The chemistry of the cosmos today is not what it used to be. The stars and planets and interstellar gas around us are laced with carbon, oxygen, and many other elements heavier than hydrogen and helium—the only substances to have existed for a few hundred million years after the big bang. Until recently, nobody had seen any of that primordial gas from those early days of cosmic history. Now, astronomers have discovered two clouds of that pristine material  in a distant corner of the universe more than 11 billion light-years away.
"We've searched carefully for oxygen, carbon, nitrogen and silicon—the things that are found on Earth and the sun in abundance," Michele Fumagalli, an astronomer at the University of California, Santa Cruz, says in a press release issued by the university. "We don't find a trace of anything other than hydrogen and deuterium (an isotope of hydrogen)." Fumagalli and his colleagues report the discovery online today in Science.
The finding provides key observational evidence for a widely accepted account of the universe's early evolution. When the universe began, researchers believe, it was made up of gas containing light elements—mostly hydrogen and helium. The first stars formed from this material, some 300 million years after the big bang. As these early stars burned their fuel, they fused the lighter atoms to produce heavier elements like carbon and oxygen, which astronomers refer to as metals. These heavier elements were spewed into interstellar space when the stars exploded as supernova. The birth and death of later generations of stars—made from gas polluted with these heavier elements—served to add even more heavy elements into the mix, making the overall chemistry of the universe increasingly metal-rich.
Fumagalli and his colleagues used the Keck telescope to look at the faraway universe, dating back to a mere 2 billion years after the big bang. Because the universe was still young at the time and hadn't yet been so heavily polluted by metals, the researchers hoped to detect relatively pristine gas clouds. To figure out the chemical composition of the clouds, they studied the spectrum of light from a background quasar that had traveled through the gas on its way to the telescope.
"For these two clouds, we clearly see multiple lines associated with the presence of hydrogen and, in one of the clouds, also lines associated with deuterium," hydrogen atoms with extra neutrons on board, Fumagalli tells ScienceNOW. "But we do not see any line characteristic of metals (e.g. carbon, oxygen, silicon) in either of them. The lack of these lines tells us that the gas is metal-free."
One implication of the finding is that the explosions of the earliest stars, believed to have occurred between 800 million and 1 billion years after the big bang, did not scatter heavy elements throughout the universe. "The fact that we see gas without metals 2 billion years after the big bang is telling us that the ejection and dispersion of metals in the Universe is not a completely efficient process," Fumagalli says.
Beatriz Barbuy, an astrophysicist at the University of São Paolo in Brazil, says the discovery supports the idea that the first stars "did form from non-metal clouds." She says the finding will help astronomers build better models of early star formation.
Fumagalli says he and his colleagues will follow up with studies to characterize the environment of the two clouds. "At the moment, we do not know if they lie in empty regions or in proximity of galaxies and we need to take images and other spectra of that part of the sky to answer this question."