detections that include matter.”
LIGO is paid for by the National Science Foundation and operated by MIT and the California Institute of Technology.