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April 16, 2024

CLASS furthers understanding of the Cosmic Microwave Background

By ZACHARY BAHAR | March 12, 2024



A recent analysis of the polarization of the Cosmic Microwave Background sheds light on the universe's development.

A recent paper published by a team of Hopkins physicists and astronomers has permanently changed the landscape of the Cosmic Microwave Background (CMB) — in this case, literally. The paper, published in The Astrophysical Journal on Monday, March 4, includes the most detailed observations of the CMB taken from Earth.

The CMB is the first light in the early universe, originating from around 380,000 years after the Big Bang. Furthermore, the CMB is not a local phenomenon; rather, it emanates from everywhere in the universe. As such, the patterns it displays serve a key role in our understanding of cosmological development and the makeup of the universe.

The Cosmology Large Angular Scale Surveyor (CLASS) project is taking highly detailed measurements of the CMB using three telescopes located in Chile’s Atacama Desert — a location renowned in the astronomy community for its high altitudes and lack of light pollution. The recent paper used data from one of these telescopes (the 40 GHz channel) to construct its map. 

A key feature of CLASS’ methods is the emphasis on studying the polarization of CMB light. Light can be polarized in two ways: linear polarization — oscillations in the vertical or horizontal directions — and circular oscillation — traveling in circles either clockwise or counterclockwise. These oscillations must be taken into account when studying the CMB. Hopkins Associate Research Scientist Joseph Eimer, the first author of the paper, explained this process in an interview with The News-Letter.

“The light from the CMB, it turns out, is partially polarized. The reason why is special scattering processes of light bounce off of the early distribution of matter in the universe,” Eimer said. “That polarized signal tells us something about what was going on in the physics of the day. This data tells us something different about the universe than just the overall temperature setting or intensity. So, if we measure how much the polarization signal is changing, the statistics of that pattern actually hold the signal that we're trying to understand.”

A key reason to study CMB polarization is that it plays a role in understanding cosmic inflation, a brief period when the early universe expanded faster than the speed of light. While this theory is a bedrock of modern cosmology, it remains unproven; however, it predicts that certain patterns would have been left behind by the rapid expansion, the polarization of the CMB among them.

The most impressive technical feat performed by the team is the isolation of CMB data from confounding factors. There are two primary sources of interference in collecting data from the Earth’s surface: issues caused by the Earth including the atmosphere and light pollution, and more significantly, issues caused by the light emanating from the Milky Way Galaxy. While methods for dealing with the first issue are well known, CLASS developed new means of handling the second.

“We're trying to say something about the universe, but it turns out we're in this group of 100 billion stars, gas and dust. All of that is shining at us too, so we have to distinguish what's coming from our little neighborhood and what's coming from the background,” Eimer said. 

Part of this background includes synchrotron radiation, a form of radiation caused by ultra-relativistic electrons moving along the galaxy‘s magnetic fields. To measure the CMB, one needs to factor out the synchrotron radiation by first taking careful measurements of it.

Additionally, the paper serves as a role model for future Earth-based studies of the CMB. The methods developed offer more complete data than those generated from comparable space-based telescopes. Ground telescopes offer a variety of advantages over satellites, including lower costs and a greater ability to make repairs.

“The purpose of this paper wasn't to show all of our plans to do things better, but we think we understand a lot about what was limiting the measurement. A lot of that has to do with the complexity of trying to do this from the ground instead of space,” Eimer said. “This is the first time anybody's tried to do this; it's not shocking that we didn't hit it out of the park on our first time at bat.”

Looking to the future, CLASS will be utilizing the other existing telescopes — operating at 3 mm, 2 mm and 1.4 mm wavelengths — in addition to a fourth yet-to-be-built telescope to gather further data. Professor Tobias Marriage shared CLASS’ future research goals in a conversation with The News-Letter.

“This is a two-pronged effort where we're doing a lot more data analysis and crunching our multifrequency dataset, and will further develop the tools that we had with the 40 GHz analysis in this paper... The CMB has been measured with awesome precision in terms of the intensity fluctuations, and the polarization measurements are coming along very well, but there is this question of what medium you’re looking through. To backtrack to what the CMB looks like, we need to have that medium characterized very well,” Marriage said. 

He also highlighted the role of student researchers in this over 13-year-long project.

“One of the most compelling aspects of this story is just how much this telescope out in the Chilean desert was designed, built, characterized and operated by students and postdocs,” he said. “These are really young undergraduates, graduate students and people who just got their PhDs. It's one of the cooler aspects and the students are making a real fundamental difference.” 

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