After Three Years of Upgrades, LIGO is Fully Operational Again

Have you noticed a lack of gravitational wave announcements the past couple of years? Well, now it is time to get ready for an onslaught, as the Laser Interferometric Gravitational-Wave Observatory (LIGO) starts a new 20-month observation run today, May 24^th after a 3-year hiatus.

LIGO has been offline for the last three years, getting some serious new upgrades. One upgrade, called “quantum squeezing,” reduces detector noise to improve its ability to sense gravitational waves.

Astronomers expect this upgrade could double the sensitivity of LIGO. This will allow black hole mergers to be seen more clearly, and it could also allow LIGO to see mergers that are fainter or farther away. Or, perhaps it could even detect new kinds of mergers that have never been seen before.

Making LIGO even stronger is that it will be joined by two other gravitational wave facilities, Europe’s Virgo instrument and the new Japanese Kamioka Gravitational Wave Detector, or KAGRA.

Gravitational waves are tiny ripples in space itself that travel through the universe. They are caused by massive objects moving with extreme accelerations, such as colliding black holes, merging neutron stars, or exploding stars.  

When a gravitational wave passes through an object, the relative positions of the particles in the object shift slightly, and it’s only through those shifts that we can detect the gravitational waves. But that shift is minuscule.

LIGO has two detectors, one located in Hanfrod, Washington, and the other in Livingston, Louisiana. Each detector consists of two concrete pipes that are joined at the base (forming a giant L-shape) and extend perpendicular to each other for about 4 km (2.5 miles). Inside the pipelines, two powerful laser beams that are bounced off a series of mirrors can measure the length of each arm with extreme precision. When a strong gravitational wave passes LIGO, the mirrors shift at the subatomic level, by only a few thousandths of the width of a proton.

Since 2015, LIGO has completed three observation runs. The first run lasted about four months; the second about nine months; and the third ran for 11 months before the COVID-19 pandemic forced the facilities to close. Starting with the second run, LIGO has been jointly observing with Virgo.

By the end of the third run in March 2020, researchers in the LIGO and Virgo collaboration had detected about 90 gravitational waves from the merging of black holes and neutron stars.

Writing in The Conversation, LIGO team member Chad Hanna explained the new quantum squeezing upgrade which involved adding a 300 meter (1,000-foot) optical cavity. Squeezing allows scientists to reduce detector noise using the quantum properties of light. Hanna said with this upgrade, as well as improvements in the software they use, the LIGO team should be able to detect much weaker gravitational waves than before

“My teammates and I are data scientists in the LIGO collaboration, and we have been working on a number of different upgrades to software used to process LIGO data and the algorithms that recognize signs of gravitational waves in that data,” Hanna wrote. “These algorithms function by searching for patterns that match theoretical models of millions of possible black hole and neutron star merger events. The improved algorithm should be able to more easily pick out the faint signs of gravitational waves from background noise in the data than the previous versions of the algorithms.”

Kimberly Burtnyk from Caltech said with the upgrades, the LIGO team has a “sensitivity goal” of 160-190 megaparsecs (Mpc) for binary neutron star mergers, which means, that is how far away LIGO can expect to detect two neutron stars colliding. Virgo has a target sensitivity of 80-115 Mpc, while KAGRA, “which employs some unique forward-looking but challenging detection technology,” should be running with greater than 1 Mpc sensitivity.  

“Of course, more violent or larger events, such as black hole collisions, are detectable from much deeper reaches of the Universe,” Burtnyk said, “but we use the distance at which we can detect neutron star mergers as a means to describe our sensitivity to all gravitational waves.”

Hanna also said that the coming months should result in several “multi-messenger” observations between the three facilities that will push the boundaries of modern astrophysics.