For the first time, scientists have used Earth-based telescopes to peer back into the cosmic dawn — an era more than 13 billion years ago when light from the first stars began reshaping our universe.
The residual light from this ancient epoch is millimeters in wavelength and extremely faint, meaning that although space-based observatories have been able to peer into it, the signal is drowned out by the electromagnetic radiation in Earth's atmosphere before ground-based telescopes can detect the primordial light.
But now, by deploying a specially designed telescope, scientists at the Cosmology Large Angular Scale Surveyor (CLASS) project have detected traces that the first stars left on the background light of the Big Bang. They published their findings June 11 in The Astrophysical Journal.
"People thought this couldn't be done from the ground," study co-author Tobias Marriage, CLASS project leader and a professor of physics and astronomy at Johns Hopkins University, said in a statement. "Astronomy is a technology-limited field, and microwave signals from the Cosmic Dawn are famously difficult to measure. Ground-based observations face additional challenges compared to space. Overcoming those obstacles makes this measurement a significant achievement."
The CLASS observatory sits at an altitude of 16,860 feet (5,138 meters) in the Andes mountains of northern Chile's Atacama desert. The telescope, which obtained its first light in 2016, is tuned to survey the sky at microwave frequencies. Besides enabling it to map 75% of the night sky, the telescope's unprecedented sensitivity lets it receive microwave signals from the cosmic dawn, or the first billion years of the universe's life.
For the first 380,000 years after the Big Bang, the universe was filled with a cloud of electrons so dense that light couldn't travel across it. But our cosmos eventually expanded and cooled, and the electrons were captured by protons to form hydrogen atoms.
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These hydrogen atoms not only enabled microwave-wavelength light to move freely — filling space with the cosmic microwave background (CMB) — but also, where it was dense enough, collapsed under gravity and ignited to form the first stars. The light from these stars then reionized pockets of unclumped hydrogen gas, separating their electrons so that some collided with light from the CMB, causing it to become polarized.
The signal from this polarized portion of the CMB is a vital part of the cosmological puzzle; without it, our picture of the early universe remains muddy.
And while efforts from past space-based telescopes, such as NASA's Wilkinson Microwave Anisotropy Probe (WMAP) and the European Space Agency's Planck space telescope, have filled in parts of this gap, their pictures contain noise and, being satellites, could not be tweaked and improved once deployed in orbit.
"Measuring this reionization signal more precisely is an important frontier of cosmic microwave background research," co-author Charles Bennett, a physics professor at Johns Hopkins who led the WMAP space mission, said in the statement.
To arrive at these observations, the researchers compared CLASS telescope data with that from the Planck and WMAP missions, narrowing down a common signal for the polarized microwave light.
"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," Bennett added. "By analyzing additional CLASS data going forward, we hope to reach the highest possible precision that's achievable."
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