Ground-Based Telescopes Illuminate Cosmic Dawn's Secret Timeline

In a groundbreaking achievement, a team of astrophysicists operating the Cosmology Large Angular Scale Surveyor (CLASS) in Chile has successfully detected cosmic polarization signals, offering new insights into the timeline of the universe’s first stars. This significant advancement was announced on June 15, 2025, and marks a pivotal moment in cosmological research, enabling scientists to refine their understanding of cosmic reionization.

The cosmic microwave background (CMB), the faint afterglow of the Big Bang, plays a crucial role in understanding the universe’s early history. Researchers have focused on the polarization of this signal, which holds vital clues regarding the formation of the universe's first stars during an era known as the Cosmic Dawn. According to Dr. Tobias Marriage, an astrophysicist at Johns Hopkins University and lead researcher on the CLASS project, the detection of these signals represents a substantial leap in observational cosmology. "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.

The CLASS telescope, situated at an elevation of 17,000 feet on Cerro Toco in the Atacama Desert, operates under conditions that minimize atmospheric distortion. Utilizing four small telescopes, the project employs a specialized optical element called a variable-delay polarization modulator to isolate cosmic signals from background noise. This innovative approach enables the team to measure the optical depth (τ), a critical parameter that quantifies the efficiency with which high-energy radiation from newborn stars scatters off surrounding gas.

Previously, the Wilkinson Microwave Anisotropy Probe (WMAP) provided an initial τ estimate of about 0.089, which was later refined to approximately 0.054 by the Planck satellite. The CLASS team has now reported a new measurement of τ at 0.053 (+0.019/–0.018), closely aligning with Planck's findings and underscoring the capability of ground-based observations to rival those conducted in space.

Dr. Charles Bennett, a veteran of the WMAP mission, emphasized the importance of this development, stating, "Measuring this reionization signal more precisely is an important frontier of cosmic microwave background research." The precision of the new result not only enhances our understanding of early star formation but also contributes to ongoing debates regarding the composition of the universe, including the roles of ordinary matter, dark matter, and elusive neutrinos.

The CLASS project, funded by the U.S. National Science Foundation, is poised for further enhancements, including the addition of a second 90 GHz dish and advanced detectors tuned to higher frequencies. These upgrades are expected to improve the measurement of τ and aid in the search for primordial gravitational waves, which may further illuminate the universe's inflationary period.

As CLASS continues its observations, the implications for cosmological research are profound. By doubling its observing time and refining its filtering algorithms, the team aims to achieve even greater precision in measuring τ, potentially shedding light on the polarized fingerprints of cosmic inflation. Nigel Sharp, a representative from the U.S. National Science Foundation, remarked, "No other ground-based experiment can do what CLASS is doing." This statement highlights the unique position of CLASS in the field of cosmology, as it opens up new avenues for understanding the universe's earliest moments.

The study detailing these findings has been published in The Astrophysical Journal, further solidifying the role of ground-based telescopes in cutting-edge astronomical research. The ongoing efforts of the CLASS team may well redefine our understanding of the universe's formation and the intricate processes that have shaped it since the Cosmic Dawn.