May 17, 2023
Most people seek warmer destinations for travel to escape the tough Chicago winters. Brad Benson, on the other hand, sets his sights on the South Pole nearly every year. On his list of top reasons to visit: the intense cold creating extremely low levels of moisture in the air, dryer even than the highest mountain peaks in the world.
Benson, an Associate Professor at the University of Chicago and scientist at Fermilab, is an experimental cosmologist who develops detectors and massive telescopes that enable the study of the origins of the Universe.
Benson isn't the only one paying attention to the water vapor levels in the South Pole; typically, water vapor absorbs microwaves in the atmosphere, but in the early eighties, researchers identified the South Pole as an ideal location because it is dryer than even the highest mountain tops. For these researchers, the South Pole became a unique destination for the study of the oldest radiation waves in the universe, the cosmic microwave background (CMB).
"[The CMB] had only been discovered technically in the sixties, but there was a lot of active research and new measurements going on, especially when I was starting in school," said Brad. "So it was an exciting topic at the time that captured my interest. It was just perfect timing to get involved."
In 2007, Benson and collaborators from the University of Chicago deployed the South Pole Telescope (SPT), the first of many advancements in detector technology that was sensitive enough for scientists to study the CMB and other astronomical phenomena at great precision.
Benson compares the detector to a camera. In many digital cameras we normally use, there are glass mirrors that reflect light waves to a sensor. When light reaches it, the sensor tells your phone or camera which pixels to light up and how to light them, which produces the photographic image. SPT and other millimeter wave telescopes operate similarly, but the mirror is aluminum and, instead of light waves, the sensors detect microwaves or radio waves.
The temperature of the South Pole is another selling point for this type of research. The telescope detectors are cooled down to near absolute zero so that when microwaves are reflected onto them, heat is generated. The sensitivity of the telescopes, then, is a measure of how much difference in heat can be detected.
The 'image' generated by the South Pole Telescope is a time capsule recording of the brightest event in the sky, the Big Bang. The signals observed are mostly uniform, but the variations in that signal paint a picture of the state of the early universe, like the distribution of matter and light.
Benson and his collaborators are studying the CMB to answer the big questions in the field: What is the content of the Universe? What is dark energy? What Physics was responsible for the Big Bang?
"Most of the mass in a galaxy is dark matter," said Benson. "When you look at the clusters of galaxies that are gravitationally bound together, almost 90% of that mass is dark matter."
"Observing dark matter is challenging, because by definition it's a component of the universe that doesn't emit or “absorb light,” Benson continued. “However we can measure its gravitational influence on the formation of cosmic structure, including its effect on patterns in the CMB that SPT can measure."
Dark energy on the other hand, is wrapped in more mystery. It was originally proposed when people started to measure the expansion rate of the universe and discovered that the expansion rate has been accelerating over the last several billion years.
So, dark energy emerged as one potential explanation for the 'new physics' necessary to account for the unexpected phenomenon.
"By comparing what the universe looked like nearly 14 billion years ago via the CMB to what it looks like today, we are able to constrain properties of dark energy and its effect on the evolution of the universe on cosmological time scales,” said Benson.
Looking to the future, Benson and collaborators worldwide are planning to build a new telescope, CMB-S4, that is on the order of 30 times larger than the current generation of SPT telescopes, with 500,000 detectors. There will be two observatory sites, the South Pole and Chile, to observe a broader swath of the sky, including parts of the northern hemisphere.
Additionally, since the celestial sky never rises or sets in the South Pole, CMB-S4 will be able to continuously monitor and make more sensitive observations to smaller signals over a fraction of the sky.
When Brad started he was interested in "building things and tinkering," he said. "It was really cool to me, especially as an undergrad, learning that you can build things in the lab and build experiments to actually measure the light left over from the Big Bang. Now, it's not so much just a couple people tinkering in a lab anymore but collaborations of people."
CMB-S4 represents this type of massive collaboration of people. “CMB-S4 will enable the search for signatures of primordial gravitational waves (predicted to have been generated shortly after the Big Bang), probe the nature of dark matter and dark energy, map the distribution of matter in the Universe, and capture transient phenomena in the microwave sky,” said Benson.