Taking It All In: The Depth and Breadth of Survey Science

Starting in the 1980s, newly automated telescopes and data repositories enabled an approach dubbed survey science that began soaking up vast swathes of sky, concomitantly capturing stars, galaxies, and other phenomena by the millions. Thus was born the ability to study the universe in a statistically powerful and insightful manner—an approach that continues through today in ever grander scope.

"Ultimately by taking observations of large areas of the astronomical sky, you can potentially learn many different things about the universe," says Bradford Benson, an associate professor of astronomy and astrophysics at the University of Chicago and a senior member of the Kavli Institute for Cosmological Physics (KICP).

Researchers at UChicago played key roles in one of the first major efforts in this vein, the Sloan Digital Sky Survey. SDSS collected its first images in 1998. Upon the founding of the KICP at UChicago a few years later in 2001, these scientists continued with SDSS observations and analysis.

It could accordingly be said that survey science was baked into the KICP from the very beginning, and indeed has remained a primary science theme at the institute since.

Benson himself is carrying on this survey science tradition and has witnessed firsthand its tremendous growth and future potential. He primarily works on gathering measurements of the cosmic microwave background, or CMB. This relic radiation of the Big Bang, emitted nearly 14 billion years ago, offers an incomparable snapshot of what the universe looked like in its primordial state. In this way, the CMB offers a critical window into when a mysterious substance called dark matter formed the structure upon which galaxies made of ordinary matter later developed. The CMB also reveals how another mysterious entity called dark energy, and which is now accelerating the universe's expansion, may have behaved from the get-go.

As Benson explains, when he started in this research area as a Ph.D. student a bit over 20 years ago, the amount of data was glaringly sparse vis-à-vis to what researchers can work with nowadays. "The measurements I was doing at the time were definitely considered cutting edge, but comparing those measurements to what we can do today, is nearly unrecognizable," says Benson.

For the last several years, Benson has conducted survey science with a microwave camera he helped personally install on the 10-meter diameter South Pole Telescope (SPT) in the heart of Antarctica. That camera—called SPT-3G, per it being the third-generation camera of its type—is orders of magnitude more sensitive to microwave CMB light than predecessors.

"SPT-3G can make the measurements I did for my Ph.D. thesis in basically an hour," says Benson. "In that seemingly short period of years, it's just remarkable to me how much better the measurements are, and how much more we've learned about the physics of the universe. It definitely makes me excited for the next 20 years, and how much more we might learn."

Bigger and better has been and will continue to be the modus operandi in survey science. After SDSS, KICP researchers spearheaded the next major astronomical optical survey, known as the Dark Energy Survey (DES). The researchers conceived the survey strategy, designed and built the massive camera at its heart, DECam, and still continue to parse the data DECam hauled in during a six-year observing campaign that started in 2012.

In total, DES observed approximately 300 million galaxies. By gathering up huge sample sizes such as these for analysis, astronomers have been able to apply "big data" techniques to drill down into the fundamental nature of processes, such as galaxy formation, that are propelled by underlying physics involving matter and dark matter interactions. This approach can also trace the influence of dark energy over eons of cosmic history, along with helping answer a host of other astrophysical and cosmological questions.

"It all comes down to statistics," says Benson. "One major goal of DES is to effectively use the galaxies found in the survey as test particles that probe the structure of the universe, and whose distribution we should be able to statistically characterize with only a handful of cosmological parameters, for example like the density of dark energy and dark matter."

Benson explains that "big data techniques can help us see 'patterns' in the data, that might be difficult to see visually, or perhaps reveal something unexpected, that might be indicative of some systematic error in the measurement, or maybe more excitingly signatures of new physics that we weren't expecting."

Befitting KICP's legacy in survey fields, its members, alongside members of other Kavli Institutes and fellow scientists worldwide, are deeply involved in the next major optical survey. Called the Legacy Survey of Space and Time, LSST will be conducted by the Vera C. Rubin Observatory over the next decade. LSST will scan the entire Southern Hemisphere sky every few nights, bringing in a gobsmacking amount of data encompassing on the order of 10 billion galaxies.

In Benson's specialty area of microwave astronomy, the survey science successor to SPT is also in the works and with heavy KICP involvement. Known as CMB-S4, the experiment involves placing half a million super-cooled detectors at 12 new telescopes at the South Pole and Chile.

CMB-S4 builds on SPT's big data, by making much more sensitive measurements from the South Pole, while compiling additional CMB statistics across 70% of the sky from Chile.

Given the ever-growing power of survey science to take it all in, so to speak, researchers expect significant advances with this next generation of instrumentation.

"I think the data sets from these upcoming experiments over the next several years are going to be very rich, enabling science across a remarkably broad range of astrophysics and cosmology, and will certainly lead to both new discoveries and reveal new surprises," says Benson.

Overall, belonging to KICP has provided Benson with a special connection to survey science projects of the past, present, and future.

"I feel very fortunate to be at a place like the KICP, with so many amazing colleagues trying to answer some of these big questions," says Benson. "I'm just very excited to see what comes next and learn about the physics of the universe."