Big Bang Vision: Exploring the Cosmic Microwave Background at Simons Observatory

What if you could look back in time—not just decades or centuries, but all the way to the beginning of the universe? That’s exactly what scientists at the Simons Observatory are doing by studying the Cosmic Microwave Background (CMB)—the faint afterglow of the Big Bang. In this case story, we’ll explore the science, instruments, and innovations behind the observatory—from its boundary-pushing research deepening our understanding of the universe, to the extreme engineering required to build its unique telescopes.

Simons Observatory in a Nutshell

Simons Observatory is a powerful new experimental cosmology facility – a highly ambitious project bringing together more than 350 scientists, engineers, and technologists from over 50 institutions worldwide. Their mission: to make the most precise measurements yet of the early universe and bring us closer to answering some of the biggest questions in cosmology—how the universe began, what it’s made of, and how it evolved into what we see today.

Located high in Chile’s Atacama Desert at 5,190 meters (17,000 feet) near the summit of Cerro Toco, the observatory sits in one of the driest and most stable climates on Earth—making it one of the best places on Earth to observe the cosmos.

The observatory’s instrumentation consists of a 6-meter Large Aperture Telescope (LAT) and three half-meter Small Aperture Telescopes (SATs), all equipped with cryogenically cooled detectors to capture the invisible CMB signals. Two of the SATs began observing in fall of 2023, followed by the third in August 2024. The LAT saw first light in March 2025.

Together, these cutting-edge telescopes are mapping the sky at millimeter wavelengths with unprecedented sensitivity. These observations are designed to reveal subtle signals embedded in the CMB that provide insight into dark matter, neutrinos, cosmic acceleration, and the formation of galaxies.

To understand the science and engineering driving the Simons Observatory (SO), we spoke with two researchers who have been deeply involved in building it.

Dr. Nicholas Galitzki, Assistant Professor of Physics at the University of Texas at Austin, is an experimental cosmologist who studies the early universe by searching for primordial gravitational waves—faint signals from the universe’s first moments—hidden in the polarization of the CMB . His work spans everything from designing cryogenic cameras for millimeter-wave astronomy to analyzing data from on-sky observations – astronomical observations made from Earth.

At SO, Nicholas has played a key role in developing the Small Aperture Telescopes (SATs), which are designed to detect the extremely subtle polarization patterns in the CMB. “What attracted me to the project was the ability to start from scratch,” he says. “It was like starting with a blank slate and building the best instrument you can. It’s exciting—we’re working in an open design space, free to be as ambitious as possible! ”

We also spoke with Dr. Jack Orlowski-Scherer, Staff Scientist at the University of Pennsylvania. During his PhD research, Jack led the mechanical and thermal simulations that helped shape SO’s Large Aperture Telescope Receiver (LATR). He also worked closely on the predecessor of LAT – The Atacama Cosmology Telescope (ACT). His research focuses on galaxy clusters, which are the largest structures in the Universe.

Jack explains, “I work with the cosmic microwave background, but more indirectly. We are able to detect galaxy clusters using the CMB, because they’re backlit by this ancient light. As the CMB passes through a cluster, it gets subtly altered, revealing the cluster’s size and properties. And because this effect doesn’t fade with distance like light does, we can spot clusters from very far away. It’s a powerful tool for studying the Universe.”

What is the Cosmic Microwave Background?

If you point a sensitive radio antenna anywhere in the sky, you’ll detect a faint signal that you just can’t get rid of. It’s everywhere. That signal is a relic of the Big Bang – and it’s packed with clues about the universe’s origins, structure, and evolution. We know it as the Cosmic Microwave Background (CMB).

Our observable universe is surrounded by the CMB. Nicholas Galitzki explains: “The cosmic microwave background is the oldest light we can look at, and the farthest back in the universe’s history we can see. If we want to understand how the universe came to be — what set everything in motion, from galaxies to planets to us — looking at the earliest light and squeezing as much information from it as possible is an opportunity we can’t pass on.”

The CMB is a kind of ‘baby picture’ of the universe when it was only about 375,000 years old – very young compared to its 13.7-billion-year age – long before stars or galaxies had formed. At that time, the universe was a hot, dense soup of particles with too much energy to form neutral atoms. Photons constantly scattered off free electrons, preventing light from traveling far and making the universe opaque.

