Earth Telescopes First to See Universe’s Cosmic Dawn

Ground-based telescopes have successfully detected signals from the universe’s Cosmic Dawn for the first time, measuring polarized microwave light that carries information about the epoch when the first stars ignited over 13 billion years ago.

Using telescopes perched high in Chile’s Atacama Desert, scientists overcame enormous technical challenges to capture signals a million times fainter than ordinary cosmic microwaves, demonstrating that Earth-based observatories can probe this mysterious period previously accessible only to space missions.

The achievement marks a new frontier in astronomy. Until now, only space-based telescopes like NASA’s WMAP and the European Space Agency’s Planck had successfully measured these ancient signals, which reveal how the first stars transformed the universe from a dark, neutral fog into the ionized cosmos we see today.

“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.

Breaking Through Cosmic Static

The researchers faced daunting obstacles. Cosmic microwaves measure mere millimeters in wavelength and arrive at Earth incredibly faint. Their polarized components—the key to understanding the Cosmic Dawn—register a million times weaker still.

Ground-based observations must contend with radio interference from broadcasts, radar systems, and satellites. Atmospheric changes, weather fluctuations, and temperature variations further distort the delicate signals. Yet the Cosmology Large Angular Scale Surveyor (CLASS) project succeeded where others thought impossible.

Polarization occurs when light waves encounter matter and 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,” explained first author Yunyang Li, who conducted the research as a PhD student at Johns Hopkins and fellow at the University of Chicago.

Cosmic Timeline Revealed

The measurements probe a transformative period in cosmic history. After the Big Bang, the universe existed as an opaque fog of electrons so dense that light couldn’t escape. As expansion cooled the cosmos, protons captured electrons to form neutral hydrogen atoms, freeing microwave radiation to travel through space.

Then came the Cosmic Dawn. The first stars blazed to life with such intense energy that they ripped electrons from hydrogen atoms, reionizing vast regions of space. The research team measured the probability that ancient photons from the Big Bang encountered these freed electrons and scattered, leaving telltale polarization signatures.

What wasn’t highlighted in initial reports is the sophisticated mathematical framework the researchers developed to correct for systematic errors. The team created what they call a “pixel-space transfer matrix”—a computational tool that models how their ground-based filtering operations affect the cosmic signals. This innovation enables them to recover unbiased measurements even when their instruments must aggressively filter out terrestrial interference.

Key scientific accomplishments include:

• First ground-based detection of cosmic reionization signals
• New methods for correcting systematic errors in ground observations
• Independent verification of space-based measurements
• Advanced techniques for isolating cosmic signals from interference
• Pathways toward cosmic variance-limited precision

Implications for Dark Universe

The findings carry significance beyond cosmic archaeology. Precise measurements of the reionization epoch help break degeneracies between fundamental cosmological parameters, potentially resolving tensions in our understanding of dark matter and the expansion rate of the universe.

“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,” said Charles Bennett, a Bloomberg Distinguished Professor at Johns Hopkins who led the WMAP space mission.

The research also provides crucial calibration for detecting primordial gravitational waves—ripples in spacetime from the universe’s first moments. These “B-mode” signals hide beneath the reionization signatures that CLASS now measures with unprecedented precision from the ground.

Looking Forward

The achievement validates CLASS’s unique approach using rapidly spinning polarization modulators that suppress systematic errors. By comparing their measurements with data from Planck and WMAP missions, the team identified common signals while filtering out instrument-specific artifacts.

“No other ground-based experiment can do what CLASS is doing,” says Nigel Sharp, program director in the NSF Division of Astronomical Sciences. The project demonstrates that ground-based observatories can compete with space missions for some of astronomy’s most challenging measurements.

Future improvements could push CLASS to cosmic variance limits—the ultimate precision boundary set by the finite number of observable cosmic structures. The team projects that modest improvements in their filtering techniques could approach this fundamental limit, opening new windows into the universe’s earliest epochs from Earth-based platforms.

As astronomers await next-generation space missions, CLASS proves that innovative ground-based approaches can illuminate cosmic dawn from our planetary vantage point, expanding humanity’s ability to study the universe’s infancy.

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