Last Dance of Neutron Star Pair
The gravitational wave observatory near Richland is again searching for ripples through time and space that may have traveled for a billion years after violent collisions to reach the Earth.
This time the detection of evidence of black holes and neutron star collisions could be made more frequently, possibly a couple of times a month.
The operating run that began last week will be the second one since the observatory and its twin in Livingston, La., detected gravitational waves for the first time in September 2015 — 100 years after Albert Einstein’s general theory of relativity predicted their existence.
The two U.S. LIGOs detected the gravitational waves from two black holes that spiraled toward each other and collided in a distant galaxy more than 1 billion light years from the Earth.
The Laser Interferometer Gravitational-wave Observatory, or LIGO, is a new kind of scientific observatory.
Instead of observing the heavens with light, it looks for evidence of past events, such as the collision of black holes, that create gravitational waves, or ripples through space and time.
It has opened up a new way for scientists to learn about the universe, much of which may be made up of matter unlike what we’re familiar with on Earth.
The gravitational waves detected in 2015 were from the violent collision two black holes.
Researchers expect more frequent detections
The LIGO detectors had operated from 2002-2010 without a detection, when operations were stopped for five years to update the observatories, giving them the capability to be 10 times more sensitive and leading to the initial detection of gravitational waves.
Since the initial Advanced LIGO run in 2015, the observatories have been shut down two more times for incremental improvements.
During the last shut down, the LIGO detectors’ sensitivity was significantly enhanced for the current operating run.
Scientists estimate they could make as many as 20 detections of black hole mergers in the planned year-long operating run, said Michael Landry, head of LIGO Hanford.
They also could observe a couple of neutron star mergers like the fiery collision detected in August 2017, he said.
Neutron star collisions create not only gravitational waves but also light, which was observed by dozens of telescopes in space and on the ground. It was the first time a cosmic event was viewed in both gravitational waves and light, marking the inception of multi-messenger astronomy.
Neutron stars are the smallest, densest stars known to exist, weighing a billion tons per teaspoon. When they collide, the crash spews material that radioactively decays creating metals like platinum and gold.
Hopes for another LIGO “first”
The initial detection of a collision of neutron stars confirmed that heavy elements in our universe, such as platinum and gold are created in neutron star smashups.
Scientists are hoping that the current operating run will lead to another first — the first detection of gravitational waves from a collision between a black hole and a neutron star.
“All of our upgrades mean that LIGO can now see farther into space to find the most extreme events in the universe,” said Calum Torrie, LIGO’s mechanical optical engineering head based at the California Institute of Technology.
LIGO is funded by the National Science Foundation and operated by Caltech and the Massachusetts Institute of Technology.
The improved sensitivity allows LIGO to observe neutron star events that created gravitational waves out to an average of 550 million light-years away, or more than 190 million light-years farther out than before.
The world has a third gravitational wave observatory, Virgo, in Italy, which also was upgraded to almost double its sensitivity and began an operating run with the two U.S. LIGOs at 8 a.m. Monday.
All three “are performing beautifully,” Landry said.
When all three observe gravitational waves passing through the Earth, scientists can triangulate data to better determine the location of the collision in the sky.
For neutron star collisions that emit fireballs of gamma rays, information about location can help astronomers at other types of observatories to find the collision site and collect data.
Before the new operating run is completed, a fourth observatory, the Kamioka Gravitational Wave Detector, or KAGRA, in Japan could also begin operating.
How gravitational waves are detected
The Hanford LIGO northwest of Richland was built on unused land at the Hanford nuclear reservation to detect gravitational waves that stretch space in one direction and contract it in another.
The LIGO observatories look for that distortion, which effectively turns a circle into an ellipse.
At LIGO Hanford two vacuum tubes, each 2.5 miles long, extend across the shrub steppe landscape north of Richland at right angles. At the end of each, a mirror is suspended on fine glass fibers.
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 if a powerful enough gravitational wave is pulsing through the Earth, the beam will be disturbed as the waves slightly stretch one detector arm and compress the other. The movement is so small that it would take 10 trillion such movements to equal the width of a a human hair.
Having a matching LIGO in Louisiana has helped confirm when data collected matches to show that movements detected were from gravitational waves. 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 across Washington state on the Pacific Coast.
Improvements made before LIGO restart
The upgrades to LIGO Hanford made in the last year included replacing three of the four mirrors at each end of the vacuum tubes.
“We had to break the fibers holding the mirrors and very carefully take out the optics and replace them,” Torrie said. “It was an enormous engineering undertaking.”
Engineers also put in new baffles to eliminate scattered light that can reflect back into the interferometer and interfere with data.
Additionally, they began upgrades designed to reduce levels of quantum noise. Quantum noise occurs due to random fluctuations of photons, which can lead to uncertainty in measurements and can mask faint gravitational-wave signals.
In February the National Science Foundation announced the award if $20.4 million for the next major upgrade to create what will be called upgrade to Advanced LIGO Plus.
UK Research and Innovation and the Australian Research Council also will contribute funds.
More work will be done to refine laser light with the use of techniques from quantum mechanics. The work and new mirror coating technology is expected to increase the volume of deep space the observatory can survey by as much as seven times.
Advanced LIGO Plus will begin operating in 2024 — following one more planned run of Advanced LIGO beyond the one just started.
A 12th detection?
The current operational run was preceded by two engineering runs to test equipment.
LIGO Hanford operated for a couple of weeks in December and for the month of March.
The first black hole merger detection in 2015 was made at the end of an engineering run, just days before the operating run started.
LIGO scientists kept it a secret as they confirmed data, announcing it in February 2016.
It’s possible that the recent engineering runs also could have made a detection that has yet to be announced.
Starting with the current operating run immediate announcements are expected to be sent to astronomers when a likely detection is made.
The observatory in the Tri-Cities backyard is making scientific history, but it still is accessible to the public.
Mid-Columbia students make field trips to LIGO Hanford each spring and public tours also are offered.
The next free monthly tour of LIGO Hanford will be April 13. Walking tours that last about about an hour start at 1:30 and 3:30 p.m. A LIGO staff member will give a talk at about 3 p.m.
To reach LIGO, search for “LIGO Hanford Observatory” on Google Maps. Or drive northwest from Richland on Highway 240 and turn right on Hanford Route 10 at the and drive about five miles.