As the universe expanded and cooled, conditions changed: electrons combined with nuclei to form the first atoms, and the once-opaque fog began to clear. This epoch, known as recombination, marked the first time light could travel freely across space.

The CMB is essentially a snapshot of that transition, and it’s also known as the “surface of last scattering”—the last moment when photons scattered off matter before streaming freely through the universe. So, CMB is the light of the primordial plasma that gave way to the universe we live in today.

That 13.7-billion-year old light, the CMB, is still reaching us now. Over billions of years, the universe’s expansion has stretched that light into longer wavelengths, making it appear now only in the microwave portion of the electromagnetic spectrum. Invisible to our eyes, but detectable with our telescopes.

The CMB was first discovered by accident in 1964 by radio astronomers Arno Penzias and Robert Wilson. What they initially thought was just noise from their antenna turned out to be a faint, uniform glow coming from every direction in the sky—a signal that became landmark evidence for the Big Bang.

Since then, research on the CMB has uncovered an astonishing variety of previously invisible features of our cosmos. A series of space missions—including NASA’s COBE and WMAP, and the European Space Agency’s Planck satellite—have mapped this ancient light in increasing detail. Planck, in particular, delivered the most detailed images yet of the tiny temperature fluctuations in the CMB. These minute variations (less than 0.01%) reveal the early seeds of galaxies and the large-scale structure of the universe.

Simons Observatory and the Next Era of CMB Science

The Simons Observatory is building on the legacy of past CMB missions—mapping the cosmic microwave background in unprecedented detail.

One of its main goals is to study the universe in its earliest fraction of a second. Scientists believe the universe went through a rapid expansion called inflation during this time—stretching faster than the speed of light. While this theory explains much about the structure of the universe, it hasn’t been confirmed. The Simons Observatory’s Small Aperture Telescopes (SATs) will search for subtle signatures of this event in the CMB, including patterns in its polarization and traces of gravitational waves.

“Overall, Simons Observatory is pushing into greater sensitivity,” Nicholas explains. “Satellites like Planck had smaller mirrors and weren’t optimized for measuring the polarization of the CMB. That’s the really compelling science we’re going after.”

Another major goal is to find hints of new particles that may have existed when the CMB formed. During this energetic period, particles called light relativistic particles may have played a key role—ones we haven’t detected with experiments on Earth. The observatory’s Large Aperture Telescope (LAT) may yet uncover signs of them in the CMB’s detailed patterns.

The observatory will also improve measurements of known cosmic properties, such as how fast the universe is expanding. It will produce sharper CMB maps and better data on gravitational lensing—the way that massive objects bend light—and Sunyaev–Zel’dovich effects, which help us understand galaxy clusters and the large-scale structure of the universe.

“Galaxy clusters play a key role in improving our understanding of the universe,” Jack explains, “because we can see clusters from different points in cosmic history. The number and size of these clusters depend on the underlying structure of the universe. By studying them, we can piece together how the cosmos has evolved—and even test whether the laws of physics have stayed the same over time.”

He also points out a major puzzle: measurements of the universe’s expansion rate from the early CMB don’t match present-day observations. “For a long time, it was assumed that the Hubble constant stayed the same, but it seems to have shifted along the way—but when, and why? Galaxy clusters might help us find out. They offer a bridge of information between the early universe and what we see now.”

Capturing the Big Picture

To collect their targeted range of data, the Simons Observatory uses two types of telescopes. “The reason we have both the LAT and the SATs,” Jack says, “is because each telescope is sensitive to different angular scales on the sky. The LAT focuses on details, and the SATs capture the large-scale patterns.”

It’s nearly impossible to do both with one instrument – the challenge comes down to a trade-off between resolution and field of view. “To see small details, you need a large dish. But the bigger the dish, the narrower your view becomes—just like binoculars. You can zoom in to see fine detail, but you lose sight of the bigger picture.” Jack says.

At Simons Observatory, the LAT is the high-resolution instrument, designed to capture small-scale features in the CMB. “It has a resolution of about 1 arcminute, which means it can distinguish details just 1/60th of a degree apart,” Jack says. “To visualize that: if you hold your pinky at arm’s length and look at it, one arcminute is roughly a quarter of the width of your pinky.”

The LAT has a relatively wide field of view for its size—about seven degrees across—but it’s still limited in the largest area of sky it can view at once.

In contrast, the SATs have a much lower resolution—about 30 arcminutes, or 30 times less detailed than the LAT—but they make up for it with a significantly wider field of view, covering roughly 35 degrees . This allows them to detect signals on the largest angular scales in the sky, which are beyond the reach of the LAT.

