In a groundbreaking feat, telescopes in Chile have detected 13-billion-year-old light from the Big Bang, scattered by the universe’s first stars. This achievement, once thought impossible from Earth, offers vital insights into the Cosmic Dawn and the early evolution of our universe
Using ground-based telescopes, researchers in the Chilean Andes have achieved what was once deemed impossible: detecting ultra-faint, polarised light from the Big Bang, scattered by the universe’s very first stars over 13 billion years ago. This groundbreaking feat, spearheaded by the U.S. National Science Foundation’s Cosmology Large Angular Scale Surveyor (CLASS) project, offers an unprecedented lens into the universe’s infancy, an epoch known as the Cosmic Dawn.
For the first time, researchers have used Earth-based instruments to peer back more than 13 billion years, revealing how the universe’s primordial stars influenced the light emitted from the Big Bang. This monumental achievement, detailed in a new study published today in The Astrophysical Journal, was led by a collaborative team from Johns Hopkins University and the University of Chicago.
Overcoming terrestrial challenges
The challenge of isolating such an ancient and minuscule signal from Earth is immense. “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,” stated Tobias Marriage, project leader and a Johns Hopkins professor of physics and astronomy. He emphasised the additional obstacles faced by ground-based observations compared to space-based missions, making this measurement a significant triumph.
Cosmic microwaves, mere millimeters in wavelength, are incredibly faint, with the polarised signal from the Cosmic Dawn being a million times fainter still. On Earth, these delicate signals are easily drowned out by omnipresent broadcast radio waves, radar, and satellite transmissions. Furthermore, atmospheric fluctuations, weather patterns, and temperature changes can distort the signal. Even under ideal conditions, detecting this type of microwave light demands extraordinarily sensitive equipment.
The CLASS advantage: A terrestrial breakthrough
Previous detections of this type of relic Big Bang light, specifically the polarised cosmic microwave background, had only been accomplished by space-borne instruments like NASA’s Wilkinson Microwave Anisotropy Probe (WMAP) and the European Space Agency’s Planck space telescopes. The CLASS team’s innovative telescopes, strategically positioned high in the Andes mountains of northern Chile, were uniquely designed to identify the subtle “fingerprints” left by the first stars in this ancient light.
To isolate the ancient signal, the researchers meticulously compared data from the CLASS telescopes with readings from the Planck and WMAP space missions. This comparative analysis allowed them to identify and filter out interference, homing in on a common signal from the polarised microwave light.
Yunyang Li, first author of the study and a former PhD student at Johns Hopkins, explained the concept of polarisation: “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.” She added, “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.”
Peering into the cosmic dawn
The scientific significance of this detection lies in its ability to illuminate the Cosmic Dawn. Following the Big Bang, the universe existed as an opaque “fog” of electrons, preventing light from escaping. As the universe expanded and cooled, protons captured electrons, forming neutral hydrogen atoms, which allowed microwave light to traverse the cosmos. However, during the Cosmic Dawn, the intense energy from the first stars re-ionised these hydrogen atoms, stripping away their electrons once more. The CLASS team measured the probability that a photon from the Big Bang would encounter one of these freed electrons and be scattered off course.
These findings will provide a more precise definition of the signals emanating from the cosmic microwave background, painting a clearer picture of the universe’s earliest moments. Charles Bennett, a Bloomberg Distinguished Professor at Johns Hopkins and leader of the WMAP mission, underscored the importance of this work: “Measuring this reionisation signal more precisely is an important frontier of cosmic microwave background research.” He added, “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 analysing additional CLASS data going forward, we hope to reach the highest possible precision that’s achievable.”
This latest research, building upon a previous CLASS project that mapped 75% of the night sky, further validates the team’s innovative observational approach. Nigel Sharp, program director in the NSF Division of Astronomical Sciences, which has supported CLASS since 2010, lauded the achievement: “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.”
