Early light

By Yaakov Zinberg

It’s a familiar experience for air travelers: your suitcase is pulled aside for inspection by an airport security officer after the X-ray machine flags something inside. The culprit is often something simple, like a forgotten water bottle or a tube of toothpaste slightly over the size limit, and you continue on your way without needing to explain much about the contents of your bags.

But when Daniel Dutcher, associate research scholar in Princeton’s physics department, recently traveled through the airport on business, his luggage raised more eyebrows than usual. Dutcher is advised by Princeton cosmologist Suzanne Staggs, whose specialty is the early universe and who is designing telescope components that allow scientists to study it. On a trip to Chile, where the Staggs research team is involved in building a new observatory, Dutcher was tasked with transporting the group’s delicate handiwork: hexagonal stacks of silicon and copper, each housing about 2,000 ultra-sensitive detectors, worth about $100,000 apiece, that sit within telescopes and can detect faint light signals from deep space.

After briefly explaining all this to slightly confused TSA officers, Dutcher was allowed to proceed with his precious cargo in hand, which he carefully stowed under the seat in front of him once onboard.

This was one of the more minor challenges of the Simons Observatory, an ambitious project that consists of four next-generation telescopes situated on a barren plateau 17,000 feet above sea level in northern Chile’s Atacama Desert. The area’s skies are free of the moisture that can interfere with observations of outer space, making it one of the only places on Earth suitable for imaging the oldest observable light in the universe, the cosmic microwave background, or CMB. The Simons Observatory aims to detect this primordial light with unprecedented sensitivity to arrive at fundamental insights about how the universe was formed and has evolved.

“A huge amount of information about our universe, and our standard cosmological model that we have now, has come from doing studies of the CMB radiation,” said Staggs, Princeton’s Henry DeWolf Smyth Professor of Physics and a member of the Simons Observatory Executive Board. “We’ll be able to do even better with the Simons Observatory.”

Staggs juggles several leadership roles within the observatory, which is primarily supported by the Simons Foundation — a private organization that supports scientific research — and is a collaboration among the University of Pennsylvania, Princeton University, University of California campuses at San Diego and Berkeley, University of Chicago, Lawrence Berkeley National Laboratory, and institutions around the world. As one of the two co-directors of the Simons Observatory (the other is Mark Devlin, a former Princeton postdoctoral fellow who is now a professor at the University of Pennsylvania), Staggs is charged with overseeing the research efforts of the project’s more than 300 collaborators working from labs across the globe. And over the past 25 years, Staggs has mentored the next generation of cosmologists — some who themselves have assumed leadership roles within the Simons Observatory.

“The thing about it that’s incredibly interesting,” said Staggs of her research, “is we’re just asking a ridiculously bold type of question, which is: what are the large scale structure, dynamics and contents of the universe?” Staggs is optimistic that the Simons Observatory’s detection of light signals from the birth of the universe will provide answers. “The goal is to explain the evolution of the universe over 14 billion years into the thing it is today by making measurements of this primordial signal,” said Staggs. When asked if this research is worthwhile despite its challenges, Staggs was unequivocal: “Heck yeah, how could we stop now?”

Strange background

Though light travels a speedy 186,000 miles per second through space, the stars and galaxies we can see in the night sky are distant enough from Earth that the light they emit takes time to reach us. Whenever we look at a faraway light source, we’re therefore, in a sense, looking backwards in time. Light from our sun, for example, travels through space for eight minutes before reaching Earth, so the sun always appears to us as it was eight minutes ago. And because the speed of light is constant throughout the universe, the same idea applies to the cosmic microwave background — on a vastly larger scale.

Unlike sunlight, the CMB is invisible to the human eye; the “microwave” in its name refers to its wavelength, which is outside the narrow range of visible light (microwave ovens use a similar wavelength). And rather than originate from our solar system, the CMB comes from the deepest part of space: it travels for 13.8 billion years, since nearly the beginning of the universe, before reaching Earth.

Some cosmologists refer to the CMB as the “baby picture” of the universe because it’s like a snapshot of what the universe looked like in its earliest days. According to the prevailing theory, all matter, light and space sprang into existence in an event known as the Big Bang, in which a single point of infinite density began spreading and cooling into the universe we know today. By about 380,000 years into its existence, the universe cooled enough to allow light to propagate freely into the expanding void. This light is the cosmic microwave background, and it is considered “background” because it now fills all the empty space between galaxies in the universe.

Staggs and her colleagues wouldn’t need to build state-of-the-art telescopes in the Chilean desert if they were just trying to view the CMB. In fact, it was discovered accidentally in 1964 in Holmdel, New Jersey — roughly 40 miles from Princeton — by two researchers, Robert Wilson and Arno Penzias, both at Bell Laboratories, who kept receiving a strange background signal from their antenna. The two later received the Nobel Prize for their discovery.

The Simons Observatory, however, will capture the cosmic microwave background radiation in incredible detail, tracking minute temperature fluctuations of a few millionths of a degree Celsius across different parts of the sky. The intensity and location of these ever-so-slightly hot and cold spots encode information about the age of the universe and how the hot, dense conditions present after the Big Bang evolved into today’s galaxies, a relationship pioneered by another Princeton physicist, James Peebles, the Albert Einstein Professor of Science, Emeritus, who won the 2019 Nobel Prize in Physics. Peebles was also one of four Princeton astrophysicists who in 1964 published a companion paper to the work of Wilson and Penzias to explain the origins of the mysterious signal.