This combination is crucial because the CMB contains meaningful structure at every scale, with each scale revealing different aspects of the universe and the physics that shaped it.

Inside the SAT Cryogenic System

Ultra-low temperatures are essential to the Simons Observatory’s ability to precisely measure the cosmic microwave background (CMB). “The CMB is effectively at 3 kelvin. Anything that’s above 3 kelvin is hotter than the CMB, and it’s a signal we don’t want,” explains Nicholas. “If you think of a camera lens, you have baffles to block stray light. Similarly, we want any stray light—the light that’s not coming from the CMB—to end up somewhere cold.”

That’s why it’s not just the detectors that need to be cold—the entire optics tube, through which CMB light travels, must also be cooled to prevent warmer background signals from interfering. This presents a major engineering challenge, simply because of the sheer physical size involved. Instead of cooling a small, contained area as in many lab systems, the cold must be maintained across large telescope components. Nicholas explains how this is possible: “Similar to the way we wear layers to stay warm, we use multiple thermal ‘shells’ to block out heat and keep the detectors cold.”

The SATs have four layers of cold. The first two—at 40 Kelvin and 4 Kelvin—are cooled using an individual Cryomech PT420-RM pulse tube cryocooler and the pulse tube cryocooler of the dilution refrigerator (also PT420-RM). “We love pulse tubes because they have no moving parts, so they’re reliable and low maintenance. Plus, they give us a lot of cooling power at just the right temperatures,” Nicholas says.

To get even colder, the SATs use a Bluefors SD400 dilution refrigerator, which takes the telescope down two more stages: about 1 Kelvin for the optics, and below 100 millikelvin for the superconducting detectors.

There are seven detector arrays in each of the small aperture telescopes, arranged in a hexagonal shape. Combined, the 7 arrays have a total of about 12 000 detectors inside them, bringing the total number of detectors across the three small-aperture telescopes to over 30,000.

The superconducting detectors are essentially used as exquisitely sensitive thermometers. “When a 3 Kelvin photon from the CMB hits a detector, it warms it up just slightly. That small temperature change pushes the superconductor slightly out of its superconducting state. This causes a sharp change in its electrical resistance, which we are then able to read out,” Nicholas explains.

But while the detectors are carefully shielded and cooled, the telescope still needs to do its job: let light in from the sky. And that presents a unique design challenge. “Unlike a quantum computer, which stays completely sealed, our system needs to let light in,” Nicholas says. “So, after designing this super carefully insulated cryogenic system, we have to cut a big hole in it! To handle this, we use special filters that only let the light that we want through, while blocking out everything else that would heat up the system,” Nicholas explains.

On top of that, all of this must work in one of the most extreme scientific environments on Earth. “That’s another unusual and tricky challenge we’ve had to tackle—dealing with snow, wind, and especially the intense UV radiation from the sun at high altitude,” Nicholas says. “Most dilution refrigerators are used indoors—in basement labs or clean rooms—where everything is carefully controlled. But in our case, we’re operating up on a mountaintop in Chile, pretty much outside.”

To protect the system, the team built environmental shields around it. Still, the core equipment must function reliably and survive for years under harsh conditions. “We’ve had to be really careful about what materials we use and how everything is built, so it can stand up to that for at least five years—and hopefully ten or more,” Nicholas says.

A High-Tech Spin on Cosmic Exploration

The signal from the cosmic microwave background (CMB) is incredibly faint—and hidden beneath the much brighter, constantly shifting emissions from Earth’s atmosphere. What makes the CMB stand out is its subtle polarization, a pattern imprinted on the light as it scattered off particles in the early universe.

Luckily, Earth’s atmosphere is mostly unpolarized. It emits equally in all directions, and that contrast can be used to distinguish the two signals.

To separate the polarized CMB from the unpolarized atmospheric noise, the SATs modulate the incoming polarization signal. Inside the cryostat, a stack of sapphire plates rotates at a constant rate, working as a broadband polarization modulator. This rotating stack—the cryogenic half-wave plate—also rotates the incoming polarization signal before it reaches the detectors.

Since the detectors are tuned to this rotation frequency, they can “lock in” to the specific pattern of the CMB signal. Any signal that doesn’t match that frequency, such as noise from the atmosphere, is effectively filtered out.

