Scientists have captured an unprecedented glimpse of cosmic dawn, an era more than 13 billion years ago, using telescopes on the surface of the Earth. This marks the first time humans have seen signatures of the first stars interacting with the early universe from our planet, rather than space.
This ancient epoch when the first stars lit up the universe has been probed by space-based observatories, but observations captured from telescopes in Chile are the first to measure key microwave signatures from the ground, reports a study published on Wednesday in The Astrophysical Journal. The advancement means it could now be much cheaper to probe this enigmatic era, when the universe we are familiar with today, alight with stars and galaxies, was born.
“This is the first breakthrough measurement,” said Tobias Marriage, a professor of physics and astronomy at Johns Hopkins University who co-authored the study. “It was very exciting to get this signal rising just above the noise.”
Many ground and space telescopes have probed the cosmic microwave background (CMB), the oldest light in the universe, which is the background radiation produced by the Big Bang. But it is much trickier to capture polarized microwave signatures—which were sparked by the interactions of the first stars with the CMB—from Earth.
This polarized microwave light is a million times fainter than the CMB, which is itself quite dim. Space-based telescopes like the WMAP and Planck missions have spotted it, but Earth’s atmosphere blocks out much of the universe’s light, putting ground-based measurements of this signature out of reach—until now.
Marriage and his colleagues set out to capture these elusive signals from Earth for the first time with the U.S. National Science Foundation’s Cosmology Large Angular Scale Surveyor (CLASS), a group of four telescopes that sits at high elevation in the Andes Mountains. A detection of this light would prove that ground-based telescopes, which are far more affordable than their space-based counterparts, could contribute to research into this mysterious era.
In particular, the team searched for a particular polarization pattern ignited by the birth of the first stars in the universe, which condensed from hydrogen gas starting a few hundred million years after the Big Bang. This inaugural starlight was so intense that it stripped electrons off of hydrogen gas atoms surrounding the stars, leading to what’s known as the epoch of reionization.
Marriage’s team aimed to capture encounters between CMB photons and the liberated electrons, which produce polarized microwave light. By measuring that polarization, scientists can estimate the abundance of freed electrons, which in turn provides a rough birthdate for the first stars.
“The first stars create this electron gas in the universe, and light scatters off the electron gas creating a polarization,” Marriage explained. “We measure the polarization, and therefore we can say how deep this gas of electrons is to the first stars, and say that's when the first stars formed.”
The researchers were confident that CLASS could eventually pinpoint the target, but they were delighted when it showed up early on in their analysis of a key frequency channel at the observatory.
“That the cosmic signal rose up in the first look was a great surprise,” Marriage said. “It was really unclear whether we were going to get this [measurement] from this particular set of data. Now that we have more in the can, we're excited to move ahead.”
Telescopes on Earth face specific challenges beyond the blurring effects of the atmosphere; Marriage is concerned that megaconstellations like Starlink will interfere with microwave research more in the coming years, as they already have with optical and radio observations. But ground telescopes also offer valuable data that can complement space-based missions like the James Webb Space Telescope (JWST) or the European Euclid observatory for a fraction of the price.
“Essentially, our measurement of reionization is a bit earlier than when one would predict with some analyzes of the JWST observations,” Marriage said. “We're putting together this puzzle to understand the full picture of when the first stars formed.”