Ground-based telescopes detect light from the Cosmic Dawn

The cosmic microwave background (CMB) is the faint afterglow of the Big Bang that still bathes the sky in microwave light. For decades, scientists have combed this signal for tiny twists called polarization that carry clues to the universe’s earliest chapters.

Tobias Marriage of Johns Hopkins University and his team now report that the ground-based Cosmology Large Angular Scale Surveyor (CLASS) in Chile has read those twists all the way back to the Cosmic Dawn – the era when the first stars switched on.

Observing the Cosmic Dawn

Newborn suns pumped high-energy radiation into surrounding gas and knocked electrons loose, a phase known as reionization. The efficiency of that scattering is described by an “optical depth” (τ).

A bigger τ means more electrons and a longer or earlier reionization period, so nailing down its value helps pin the timeline of early star formation.

Spacecraft such as the Wilkinson Microwave Anisotropy Probe (WMAP) first measured τ at about 0.089, but later analyses by Europe’s Planck satellite trimmed it to roughly 0.054.

Until now, no Earth-based telescope had the sensitivity and noise control to weigh in on this debate.

Climbing above the static

CLASS operates four small (five-foot-wide) telescopes on Cerro Toco, 17,000 feet up in the Atacama Desert, where the air holds little water vapor.

Even there, radio chatter from satellites, radar, and the occasional passing truck can drown a million-times-fainter cosmic signal.

“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 Marriage.

Cosmic waves reveal polarization patterns

The team focused on microwaves just 0.13 inches (3 mm) long. When these waves scatter off free electrons, their electric fields line up, creating a telltale polarization pattern called an E-mode.

“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 Yunyang Li of the University of Chicago.

CLASS’s 90 GHz instrument spun a reflective modulator 10 times a second, flicking the wave orientation back and forth and separating sky polarization from local noise. 

CLASS used a special optical element called a variable-delay polarization modulator to isolate sky polarization from background interference. This approach has proven effective in filtering out slow temperature drifts and motion-related artifacts from Earth-based observations.

Sharpening the cosmic clock

The new result yields an optical depth of 0.053 (+0.019/–0.018). That closely matches Planck’s space-based estimate and tightens Earth’s grip on one of cosmology’s most stubborn numbers.

“Measuring this reionization signal more precisely is an important frontier of cosmic microwave background research,” said Charles Bennett, a veteran of the WMAP mission.

Lower uncertainty in τ feeds directly into sharper estimates of how much ordinary matter, dark matter, and elusive neutrinos occupy today’s universe.

Ground telescopes can rival satellites

Unlike satellite missions that operate above Earth’s atmosphere, CLASS must correct for atmospheric distortion and ground interference.

That challenge has made the precision of this result particularly noteworthy, showing that well-designed ground instruments can rival space-based platforms for specific cosmological goals.

The CLASS team built on insights from earlier satellite missions, using data processing methods refined during the WMAP and Planck projects to guide their approach to mapping polarization.

The researchers also carefully selected their observing frequencies and signal modulation techniques to minimize interference from foreground sources, especially at the largest cosmic scales.

CLASS improves polarization precision

“No other ground-based experiment can do what CLASS is doing,” noted Nigel Sharp of the U.S. National Science Foundation, which funds the project.

Future upgrades include a second 90 GHz dish and detectors tuned to 150 GHz and 220 GHz, frequencies that help peel away galactic dust and improve polarization precision.

A cleaner τ also boosts the quest for primordial gravitational waves, the faint “B-mode” swirl pattern inflation may have imprinted on the CMB.

With cosmic variance (the ultimate statistical floor) sitting near ±0.003, CLASS’s mountaintop campaign is now within a factor of three of fundamental limits.

Cosmic polarization and dark matter research

Over the next few Chilean winters, CLASS plans to double its observing time and refine its filtering algorithms to rescue even more low-frequency modes from ground pickup.

If successful, the project could lock τ to cosmic-variance precision and clear a path for ground arrays to chase the polarized fingerprints of inflation itself.

Scientists also expect CLASS to improve constraints on the sum of neutrino masses, which influence how structures form in the universe. Precise τ measurements help to reduce uncertainty in the overall amplitude of early fluctuations – a key parameter in both dark matter and neutrino research.

For now, the new window into the Cosmic Dawn shows that high-altitude hardware can rival satellites in teasing secrets out of the sky’s oldest light. And it all started with a few hardy telescopes braving thin air in the Atacama.

The study is published in The Astrophysical Journal.

Image credit: ESA/Webb, NASA & CSA

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