The height of advanced technology: SPIDER's high-altitude balloon flight collects new data on the CMB

Illinois physicists led an ambitious flight to the furthest reaches of the atmosphere. From that perch, they grabbed data that will reveal new details on the cosmic microwave background radiation, as well as the galactic dust that conceals it.

The air gives the stars their romantic twinkle. But it also gives physicists and astronomers headaches, obscuring their view of distant objects and ancient light.

Space-based telescopes are one way of curing those headaches, putting scientists’ instruments beyond the atmosphere and its interference. Such telescopes are expensive, decades-long undertakings, however. The James Webb Space Telescope, for example, cost more than $10 billion by the time it launched in 2021—about 25 years after initial design studies on the project began.

High-altitude balloon flights offer a much quicker and less costly alternative. NASA offers rides—competitively, through a process similar to getting research funding from the agency—on 10 to 15 flights each year from sites around the world.

A team took advantage of one of those flights in late December 2022 to loft a floating telescope called SPIDER (Suborbital Polarimeter for Inflation, Dust, and the Epoch of Reionization). Led by Illinois Physics Professor Jeff Filippini, SPIDER captured three frequencies of light that will help researchers better understand the cosmic microwave background (CMB), pervasive radiation dating back to the very beginnings of the universe.

“These balloons get you an observing environment that is very similar to that of space. Not quite above the entire atmosphere—but above more than 99 percent. For microwaves, which we’re observing with SPIDER, it’s very good,” Filippini says.

Primordial gravitational waves

As Elle Shaw, a PhD student in Filippini’s lab, describes it, the CMB “is the oldest light we can observe. The glow emitted back when the entire universe was an opaque, hot plasma—sort of like the surface of the Sun, but everywhere.” The CMB provides a unique “baby picture” of our universe, revealing information about the universe’s composition and behavior billions of years before the birth of anything we see through our telescopes today.

Elle shaw wrote about her experience testing spider everywhere from Champaign to Texas to the South Pole. Read her blog post here.

Beyond the atmosphere, another natural phenomenon complicates observations of the CMB. It’s galactic dust, and the universe is full of it.

Think of that interstellar dust as soot cast off from dying stars. Large molecules containing carbon, water, and ice, some silicon and iron atoms mixed in—all of it slowly coalescing into new stars. Like the motes floating in sunbeams coming through your window, they all dance within and between galaxies. And they all emit and reflect light that contaminates measurements of the CMB.

“We’re embedded in a galaxy, this big whirlpool of gas and dust and stars. In any direction we look, there are wisps of gas and dust and stars. If you make sensitive enough measurements, those get in the way,” Filippini says.

Most notably, galactic dust prevents the possibility of physicists observing primordial gravitational waves.

Many gravitational waves, including those that have been detected to date, are created by catastrophic astronomical events like supermassive black holes colliding. Physicists hypothesize another category of gravitational waves, ripples in spacetime created by the very rapid inflation of the universe in its earliest moments. No instrument can measure those very old ripples themselves, and no particle accelerator is powerful enough to replicate that moment. But researchers believe they should be able to see the impact the waves have on the CMB.

“That epoch of cosmic inflation created the over- and under-densities that grow into galaxy clusters. It also imprinted the universe with quantum noise on very large scales,” Filippini explains. “We see this noise as tiny hot and cold spots in the CMB. Over- and under-densities that have since grown into galaxies.”

“But inflation should also have filled the universe with a hum of gravitational waves. You should be able to detect that hum as a unique—but incredibly faint!—pattern in the polarization of the CMB’s light. If we can detect that pattern, it would provide unique information about the early universe that we have no other way to access.”

SPIDER’s third eye

Parts of SPIDER were designed, built, and tested at institutions around the country, including Case Western Reserve University, Princeton University, and Washington University in St. Louis. Filippini’s team at the University of Illinois Urbana-Champaign focused on a set of telescopes to observe light at 280 gigahertz.

Those 280-gigahertz telescopes focus light onto arrays of superconducting detectors built by colleagues at the National Institute of Standards and Technology and cooled to just 0.3 degrees above absolute zero. They are particularly important to getting around that pesky galactic dust, because that is where the light emitted by galactic dust is particularly bright. Other telescopes at 90 and 150 gigahertz collect data that is central to understanding the CMB itself.

Jeff filippini described working on the gondola that held the SPIDER instrument and the cryostat that keeps it cold. Read his blog post here.

The Earth’s atmosphere clouds observations at 280 GHz from the ground. But, because SPIDER and its telescopes operate in the exceptional observing environment of the stratosphere, researchers expect a much clearer set of measurements. The team spent years building these instruments and fully characterizing how they would perform. They tested and retested features like the telescope’s optical efficiency (how much of the light that shines in the front of the telescope ends up hitting the detector at the back), as well as internal loading (the glow on the detectors from the instrument itself).

“You have to ensure that the instruments perform to their expected level, so they can do the thing you want them to do at altitude,” says Filippini. “But there’s also an important engineering feedback loop. If your instrument isn’t performing the way you expect it to, you want to figure out why and improve it.”

“Nothing works right the first time. Maybe not even the 21st time. You need to mess with things and fine tune all the little things you discover along the way,” he adds.

Touchdown

SPIDER returned to Earth in early January 2023, after a 16-day meander 20 miles above Antarctica.

It touched down near (by local standards) Hercules Dome, a scientific camp where researchers drill ice cores to better understand climate change. Hercules Dome sits nearly 1,000 miles from McMurdo station where the balloon lifted off in December. Recovery teams arrived a couple of weeks later, grabbing the hard drives that hold SPIDER’s observations. The drives are now making their way back to North America, and scientists around the world will study the results for years to come.

Once the 280 gigahertz data is in hand, Filippini and the rest of the SPIDER team plan to create what they call a “dust map” to better understand the dust’s glow. Crucially, the map will also allow them to subtract the dust’s glow from their broader readings and see the CMB more clearly.

“The pattern of the galactic dust can look like the pattern of the CMB, so we have to understand it very well, so that we don’t confuse it with the signal that we’re looking for. The maps we make will be used to inform models of galactic dust, which will benefit both our own analysis and that of other groups observing the CMB,” Filippini says, because they’ll be able to get rid of light contamination at 280 gigahertz.

“Other people want this map too. It’s useful for the whole field.”

These instruments and the balloon flights that carry them are also an investment in the future. They test the technology that will go into other experiments down the road and train the people who will do those experiments.

“You operate under the same constraints as space. You have to be very light. You have very low power requirements. You need to be very reliable. But you’re doing all of this on a much cheaper budget and shorter development cycle. You’re proving out technologies for NASA before they get put on a billion-dollar telescope,” Filippini said.

Meanwhile, “SPIDER and other balloon payloads are built by students and postdocs, who learn to put together the instruments under these constraints. Whether they end up working at NASA, at another space agency, or in industry, the experience is really unique,” Filippini asserts. “The people it produces. The science it produces. The technology it produces. It’s all really powerful.”

 

The SPIDER experiment is funded in the U.S. by NASA’s Science Mission Directorate under Grant Nos. NNX07AL64G, NNX12AE95G, NNX17AC55G, and 80NSSC21K1986. Logistical support for Antarctic deployment and operations is provided through the U.S. Antarctic Program, part of the National Science Foundation’s Office of Polar Programs under Grant No. PLR-1043515.  Additional funding in Canada is provided by the Natural Sciences and Engineering Research Council and the Canadian Space Agency. The findings presented are those of the researchers and not necessarily those of the funding agencies.

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