Have you ever thought about hearing a whisper from 13 billion years ago? Besides being almost poetic, we thought it impossible, but that’s exactly what happened. For the first time, telescopes here on Earth have managed to capture the silent echo of the universe’s first stars, born shortly after the Big Bang, in a period known as the cosmic dawn. Until then, this type of signal could only be observed with equipment in space, but scientists have broken that barrier. And the feat isn’t just technical; in fact, it could change the way we understand the birth of light in the universe.

What erased the universe’s fog? Mysterious dawn that reshaped light itself

We must remember that in the first moments after the Big Bang, the universe was a dense soup of charged particles—electrons everywhere, so tightly packed that light simply couldn’t pass through. It was like an endless cosmic fog. However, over time, the universe cooled, and protons began capturing electrons, forming neutral hydrogen atoms. And only then was light able to break free. This light is what we know today as the cosmic microwave background, a kind of fossil glow from the birth of the universe.

However, this light didn’t remain untouched. When the first stars appeared (during the cosmic dawn), they changed everything. This is because, with their power, they ripped electrons from hydrogen atoms, disrupting the path of light traveling through them. This process is called reionization; understanding how it happened is essential to deciphering cosmic history.

Can we hear the universe whisper? Cosmic signal buried in Earth’s noise

The problem is that detecting this type of light, specifically the polarized microwaves coming from the cosmic aurora, is a nearly impossible task. That’s because the signal is weak, about a million times more subtle than the interference we experience here on Earth, such as radio, radar, satellites, even weather. To put it mildly, it was like trying to hear a cricket whispering in the middle of a rock concert.

Until the US National Science Foundation’s CLASS project telescopes (different from, but just as important as, the James Webb) arrived, installed at an altitude of over 5,000 meters in the Chilean Andes. They were built precisely for this challenge. With extreme precision, they managed to isolate this cosmic whisper and, most importantly, confirmed its reality by comparing it with data from space missions like Planck and WMAP.

Their idea was to search for a “common signal”, a kind of signature that could only come from the cosmic aurora. Just as polarized sunglasses eliminate annoying reflections, the researchers used technological filters (and a lot of calculations) to eliminate noise and focus only on the light that matters.

“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”, said first author Yunyang Li.

Ground-based telescopes just did the impossible: Universe’s oldest secrets are next

We must keep in mind that this advance is not just a technical feat; on the contrary, it has enormous implications. First, it helps us understand how light behaved in the first billions of years. This helps us refine models about the birth of the first galaxies, stars, dark matter itself, and neutrinos, the particles that are ubiquitous but remain a mystery in many ways.

We now know it’s possible to take this deep reading without leaving the planet, which opens up new possibilities for terrestrial astrophysics. In other words, what was previously only possible with space telescopes is now being done by instruments down here, with greater flexibility and lower cost… It’s no wonder other signals from nearly 10 billion years ago have recently been detected.