Telescopes located in Chile have successfully detected ancient light that dates back 13 billion years, originating from the universe's first stars. This remarkable achievement sheds light on the Cosmic Dawn and the early stages of the universe's evolution.

Ground-based telescopes in the Chilean Andes have accomplished the seemingly impossible task of capturing ultra-faint, polarised light from the Big Bang that was scattered by the universe's initial stars over 13 billion years ago. Spearheaded by the U.S. National Science Foundation's Cosmology Large Angular Scale Surveyor (CLASS) project, this groundbreaking discovery offers a unique glimpse into the universe's early days, known as the Cosmic Dawn.

For the first time, researchers have utilized instruments on Earth to look back more than 13 billion years, unveiling how the universe's earliest stars influenced the light emitted during the Big Bang. This significant breakthrough, outlined in a recent study published in The Astrophysical Journal, was led by a collaborative team from Johns Hopkins University and the University of Chicago.

Overcoming terrestrial challenges

The task of isolating such an ancient and faint signal from Earth presented significant challenges. Tobias Marriage, project leader and professor of physics and astronomy at Johns Hopkins University, highlighted the difficulties of measuring microwave signals from the Cosmic Dawn, which are notoriously hard to detect. Ground-based observations face additional obstacles compared to space-based missions, making this achievement all the more remarkable.

Cosmic microwaves, with wavelengths in the millimeter range, are extremely faint, and the polarised signal from the Cosmic Dawn is even fainter by a factor of a million. On Earth, these delicate signals can easily be overwhelmed by background radio waves, radar, and satellite transmissions. Moreover, atmospheric conditions like weather patterns and temperature changes can distort the signal. Detecting this type of microwave light requires highly sensitive equipment, even under optimal conditions.

The CLASS advantage: A terrestrial breakthrough

Prior to this discovery, relic Big Bang light, particularly the polarised cosmic microwave background, had only been detected by space-borne instruments such as NASA's Wilkinson Microwave Anisotropy Probe (WMAP) and the European Space Agency's Planck space telescopes. The innovative telescopes used by the CLASS team, strategically located in the Andes mountains of northern Chile, were specifically designed to identify the subtle signatures left by the first stars in this ancient light.

To isolate the ancient signal, researchers meticulously compared data from the CLASS telescopes with information from the Planck and WMAP space missions. This comparative analysis enabled them to filter out interference and focus on a common signal from the polarised microwave light.

Yunyang Li, the study's first author and former PhD student at Johns Hopkins, explained the concept of polarisation, likening it to wearing polarised glasses to reduce glare. By using the new common signal, researchers can determine the amount of cosmic glare in the light reflected off the Cosmic Dawn's 'hood', so to speak.

Peering into the cosmic dawn

The significance of this discovery lies in its ability to illuminate the Cosmic Dawn era. Following the Big Bang, the universe was shrouded in an electron 'fog' that prevented light from escaping. As the universe expanded and cooled, protons captured electrons, forming neutral hydrogen atoms that allowed microwave light to travel through space. However, during the Cosmic Dawn, the energy from the first stars re-ionised these hydrogen atoms, stripping away their electrons once again. The CLASS team measured the likelihood of a photon from the Big Bang encountering one of these freed electrons and being deflected.

These findings will offer a more precise understanding of the signals emanating from the cosmic microwave background, providing a clearer picture of the universe's early moments. Charles Bennett, a professor at Johns Hopkins and leader of the WMAP mission, emphasized the importance of this research in advancing cosmic microwave background studies. By analyzing additional CLASS data in the future, researchers aim to achieve the highest possible precision.

This latest research, which builds upon a previous CLASS project mapping 75% of the night sky, further validates the team's innovative observational approach. Nigel Sharp, program director in the NSF Division of Astronomical Sciences, commended the achievement, noting that no other ground-based experiment can match what CLASS has accomplished.