Spotting the Universe’s oldest light from the Atacama Desert

May 2, 2024

Toby Satterthwaite. (Photo courtesy T. Satterthwaite.)

by Toby Satterthwaite

Three flights and about 26 hours from Stanford’s campus, on a cold, dry, and remote mountain high in the Atacama Desert, sits the Simons Observatory, a cosmic microwave background (CMB) experiment which will soon begin to map this ancient light with exquisite precision. 

The CMB is the oldest light in our Universe; it was emitted almost 14 billion years ago, just 400,000 years after the Big Bang (when the Universe was only 0.003% of its current age). For the first several hundred thousand years of the Universe’s history, matter was too hot and dense for light to travel freely. Only after the Universe had expanded and cooled could light begin to move unimpeded, and this first light formed the CMB that we observe today.

Studying the CMB can yield an incredibly rich amount of information about our Universe. By studying the light’s polarization, we can learn about whether there was a period of rapid inflation just after the Big Bang in which space expanded exponentially. By analyzing how the light was perturbed—or lensed—by gravity as it interacted with matter during its journey to us, we can also learn about the distribution of this matter, both visible and dark, across the cosmos. And, through a similar methodology, we can learn about the cosmological parameters which predict the Universe’s future.

Since it is the furthest light source in our Universe, however, studying the CMB is particularly difficult. Ground-based telescopes such as the Simons Observatory must be constructed in some of the most remote corners of the globe. Water vapor in our atmosphere perturbs astronomical measurements, so scientists build telescopes in the dry, thin air of high-altitude deserts. The Atacama Desert in Chile is one such location, where some areas among the Andes Mountains are nearly 50 times drier than California’s Death Valley.

The site of the Simons Observatory against the backdrop of the Atacama Desert. (Photo: T. Satterthwaite.)
The site of the Simons Observatory against the backdrop of the Atacama Desert. (Photo: T. Satterthwaite.)


Here, the observatory comprises four telescopes: three 0.5m Small Aperture Telescopes (SATs), which will look for signs of early inflation in the Universe, and one 6m Large Aperture Telescope (LAT), which will scan the sky to better understand how the CMB has been lensed along its journey. These telescopes contain 60,000 detectors which are similar to pixels in a camera except that they must be maintained at cryogenic temperatures. In other words, they are very cold—they operate at less than one tenth of one degree Celsius above absolute zero, which is colder than outer space.

These tens of thousands of super-cold detectors are necessary for achieving world-leading CMB results, but collecting data from them is no small task. The data must be stored on a computer which lives at room temperature, yet running 60,000 wires between the detectors and the computers would be both cumbersome and make achieving temperatures approaching absolute zero nearly impossible.

To solve this problem, researchers at SLAC National Accelerator Laboratory have developed a technology called SLAC Microresonator RF (SMuRF) electronics, which is able to read out information from almost 1,000 detectors on just two wires. The technology exploits quantum mechanical properties to couple each detector to a microwave-frequency resonator whose resonance frequency is slightly perturbed by the detection of incoming light. This effectively turns the telescope’s camera into an FM radio; just as you can listen to different radio stations by tuning your radio’s frequency, SMuRF can read out information from different detectors by tuning the frequency it listens to along the same wire. This technology is revolutionary for CMB physics, and the Simons Observatory is its first large-scale deployment for astronomy. While in Chile, I worked on commissioning the SMuRF system for the observatory’s LAT, which contains 30,000 of the ultracold detectors, making it the largest-ever cryogenic cosmology telescope. 

Assembled SMuRF system on a test bench in Chile. (Photo: T. Satterthwaite.)
Assembled SMuRF system on a test bench in Chile. (Photo: T. Satterthwaite.)


Working in the Atacama Desert is challenging. At 17,000 feet above sea level, which is roughly equivalent to a Mt. Everest base camp, the altitude can be punishing. Air pressure is half what it is at sea level, so even with supplemental oxygen, the effects of the altitude are unavoidable. Thinking can be difficult, food tastes duller, and even though I am a former national team rower, I would get winded climbing a flight of stairs. Not to mention the “Altiplanic Winter” snowstorms, caused by humid air from the Amazon rainforest passing across the Andes mountains, which would dump snow on the site in the middle of the Southern Hemisphere summer. But the views are incredible (and the pisco sours aren’t bad either).

The Large Aperture Telescope after a summertime snowstorm during “Altiplanic Winter” in February 2024. (Photo: T. Satterthwaite.)
The Large Aperture Telescope after a summertime snowstorm during “Altiplanic Winter” in February 2024. (Photo: T. Satterthwaite.)


All of this effort will surely be worth it. A telescope made of tens of thousands of ultracold detectors built in one of the world’s highest and driest deserts will soon tell of the Universe’s oldest light with exquisite precision. As I work on my PhD at Stanford University, I can’t wait to see what this new data reveals, and to take part in sharing it with the greater scientific community.

Additional Reading

The swirly sky: A new way the CMB may help track down dark matter

Determining the Hubble-Lemaitre parameter with the Simons Observatory