“The technological aspect of implementing this is wild, because the optical device we’re rotating weighs over 30 kilos , it’s in a vacuum and cooled to 40 kelvin. And rotating at 2 Hertz. So basically, something like a coffee tabletop spinning around two times a second. Inside a vacuum chamber. At cryogenic temperatures,“ Nicholas laughs.

How is this possible? The weight of the half-wave plate is supported by a superconducting magnetic bearing, taking advantage of the way magnets levitate above superconductors. “For our device, we cool down a superconductor until it gets ‘trapped’ in a magnetic field, creating an incredibly low-friction bearing that lets the device spin smoothly. But it’s straight out of science fiction because it’s not supported by anything, it’s levitating,” Nicholas says.

“This is something we often demonstrate in physics classes, where we levitate and spin a cooled superconductor on a magnetic track. The coolest part is that we took this classroom demo and found a practical application for it,” he adds.

LATR: From Sky to Signal

To record the incoming CMB signal, the LAT uses a custom-built camera called the Large Aperture Telescope Receiver (LATR). At 2.4 meters in diameter and 2 meters long, the LATR is the largest receiver of its kind. Structurally, the LATR is a vacuum chamber with 13 windows cut in the front to allow light to reach the detectors after reflecting off the telescope’s mirrors.

The LATR’s high sensitivity is produced by the sheer number of detectors it holds—nearly 60,000 in total. “The detector technology in our field was basically perfected about 15 years ago,” Jack says. “These detectors already catch every CMB photon that hits them. So the only way to improve is to go bigger—add more detectors. But more detectors take up space, which means larger telescopes too.”

Handling data from that many detectors required a major leap in technology. “I’d say the biggest technological upgrade is our readout system. With the previous telescope I worked on, ACT, we basically ran a pair of wires to every detector—about 5,000 wires total. Painful, but doable. But in LATR, with nearly 60,000 detectors, that was a non-starter,” Jack explains.

To solve this, the Simons Observatory telescopes switched to a technique called microwave multiplexing. “Instead of wiring each detector, we use signals that sweep through and pick up data from each one based on its unique frequency. That way, we can read out about 1,000 detectors on a single coaxial wire. It’s been a huge improvement,” Jack says.

Inside the LATR Cryogenic System

The cryogenic system in the Large Aperture Telescope Receiver (LATR) builds on the design used in the SATs but at a larger scale and with some key additions.

Like the SATs, the LATR has cooling stages at 40 kelvin and 4 kelvin, powered by Cryomech PT420 cryocoolers, along with the colder stages at 1 kelvin and 100 millikelvin, maintained by a Bluefors dilution refrigerator. But because the LATR is bigger and demands more cooling power, the team uses an LD400 dilution refrigerator and twice the number of cryocoolers—running two PT420s and two PT90s. The system is also designed with room to expand, so more cryocoolers can be added later if needed.

One major addition is an extra 80 kelvin stage, which is cooled by the two PT90 cryocoolers. This stage is right behind the window, at the front of the receiver. “To keep the cryostat sealed while still allowing CMB light in, we use special plastics that are transparent to millimeter waves but block most optical light. But even a small amount of 300 K radiation getting in is a big problem. So we spend a lot of effort filtering out everything outside our target range, and the 80 K stage plays an important role in that,” Jack explains.

Handling the Weight: A Fiberglass Solution

One of the major engineering challenges in building the LATR cryostat was figuring out how to mechanically support weight inside the system. “To provide some numbers: we suspend a metric ton (1000 kg) at 4 kelvin, and we have to hold that weight without letting in too much conductive heat”, Jack explains. Using something like steel wasn’t an option; it’s strong, but it conducts heat too well, which would compromise the cryogenic system.

There was another complication: as the cryostat cools, its parts shrink at different rates. “Our cryostat is so big that when it cools, the 4 K stage shrinks by over a centimeter compared to the 300 K stage. That means any support material has to flex as the structure contracts, without breaking,” Jack says.

To solve this, the team turned to a special type of fiberglass that performs well at cryogenic temperatures. Jack explains: “We use G10 tabs—they’re thin in the direction they need to flex, but stiff where they need to hold weight. It’s a little unnerving because you can actually bend them by hand, but believe it or not, they hold up a ton of weight.”

The team ran extensive simulations to get the design just right, balancing flexibility, strength, and thermal performance. And it’s worked: “They’ve been in there since 2019, through 30 or 40 cooldowns, and they’ve been rock solid.”

