Breakthrough in Astronomy: Chilean Telescopes Detect 13-Billion-Year-Old Light

In a remarkable achievement for ground-based astronomy, a team of researchers has successfully detected light that originated 13 billion years ago, emitted by the universe's first stars, using telescopes located in the Chilean Andes. This groundbreaking discovery, spearheaded by the U.S. National Science Foundation’s Cosmology Large Angular Scale Surveyor (CLASS) project, has provided unprecedented insights into the Cosmic Dawn, a crucial period in the universe's early evolution. Published in The Astrophysical Journal on June 13, 2025, this study highlights how terrestrial instruments can now peer back into the universe's infancy, a feat once deemed impossible.

The Cosmic Dawn refers to the epoch following the Big Bang when the universe transitioned from a dense, opaque state to one where light could travel freely. The detection of this ancient light is particularly significant as it sheds light on how the first stars influenced the primordial conditions of the universe. According to Tobias Marriage, a Johns Hopkins University professor of physics and astronomy and the project leader, isolating such faint signals from Earth presents significant challenges.

"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," Marriage stated. This polarised cosmic microwave background light, emanating from the Cosmic Dawn, is a million times fainter than other cosmic signals, making its detection a monumental scientific triumph.

Previous attempts to detect this type of light were limited to space-based missions such as NASA’s Wilkinson Microwave Anisotropy Probe (WMAP) and the European Space Agency’s Planck spacecraft. The CLASS project’s telescopes, strategically positioned in the high Andes of northern Chile, were uniquely designed to capture the delicate signatures of the first stars within this ancient light. Researchers meticulously compared data from CLASS with observations from WMAP and Planck to filter out interference and home in on the desired polarised microwave signal.

Yunyang Li, the first author of the study and a former PhD student at Johns Hopkins, explained the concept of polarisation in layman's terms: “When light hits the hood of your car and you see a glare, that’s polarisation. To see clearly, you can put on polarised glasses to take away glare.” This analogy illustrates how the CLASS team utilized a common signal to discern the cosmic glare from the Cosmic Dawn.

The implications of this research extend beyond the mere detection of ancient light; it provides a deeper understanding of the Cosmic Dawn itself. After the Big Bang, the universe was filled with an opaque fog of electrons, preventing light from escaping. As the universe expanded and cooled, protons captured electrons to form neutral hydrogen atoms, allowing microwave light to traverse. During the Cosmic Dawn, energy from the first stars re-ionised these hydrogen atoms, making this discovery critical for understanding the universe’s evolution.

Charles Bennett, a Bloomberg Distinguished Professor at Johns Hopkins and leader of the WMAP mission, echoed the significance of this work, stating, "Measuring this reionisation signal more precisely is an important frontier of cosmic microwave background research." He emphasized the need for higher precision measurements to refine our understanding of dark matter and neutrinos, elusive particles that comprise much of the universe.

The CLASS project has validated its innovative observational approach, building on prior work that mapped 75% of the night sky. Nigel Sharp, program director at the NSF Division of Astronomical Sciences, praised the CLASS team’s achievements. “No other ground-based experiment can do what CLASS is doing. The CLASS team has greatly improved measurement of the cosmic microwave polarisation signal, and this impressive leap forward is a testament to the scientific value produced by NSF’s long-term support,” Sharp remarked.

As scientists continue to analyze CLASS data, the potential for further discoveries looms large. This detection not only advances our comprehension of the universe's earliest moments but also opens new avenues for exploration in cosmology and astrophysics, highlighting the critical role of ground-based observations in contemporary science.