Researchers at MIT, the California Institute of Technology, and the University of California at San Diego have peered so far back in time that they've found matter that pre-dates the creation of heavy elements.
It's generally accepted that in the minutes following the Big Bang, protons and neutrons collided in nuclear fusion reactions to form hydrogen and helium.
As the universe cooled, fusion stopped generating these basic elements, leaving hydrogen as the main constituent of the universe. Heavier elements, such as carbon and oxygen, formed when the first stars appeared.
To make the discovery, the team analyzed light from the most distant known quasar, a galactic nucleus more than 13 billion light-years from Earth.This gives a snapshot of the universe just 750 million years after the Big Bang.
Analysis of the quasar's light spectrum provided no evidence of heavy elements in the surrounding gaseous cloud - a finding that suggests the quasar dates to around the same time as the universe's first stars.
"The first stars will form in different spots in the universe... it's not like they flashed on at the same time," says Robert Simcoe, an associate professor of physics at MIT. "But this is the time that it starts getting interesting."
Until now, scientists have only been able to observe objects that are less than about 11 billion years old. All of these show heavy elements, suggesting stars were already fairly plentiful at that point.
"Prior to this result, we have not seen regions of the universe this old and devoid of heavy elements, so there was a missing link in our understanding of how the elemental content of the universe has evolved with time," says John O'Meara, an associate professor of physics at St. Michael's College in Vermont.
"[This] discovery possibly provides such a rare environment where the universe had yet to form stars."
The research appears in Nature.