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Northwestern University

Fourteen years ago scientists started looking for something—something that now, with the help of 760 scientists from 60 institutions and 11 countries across the world looking with them, they are closer than ever to finding: gravitational waves. Gravitational waves are a theoretical consequence of Einstein's General Theory of Relativity, which posits that the universe is made out of a fabric called space-time (3-dimensional space as we think of it, plus an additional dimension of time—the same time that we read on our watches), and that this space-time can be altered by the massive objects it contains (stars, planets, black holes, etc.) The traditional three-dimensional analogue is a rubber sheet with a bowling ball on top, where space-time is the sheet and any massive cosmic object the ball. Much as a rubber sheet will sag beneath a heavy bowling ball, so too does space-time bend under the burden of massive objects. And when these massive objects move, ripples are generated across the fabric of space-time—an effect very similar to the ripples cast across a pond when a duck swims across. Hence the name gravitational waves.

The U.S.-funded part of the collaborative project in search of these waves is called the Laser Interferometer Gravitational Wave Observatory, or LIGO for short. The LIGO Scientific Collaboration has many different divisions, and this group, which is headed by Vicky Kalogera and consists of Vivien Raymond, Will Farr, Diego Fazi, Carl Rodriguez, Ben Farr, and Erik Luijten as the expert in statistical methods, is working as a part of the Data Analysis division. This Northwestern University group is part of CIERA, the Center for Interdisciplinary Exploration and Research in Astrophysics, and their section within LIGO is Compact Binaries Coalescence—the most promising source of gravitational waves. Compact binaries are any class of system in which two hugely massive objects, such as black holes and neutron stars, are in (relatively) close orbit around each other. These systems are dense enough to significantly deform the space-time around them, and when they move they should generate strong gravitational waves. So why haven't any waves been detected? The answer lies in the difficulty of detection.

LIGO consists of several laser interferometers across the globe—giant L-shaped devices that measure the time it takes for laser light to travel between two mirrors. The mirrors are suspended at the end of both arms in the "L", which are approximately 2.5 miles long each. Their lengths are identical, so that the time it takes for the laser light to travel back and forth across each arm is exactly the same. If a gravitational wave were to pass through the earth, however, the ripples in space-time would distort the distance travelled by the light, creating a slight discrepancy between the times measured in each arm. This predicted effect is so small, though, that the change in distance created in a 2.5-mile arm would be less than the diameter of a proton, which is several times smaller than the width of a human hair. Getting an instrument to make measurements on this scale is a true feat of modern engineering, because, as one could imagine, the logistics of such measurement are extremely difficult.

The interferometers are constructed in very isolated locations to avoid external interference. Still, a car driving down the highway tens of miles away, or an earthquake in Japan, or large waves hitting the Gulf of Mexico, or the 8:00am cargo train that passes within 50 miles of the device every morning, all cause the mirrors to vibrate and create distinct perturbations in the measurements of time. These myriad interferences make data analysis a gruelingly challenging affair. Kalogera and her team are part of the Parameter Estimation subgroup. They are given interesting sections of data collected from the interferometers and line them up with models of what the expected signal would look like from a passing gravitational wave. They then use the results from suspicious patterns to estimate the parameters of the objects that cause them—the size and mass of the black holes, their distance from us, and their spin (Kalogera and her team are the only group in all of LIGO that can do parameter estimation accounting for the spin of the black holes). These parameters could prove to be essential in furthering our knowledge of the universe and forming population synthesis models (explanations of how everything came to be in our universe). To do a full analysis on just one case, however, requires several months of computer power.

Kalogera and her team are working constantly on making this process as efficient and accurate as possible. They will continue analyzing older data throughout the next several years, even after the LIGO interferometers are shut off for a 6-year renovation and upgrade. The remaining time Kalogera and her team will spend preparing for the new detectors, called Advanced LIGO (AdLIGO), which are projected to increase the detection rate by a factor of more than 5,000. With the current system, gravitational waves from binary black holes are estimated to occur anywhere from once every year to once every 5,000 years (no wonder we haven't seen anything yet). With the advanced detectors in place, however, waves are estimated to occur once every 2.5 years, worst-case scenario. Kalogera and her team are at the forefront of an exciting time in the search for gravitation waves. Their team has been an integral component of what could soon be the long-awaited discovery—something that would dramatically alter our understanding of our universe and confirm the very limits of Einstein's General Theory.

Apart from the Data and Analysis division but still within LIGO and Northwestern's CIERA, professor Selim Shahriar and his team have been working on making possible further improvements to the detectors, even beyond AdLIGO. They work at the Laboratory of Atomic and Photonic Technology, where they have been investigating the feasibility of enhancing the sensitivity and bandwidth of the AdLIGO apparatus based on the principles of the White Light Cavity effect. Shariar and team have made a series of alterations to the traditional apparatus setup. The net result is that the modified configuration has larger sensitivity and much larger bandwidth than the current AdLIGO configuration. They have so far demonstrated the Wight Light Cavity effect experimentally, albeit in a different configuration, and have carried out a detailed theoretical modeling of their experimental configuration. Currently, they are working on a detailed analysis of the expected quantum noise properties of this configuration, as well as investigating different materials as candidates for producing the desired improvements. Their work could prove instrumental in further enhancing the detection capabilities of AdLIGO and advancing gravitational-wave astronomy for decades to come.


Written by: Aaron Frumkin



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