Astronomers have discovered two clouds of gas that are still in the same state as they were just minutes after the Big Bang.
By the time a few hundred million years passed, the primordial gas, consisting of hydrogen and helium, condensed to form the first stars, where heavy elements were forged.
The new discovery is the first time astronomers have discovered a part of the universe free of such 'metals', and the first evidence that the primordial gas predicted by the Big Bang theory really exists.
"As hard as we've tried to find pristine material in the universe, we have failed until now," says J Xavier Prochaska, professor of astronomy and astrophysics at the University of California, Santa Cruz.
"This is the first time we've observed pristine gas uncontaminated by heavier elements from stars."
The researchers discovered the clouds of gas by analyzing the light from distant quasars, using the HIRES spectrometer on the Keck I Telescope at the M Keck Observatory in Hawaii.
"We can see absorption lines in the spectrum where the light was absorbed by the gas, and that allows us to measure the composition of the gas," says graduate student Michele Fumagalli.
Until now, the lowest measurements of metal abundance in the universe were about one-thousandth the 'metallicity' of the sun.
"People had thought there was a 'floor' to metallicity, that nothing could be less than one-thousandth the solar enrichment. That's because the metals produced in galaxies were so widely dispersed in the universe," says Fumagalli. "So this was unexpected. It challenges our ideas about how metals are dispersed from the stars that produce them."
The researchers believe the pristine gas is about one-ten-thousandth as metallic that of the sun. At the other extreme, stars and gas with the highest metallicities are almost ten times as enriched as the sun.
"The metal abundance in different pockets of the universe covers a tremendous range," Prochaska said. "So these findings place new constraints on our understanding of how metals are distributed throughout the universe."