Smooth Sailing with Bluefors: A New Era of Cryogenics

Along with Cryomech Pulse Tube Cryocoolers, the Simons Observatory relies on Bluefors dilution refrigerators. For the researchers, working with these Bluefors systems has made cryogenic operations far more reliable and user-friendly—especially compared to the older, wet dilution refrigerators they used in past projects.

Nicholas explains the difference: “Cooling down liquid-based systems used to be a week-long, nerve-wracking process with tons of steps and constant supervision. But with a modern commercial system like a Bluefors, it’s so much simpler—you go into the lab, check a few things, press a button, and by the next day, it’s cold. It makes our work so much easier. We can just order it, plug it in, and then focus on doing all our crazy custom modifications to make it work for a telescope in this really unique environment.”

It’s not just the ease of use; Bluefors has also supported the team all the way. “Bluefors has been eager to learn from us,” Nicholas adds. “They’re curious to see how their systems perform in such unusual conditions, and they’ve been really supportive in helping us troubleshoot and adapt to challenges as they come up.”

Jack had a similar experience. Having worked on the Atacama Cosmology Telescope (ACT), he remembers the old way all too well: “At ACT, we didn’t have a Bluefors, but an older system. It worked, but it was not user-friendly in the slightest. You really had to know what you were doing, or you’d get into trouble fast—like accidentally venting your helium if you made the wrong move. That’s why only specific people operated it. I was one of three DR experts allowed to touch it.”

The contrast with the new system was striking. “When we got the Bluefors system, it was a total game changer. The end-user experience is just worlds away from what we used to deal with. I actually joked it might put me out of a job because it was push-button simple—you click, and it cools. There are built-in safety measures too, so even if someone makes a mistake, there’s only so much trouble they can get into. We even let our remote observing volunteers monitor the system. That would’ve been unthinkable with the old setup.”

And the performance? Even better. “The cryogenic performance is out of this world. The promised performance is good, but our system overperforms.  The specs said we’d get 400 microwatts at 100 millikelvin, but we’re getting easily more like 500. And if we really drive the system, we could probably get 550,” Jack says. “And adapting it to our site’s conditions was easy—we just tweaked a few numbers in the scripts, and it worked smoothly.”

New Eyes on the Sky

After years of design, testing, preparation, and some COVID-19 delays, the Simons Observatory is up and observing. Nicholas recalls the most thrilling moment so far: “The most exciting part—and I think most telescope builders would agree—was the first light. We hit that milestone on October 10th, 2023, and it was huge. We’d been working on this since 2016, through countless design, build, and test cycles. As a physicist, while I love building the system, the real goal is to get data and learn about the universe. So when everything was finally in Chile, detectors cold, the telescope running, and we were collecting data — that was incredible.”

Since that landmark day, the team has been busy. “First light was just the beginning,” Nicholas says. “The next big milestone is getting our science results out. After about a year and a half of observing with the SATSs, we’re really getting to know the instrument; characterizing its performance and collecting a huge amount of data. Simons Observatory is a massive collaboration—around 300 people with all kinds of expertise—and it’s been rewarding to see the progress: turning those initial data sets into science-quality results.”

Nicholas is excited about the near future: “We’re getting close to sharing our first findings, and hopefully within the next year, we’ll see our first papers come out. That’ll be a key validation—showing not just what we’re capable of, but the science we can do. Then it’s back to work, observing for another five years or more to reach our target sensitivity. That will be the really big moment—when we can say that since 2016, when we set our science goals, we actually got there.”

While much of the anticipation is focused on the long-term cosmological goals, researchers are also looking forward to the surprises that might come along the way. For Jack, one of the most exciting prospects is exploring what’s known as transient science—studying cosmic events that occur on relatively short, human timescales.

“The cosmic microwave background is unchanging, but things like supernovae or flaring stars evolve quickly, sometimes over days or months. With our previous telescope, ACT, we started picking up some of these transients, including a couple of objects that didn’t match anything we know. That was really intriguing because it suggests there are phenomena out there that we haven’t yet identified,” Jack explains.

“With the Simons Observatory, we’re going to be much more sensitive and much better equipped to detect these kinds of events. We expect to see many more transients, possibly right from the start of observations. It’s a pretty new area in our field, and we honestly don’t know exactly what we’ll find—which makes it all the more exciting.”