As light travels through spacetime, the gravitational pull of galaxies and other massive objects causes light to bend as if it were traveling through a lens. This gravitational lensing, predicted by Einstein’s theory of general relativity, can reveal the locations of dark matter — a substance that makes up about 85% of all matter in the universe but is invisible because it does not interact with light. As the CMB light travels, its path is bent by the gravitational pull of dark matter as well as visible galaxies and galaxy clusters. Tracking this path can reveal how the universe evolved over time, where it was “lumpy” with clusters of galaxies and dark matter, and how fast the universe expanded.

Mapping the CMB at this resolution is the job of the Large Aperture Telescope (LAT), one of the four telescopes in construction at the Simons Observatory. The LAT boasts two 20-foot-in-diameter mirrors that reflect light from space into the telescope’s camera, hitting the detector modules assembled by the Staggs group. Each module contains nearly 2,000 detectors that transmit the signal from CMB light. By taking advantage of a technique called multiplexing, thousands of detectors can be read out using only a handful of wires.

The efforts and expertise of many different groups were required to design and test the LAT. A company from Germany built the LAT’s outer structure and mirrors, while a team from the University of Pennsylvania designed the camera and the cryogenic chamber that keeps the detectors close to absolute zero, the only temperature at which they can operate. Detector components were fabricated at sites including the National Institute of Standards and Technology in Boulder, Colorado, and were tested at the University of Chicago or Cornell University before Dutcher escorted them to Chile.

The LAT builds on the discoveries of the Atacama Cosmology Telescope (ACT), which is located on the same Chilean mountaintop and recently finished collecting data. The two telescopes operate similarly, but the LAT will contain 10 times as many detectors as ACT did, guaranteeing that it will produce a higher-fidelity CMB image. “More detectors means you can just see the detail in the microwave background light better, which then leads to science we couldn’t do with ACT,” said Jo Dunkley, professor of physics and astrophysical sciences and — along with Lyman Page, Princeton’s James S. McDonnell Distinguished University Professor in Physics and a long-time CMB researcher — a member of the Simons Observatory Executive Board.

Finding inflation

Dunkley and Staggs are part of a multi-generational Princeton tradition of pushing CMB research to new frontiers. Staggs, who earned her Ph.D. at Princeton, owes her involvement in the field to one of the early pioneers, the late David Wilkinson. She initially wanted to work in nuclear physics, but the faculty member with whom she hoped to work soon left for another research institution. Staggs thought about following him, but Wilkinson personally appealed to her to stay.

The chance to explore the CMB with one of its foremost experts was too good to pass up. “Wilkinson just sold me on it,” Staggs recalled. “He was an extremely charismatic person with a great understanding of physics and cosmology.” A NASA mission that launched in 2001 and created a full-sky map of the CMB is called the Wilkinson Microwave Anisotropy Probe (WMAP) in his honor.

Princeton has also been closely associated with an important cosmology theory called inflation that stands to be validated by the Simons Observatory. To solve cosmological observations that can’t be explained by the Big Bang, scientists have proposed that in the first trillion-trillion-trillionth of a second after the Big Bang, the universe expanded exponentially — it inflated — before settling into a much slower expansion rate. Although the theory has many supporters, no hard evidence for inflation exists.

The small aperture telescopes (SATs) at the Simons Observatory may change this. These three telescopes, whose lenses measure only 20 inches in diameter, pale in size to the LAT, but their design could allow them to detect gravitational waves from the first moments of the universe: these primordial gravitational waves should exist if inflation truly happened. Light is an electromagnetic wave, and as it travels it creates an electric field in a defined direction, or polarization. The CMB light carries a distinctive pattern of polarization that can be measured by the SATs. Gravitational waves twist the patterns in certain ways, and finding evidence of the gravitational wave signature pattern would be really exciting, Staggs said.

In the decades since Wilkinson offered to advise Staggs, she in turn has advised dozens of students at all levels. Claire Lessler completed her undergraduate senior thesis with Staggs at Princeton in 2022 and is now a graduate student at the University of Chicago, where she works with Professor Jeffrey McMahon (who himself earned his Ph.D. from Princeton in 2006, under Staggs’s mentorship). Lessler, who fabricated a component of the detector module for her thesis, said that even with Staggs’s responsibilities leading the Simons Observatory, she was a diligent undergraduate thesis adviser and a supportive mentor.

“Suzanne went and checked every single one of my citations,” recalled Lessler, who was also encouraged by Staggs to pursue graduate studies. “I went to Suzanne and asked, ‘Do you think I could go to grad school?’ and she was very, very supportive. She was also excited that I was a woman and going to grad school —that was a big deal to her,” said Lessler.

Widening the pipeline

It’ll be a number of years before the Simons Observatory yields answers to the cosmological mysteries it was built to address. Though it is set to make its first scientific observations in May 2024, data collection and analysis will together take several years, and the team expects to have published substantial results by the end of the decade.

Before that happens, the Simons Observatory will go through a major upgrade, thanks to a recent $53 million grant, led by Mark Devlin, from the National Science Foundation.

It will allow the Princeton team to assemble more detector modules for the LAT, rapidly accelerate the data pipeline, and fund an array of solar panels to generate 70%of the observatory’s power, which previously relied exclusively on diesel generators.

A lot of scientists are motivated by the potential societal benefits of their research, Staggs said. “In different areas of science, you might become interested because it’s going to lead to various inventions that in the future will change things. That’s not really true, to my mind, with our area,” she said. Instead, she builds telescopes to achieve something more profound: to answer fundamental questions about the nature of the universe. “It just still seems crazy to me that we can even pursue them,” Staggs said of these questions, “and so very exciting to keep doing so.