New Methods for Detecting Astrophysical Signals

The University of Delaware’s Swati Singh was a curious child. If you polled her parents, they probably would recount the numerous times she took apart her toys while trying to understand how they worked. She broke a lot of things for the same reason.

In middle school, Singh realized that she could leverage this inquisitiveness as a scientist, after a pivotal conversation with her dad. She had asked her father how a lightbulb worked. The answer, she said, came down to electrons.

“He said, ‘But have you ever seen an electron?’ I told him no and he said, ‘You just believe it,’ ” she said of the conversation. “I still haven’t seen an electron, but there are reasons that I believe it exists.”

Singh, now a UD assistant professor of electrical and computer engineering, is still searching for answers about the unknown. These days she is focused on understanding dark matter and dark energy, the mysterious substances that make up over 95% of the universe but can’t be measured using existing devices. Together dark matter and dark energy are known as the dark sector.

It’s a fitting field for the girl who once believed in something she couldn’t see that science said was there, and the well-respected quantum physicist and electrical engineer she has become. In her work, Singh has carved a niche for herself by investigating ways to repurpose existing tabletop sensors for dark matter detection.

Now, the National Science Foundation has awarded Singh a five-year, $400,000 Faculty Early Career Development Award (NSF CAREER) to explore new methods for studying the dark sector using additional mechanical devices operating in the classical and quantum realm.

Singh hopes this groundwork will lead to new precision measurement systems to detect astrophysical signals.

Studying the dark sector

So, a quick primer: ordinary matter includes all things that emit light, such as gas, dust, stars, planets and us. Dark matter comprises everything else — it doesn’t emit light, but researchers know it exists by its gravitational effects. Dark energy, meanwhile, is an unknown form of energy/matter present throughout the universe that is responsible for the universe’s accelerated expansion.

When normal matter is coupled with dark matter or dark energy, it can manifest in a mechanical effect, such as strains, recoil kicks or accelerations. Accurately monitoring mechanical motion could give scientists unprecedented access to these minuscule signals, said Singh. She wants to incorporate new results from quantum physics, where recent work is providing interesting possible ways to look for very weak signals.

To begin, Singh plans to address some fundamental questions about the nature of dark matter itself. For instance, instead of swimming in a sea of dark matter as her research had assumed, what if there is a wave of dark matter that passes through at specific times? According to Singh, this would change the resulting signal.

She also plans to use a fiber-based broadband detector to search for dark matter in the infrasonic range, which sits below 20 hertz, the threshold for human hearing. Things that produce sound in this frequency range include earthquakes or volcanoes.

“We plan to take some of the best fiber optic cables, put them in a low-noise environment and use them as a ruler to try and measure small corrections to the length that would occur if dark matter were present,” said Singh.

In addition to the dark matter research in her group, she is teaming up with an experimental group in Alberta, Canada, that is building a superfluid helium-based detector for dark matter. The collaborative effort builds off Singh’s early work using helium-based detectors to sense gravitational waves. She and doctoral student Jack Manley will spend time at the University of Alberta as visiting scholars to learn and contribute to this detection effort through an innovation award from the American Physical Society, funded by the Gordon and Betty Moore Foundation to address pressing problems in fundamental physics.

In another arc of the work, Singh wants to know more about what dark energy could be and why it has no effect on human-made devices.

She explained that the mass of normal matter and dark matter both bend the fabric of spacetime, which we call gravity, kind of the way that a stone placed on a piece of cloth would bow the material. In the cosmos, however, the universe is also expanding at the same time. And, observations have shown the fabric of spacetime is doing more than just stretching, it’s stretching faster than it has in the past. So, something is hitting the gas.

Singh plans to use spherical microparticles made of silica and membrane-based detectors as sensors to probe what dark energy might be. She said that when scientists look carefully at the force between the sun and Earth, the Earth and moon or two balls in a laboratory, they only see Newton’s law of gravity at work pulling these masses together. Meanwhile, research tells us that galaxies are simultaneously expanding away from each other.

This cosmic acceleration is attributed to dark energy. So, why are scientists only able to detect gravity?

One possible explanation, Singh said, is that in the absence of matter — say in the empty space between two galaxies — cosmic acceleration dominates. But when matter is present from a galaxy, the solar system or earthly objects such as balls in a laboratory, this acceleration effect is “screened” or masked.

Whether an object’s density plays a role in this effect is an open question. For example, what if interstellar dust, which is lightweight, exhibits less of a screening effect than a piece of lead on Earth that has greater density? If correct, this could help explain why researchers would observe cosmic acceleration on the largest scales, but closer to home, where objects from pebbles to planets are denser, this acceleration is screened, leaving only gravity detectable.

“These are very density- and geometry-dependent ideas,” Singh said. “My doctoral student Joey Betz is working through the math of it and looking at a variety of mechanical sensors with different shapes and materials to calculate how this size knob and geometry knob can inform us about modifications to gravity because of dark energy.”

Inspiring current and future peers

By helping to put experimental bounds on various connections between the dark sector and normal matter, Singh aspires to stimulate high-energy theory research into new models of dark matter and dark energy. She also hopes the work can inspire improved collaboration and a common language across multiple fields involved in studying the dark sector, where cosmology or astronomy perspectives reach down to the lab and insights from laboratory research inform theoretical fields. To foster this type of collaboration, Singh regularly involves cosmologists and experimentalists in developing ideas that her students are working on.

Additionally, Singh wants to motivate the next generation of scientists, particularly women, to stay curious.

“I don’t want to be one of the few people working in this area, so I feel the need to open the door for everyone else, as my mentors did for me,” she said.

Singh recently had the rare opportunity to learn about her own influence when former students from disparate areas of her academic life got together to celebrate her NSF CAREER achievement.

“These weren’t students in my research lab, they were people I had taught over the years,” she said. “It was such a special moment because my research accomplishments were being celebrated by students whom I’ve only interacted with in a classroom setting.”

It’s the kind of momentum that Singh wants to keep going. To that end, she regularly interacts with students involved with engineering organizations on campus, such the Society of Women Engineers and Women in Electrical and Computer Engineering, and she plans to collaborate with the College of Engineering’s K-12 program on ways to share dark matter and dark energy research with middle- and high-school students as part of her CAREER award. Singh developed a course to help UD students understand quantum computing by developing computer games, too. The course grew out of a guest lecture Singh gave in a cryptography class that led to a project with several students, deep questions about the mathematics of game theory and even a collaboration with Ivan Todorov, a mathematical sciences professor.

“Science is not done in isolation. Instead, advances are often inspired by scientists working at the interface of different fields,” Singh said.

Article by Karen B. Roberts | Photos by Evan Krape and NASA | December 06, 2021