Groundbreaking Study Reveals Cosmic Dawn Insights Using Telescopes

In a significant advancement for astrophysics, researchers have successfully utilized Earth-based telescopes to explore the Cosmic Dawn, a pivotal era in the early universe, for the first time. This groundbreaking study, which reveals insights into the formation of the first stars over 13 billion years ago, was led by a team from Johns Hopkins University and the University of Chicago and published in *The Astrophysical Journal* on June 11, 2025.

The Cosmic Dawn, a crucial epoch following the Big Bang, is characterized by the emergence of the universe’s first stars and galaxies. Utilizing telescopes situated in the Atacama Desert of northern Chile, the researchers measured polarized microwave light, a challenging task due to its faintness and susceptibility to interference from terrestrial signals. Tobias Marriage, project leader and professor of physics and astronomy at Johns Hopkins University, emphasized the significance of this achievement, stating, "People thought this couldn't be done from the ground. Astronomy is a technology-limited field, and microwave signals from the Cosmic Dawn are famously difficult to measure. Overcoming those obstacles makes this measurement a significant achievement."

The research utilized the Cosmology Large Angular Scale Surveyor (CLASS) to detect minute signals that have been previously observed only by space-based instruments, such as NASA's Wilkinson Microwave Anisotropy Probe (WMAP) and the European Space Agency’s Planck telescope. The CLASS project, supported by the U.S. National Science Foundation (NSF), aims to refine our understanding of the cosmic microwave background (CMB), the remnant radiation from the Big Bang.

According to Yunyang Li, the first author of the study and a former PhD student at Johns Hopkins, the research involved determining the polarization of light waves emitted during the Cosmic Dawn. He explained, “When light hits the hood of your car and you see a glare, that’s polarization. To see clearly, you can put on polarized glasses to take away glare.” This analogy illustrates the scientists' method of isolating cosmic signals from terrestrial noise.

After the Big Bang, the universe was initially opaque, filled with electrons that prevented light from escaping. As the universe expanded and cooled, neutral hydrogen atoms formed, allowing microwave radiation to travel. The formation of the first stars during the Cosmic Dawn reionized the universe, freeing electrons from hydrogen atoms and leading to the signals measured in this study.

The findings provide a clearer understanding of how the first stars influenced the CMB and will help refine models of cosmic evolution. Charles Bennett, a Bloomberg Distinguished Professor at Johns Hopkins and leader of the WMAP mission, noted, “Measuring this reionization signal more precisely is an important frontier of cosmic microwave background research.”

The CLASS team’s capabilities mark a significant development in ground-based astronomy, emphasizing that "No other ground-based experiment can do what CLASS is doing," stated Nigel Sharp, NSF Division of Astronomical Sciences program director. The observatory operates under the Agencia Nacional de Investigación y Desarrollo and collaborates with various universities and research centers, including the University of Chicago, Villanova University, and the Harvard-Smithsonian Center for Astrophysics.

This research not only sheds light on the Cosmic Dawn but also sets the stage for future studies that could enhance our understanding of dark matter and neutrinos, two elusive components of the universe. The CLASS team plans to analyze additional data going forward, aiming for the highest possible precision in cosmic measurements, which could redefine our comprehension of the universe's evolution.