This article has been reviewed according to Science X’s editorial process and policies . Editors have highlighted the following attributes while ensuring the content’s credibility:

CLASS telescopes can detect cosmic microwave light signals from the Comic Dawn. Credit: Deniz Valle and Jullianna Couto

For the first time, scientists have used Earth-based telescopes to look back over 13 billion years to see how the first stars in the universe affect light emitted from the Big Bang.

Using telescopes high in the Andes mountains of northern Chile, astrophysicists have measured this polarized microwave light to create a clearer picture of one of the least understood epochs in the history of the universe, the Cosmic Dawn.

“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,” said Tobias Marriage, project leader and a Johns Hopkins professor of physics and astronomy. “Ground-based observations face additional challenges compared to space. Overcoming those obstacles makes this measurement a significant achievement.”

Cosmic microwaves are mere millimeters in wavelength and very faint. The signal from polarized microwave light is about a million times fainter. On Earth, broadcast radio waves, radar, and satellites can drown out their signal, while changes in the atmosphere, weather, and temperature can distort them. Even in perfect conditions, measuring this type of microwave requires extremely sensitive equipment.

Scientists from the U.S. National Science Foundation’s Cosmology Large Angular Scale Surveyor, or CLASS, project used telescopes uniquely designed to detect the fingerprints left by the first stars in the relic Big Bang light—a feat that previously had only been accomplished by technology deployed in space, such as the U.S. National Aeronautics and Space Administration Wilkinson Microwave Anisotropy Probe (WMAP) and European Space Agency Planck space telescopes.

The new research, led by Johns Hopkins University and the University of Chicago, appears in The Astrophysical Journal.

By comparing the CLASS telescope data with the data from the Planck and WMAP space missions, the researchers identified interference and narrowed in on a common signal from the polarized microwave light.

Polarization happens when light waves run into something and then scatter.

“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,” said first author Yunyang Li, who was a Ph.D. student at Johns Hopkins and then a fellow at the University of Chicago during the research.

“Using the new common signal, we can determine how much of what we’re seeing is cosmic glare from light bouncing off the hood of the Cosmic Dawn, so to speak.”

After the Big Bang, the universe was a fog of electrons so dense that light energy was unable to escape. As the universe expanded and cooled, protons captured the electrons to form neutral hydrogen atoms, and microwave light was then free to travel through the space in between. When the first stars formed during the Cosmic Dawn, their intense energy ripped electrons free from the hydrogen atoms. The research team measured the probability that a photon from the Big Bang encountered one of the freed electrons on its way through the cloud of ionized gas and skittered off course.

Discover the latest in science, tech, and space with over 100,000 subscribers who rely on Phys.org for daily insights. Sign up for our free newsletter and get updates on breakthroughs, innovations, and research that matter—daily or weekly.

The findings will help better define signals coming from the residual glow of the Big Bang, or the cosmic microwave background, and form a clearer picture of the early universe.

“Measuring this reionization signal more precisely is an important frontier of cosmic microwave background research,” said Charles Bennett, a Bloomberg Distinguished Professor at Johns Hopkins who led the WMAP space mission. “For us, the universe is like a physics lab. Better measurements of the universe help to refine our understanding of dark matter and neutrinos, abundant but elusive particles that fill the universe. By analyzing additional CLASS data going forward, we hope to reach the highest possible precision that’s achievable.”

Building on research published last year that used the CLASS telescopes to map 75% of the night sky, the new results also help solidify the CLASS team’s approach.

“No other ground-based experiment can do what CLASS is doing,” says Nigel Sharp, program director in the NSF Division of Astronomical Sciences, which has supported the CLASS instrument and research team since 2010. “The CLASS team has greatly improved the measurement of the cosmic microwave polarization signal and this impressive leap forward is a testament to the scientific value produced by NSF’s long-term support.”

The CLASS observatory operates in the Parque Astronómico Atacama in northern Chile under the auspices of the Agencia Nacional de Investigación y Desarrollo.

More information: A Measurement of the Largest-Scale CMB E-mode Polarization with CLASS, The Astrophysical Journal (2025). DOI: 10.3847/1538-4357/adc723 Journal information: Astrophysical Journal