For the first time, scientists have used ground-based telescopes to look back more than 13 billion years and observe how the universe’s first stars affected ancient light left over from the Big Bang.
This breakthrough gives astronomers a clearer view of a mysterious period in the universe’s history known as the cosmic dawn.
The discovery was made by the CLASS project, a team of astrophysicists supported by the U.S. National Science Foundation.
Using telescopes located high in the Andes Mountains of northern Chile, researchers were able to measure a very faint signal of polarized microwave light from the early universe—something that had only been done previously using space telescopes like NASA’s WMAP or the European Space Agency’s Planck mission.
Microwave signals from the early universe are extremely hard to detect from Earth. These waves are tiny—just millimeters in length—and the polarization signal scientists are looking for is even weaker, about a million times fainter.
On top of that, the Earth’s atmosphere, satellites, and radio waves can interfere with the measurements. That’s why this new ground-based success is such a big achievement.
The cosmic dawn happened shortly after the Big Bang, when the first stars were just starting to form. Before that, the universe was filled with a thick fog of particles that blocked light.
As things cooled down, atoms formed and light could finally travel freely.
But once the first stars turned on, their energy broke apart these atoms again, creating ionized gas. The CLASS team has now measured how likely it was that a photon—a particle of light from the Big Bang—bumped into one of these freed electrons and scattered, changing its direction.
To get these results, the scientists compared the new data from CLASS with older data from space missions. This allowed them to filter out noise and zero in on a common signal that represents the polarized microwave light caused by the early stars.
Understanding this light helps scientists study the cosmic microwave background—the faint glow of radiation left over from the Big Bang. It’s like looking at a baby photo of the universe. The more clearly we can see it, the more we learn about the early universe, including strange particles like dark matter and neutrinos.
This new study builds on earlier work by the CLASS team, who have already mapped most of the night sky. With continued support and more data, scientists hope to measure these signals with even greater accuracy, opening a window into the first moments of the cosmos—all from right here on Earth.
