Inside a 3,000-square-foot space adjacent to the UC Davis Physics Building, Nancy Aggarwal searches for invisible ripples and substances in the fabric of space-time. It’s a space uniquely suited for detecting gravitational waves and dark matter, built on its own foundation and outfitted with thick concrete walls.
“In my experiments, we need a very quiet space, and by quiet, I don’t just mean sound. I mean quiet in every way,” said Aggarwal, an assistant professor in the Department of Physics and Astronomy. “If people are walking next to my lab, that might shake things but also seismic motion — so Earth’s motion, ground motion — a lot of these movements are very disturbing to our experiments.”
An expert in quantum precision measurements, Aggarwal creates technologies to detect the universe’s unseen phenomena. In her lab, she’s developing two major experimental platforms: a first-of-its-kind gravitational-wave observatory and precision measurement systems for next-generation dark matter searches.
For Aggarwal, the tasks, and the technologies underlying them, are reminiscent of the scientific revolutions ushered in by Galileo Galilei and other early scientists when they first pointed telescopes at the stars and planets. That innovation opened the cosmos, allowing humanity to perceive things beyond the capabilities of the naked eye. But more is still out there.
"We know there's about five times more matter in the universe than what we can see. There are so many things out there that do not emit light. For example, black holes and dark matter. Gravitational waves can come from objects that emit little or no light."
— Nancy Aggarwal
“And that’s what we’ve done with gravitational wave observatories, like the Laser Interferometer Gravitational-Wave Observatory, or LIGO,” she added. “We have pointed a new type of telescope at the sky, but we need to continue innovating.”
What are gravitational waves?
Gravitational waves are invisible emanations from the universe’s most cataclysmic cosmic events, such as the merging of black holes, supernovae or colliding stars. These events send ripples through space-time, but those ripples are subatomic in size and require special instrumentation for measurement. Usually, these instruments need to be isolated and large. LIGO, twin observatories that detected the first gravitational waves in 2015, is 4 kilometers in size.
Aggarwal worked at the LIGO Lab while studying for her doctoral degree at the Massachusetts Institute of Technology.
“Gravitational wave detectors aren’t typically on university campuses, but we have a design that will allow us to detect gravitational waves with a much, much smaller detector,” Aggarwal said.
While LIGO looks at celestial objects 100 megaparsecs, or 326 million light-years, away, Aggarwal’s detector will focus on objects much closer in our cosmic neighborhood, about 10 kiloparsecs away.
“We have thought about a technique that could be sensitive to gravitational waves from dark matter and black holes in the Milky Way galaxy,” Aggarwal said.
How do scientists detect gravitational waves?
Aggarwal’s technique hinges on amplifying the signal from gravitational waves by tuning in to its resonant frequency. Think of the gravitational wave detector as a bridge and the gravitational waves as people walking on that bridge, Aggarwal explained. If the people walk at a speed matching the bridge’s resonant frequency, the bridge will start to oscillate.
“Our detector will move by a lot if the gravitational wave frequency is on resonance with the detector frequency,” Aggarwal said. “And if the detector is moving a lot, then I have made my challenge of detecting something small somewhat easier because now I’m detecting a larger motion.”
“We’re amplifying it by a million on resonance,” she added. “That’s the technology we’ll be using to bring this gravitational wave detection to tabletop size.”
The experiments will be done in two stages. First, Aggarwal and her colleagues will construct a one-meter sized detector to demonstrate the technique’s efficacy. Once fine-tuned, they’ll build a 10-meter sized detector in the space to look deeper into the universe.
Advancing the search for dark matter
Part and parcel with Aggarwal’s research on gravitational waves are her research efforts to enhance dark matter measurement techniques. For Aggarwal, dark matter represents an interesting conundrum. It makes up nearly 80% of the universe yet very little is known about it. What’s more, it’s evaded detection by our current technologies.
Aggarwal and her colleagues are building a custom cryostat to help hone the search. They’ll use the instrument to monitor the super-chilled environment of helium-3 particles. Specifically, they’re searching for axion-mediated interactions. Axions are hypothetical particles that are among the lead particle candidates for dark matter. If Aggarwal and her colleagues can detect how these proposed particles affect the spin of helium-3 particles, they can refine dark matter search techniques.
"What we want to do at UC Davis is look ahead and start studying the quantum measurement limit now, so that 10 years later when the experiments are advanced enough, they can use techniques that we have developed in our lab to go down below the quantum measurement limit," Aggarwal said.
Training the next generation of physicists and engineers
Aggarwal’s lab is a work-in-progress. The instrumentation to do the science she wants to do simply doesn’t exist yet. She must build it. Those tasks have created a unique opportunity for UC Davis students who want to be at the forefront of leading-edge physics and engineering research.
“One of my mentors told me that you can be all passionate about dark matter and gravitational waves, but if you don’t like soldering, you will never accomplish what you want to do,” Aggarwal said. “We’re building machines, building detectors, because we can’t just buy them.”
Currently, Aggarwal’s lab comprises two graduate students in physics and four undergraduate engineering students. It’s a hands-on experience from the ground up.
“The students start with designing something and then they go manufacture it, and then they go install it in the lab,” Aggarwal explained. “They go through this full cycle and then they test whether it works or not.”
As the lab is built, Aggarwal foresees more students and postdocs working in her lab.
How fundamental physics leads to new technologies
While Aggarwal’s research probes the fundamental nature of the universe, the experiments she’s developing could have broader impacts on technological innovation. Just like gravitational waves, basic science ripples out to the application side.
“Einstein discovered general relativity starting around 1915, so that’s a little over 100 years ago, and one of the technological marvels that Einstein could have never predicted was the global positioning system,” Aggarwal said. “It turns out if you don’t use general relativity, your location would be all kinds of wrong and now, we use general relativity in our mobile phones every day.”
The creation of LIGO required technological innovations to advance glass fibers, lasers, accelerometers and advance suspension techniques.
“A lot of this technology research that was done to build the gravitational wave detector is now being used in industry for other things,” Aggarwal said.
While the application side is very real, it’s Aggarwal’s curiosity that motivates her most.
I can’t just accept the fact that we don’t know what 80% of the universe is made up of. There’s so much that we have no idea about.
— Nancy Aggarwal
As humanity builds more advanced observatory and telescopic technologies, we slowly open the mysteries and unseen nature of the universe. It’s akin to going for a hike in the cosmos.
“Let’s go explore,” Aggarwal said.
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