For millions of years after the big bang, the universe was utterly dark. And then there was light.
Based on data gleaned from jets of high-energy radiation emitted long ago, scientists say they now know that stars were present at least as early as 500 million years after the big bang, which gave birth to the universe some 13.75 billion years ago.
The findings also put an upper and lower limit on the amount of light present at that 500-million-year mark, and consequently the size and number of early stars. The stellar population, the researchers confirmed, was quite small.
"We haven't been able to establish yet when the first stars began to shine, but we have peeked into the epoch of our universe when it happened," said study co-author Marco Ajello, an astrophysicist at Stanford University.
The Fermi findings, then, get us a step closer to pinning down the powering up of the first stars. That future discovery should shed new light on the dark period that preceded first light—and on the big bang that preceded everything.
The telltale new data comes from the first ever direct measurement of long-theorized "extragalactic background light," according to the study, published Thursday by the journal Science.
Carrying information about its origins, "the optical and ultraviolet light from stars continues to travel throughout the universe even after the stars cease to shine, and this creates a fossil radiation field," Allejo said.
To measure this light, the Fermi team studied blazars—ancient galaxies that appear superbright because the high-energy jets shooting from their central black holes are aimed directly at Earth. (See "Ultrabright Gamma-ray Burst 'Blinded' NASA Telescope.")
Gamma rays—high-frequency electromagnetic radiation—from blazar jets shine like lighthouse beacons through the "fog" of the cosmic background light, the researchers say.
Speeding through space, the gamma rays collide with that remnant starlight and lose some of their energy—rays that have lost most of their gamma radiation, then, must be from the oldest blazars.
It's these old rays that the Fermi satellite has used to determine the "thickness" of the cosmic light fog, all the way back to 500 million years after the big bang—a stand-in for measuring the stars themselves, which is currently impossible.
Pushing Back to the First Stars
The new study is part of a concerted global scientific effort to "push back to the first stars," in the words of astrophysicst Justin Finke.
Another astrophysicist involved in that "pushing back" effort is Avi Loeb, chair of the astronomy department at Harvard University. He said the new paper's approach is both "novel" and "convincing."
And it adds to evidence that the early universe was relatively star poor, said Loeb, who has quite literally written the textbook on early star formation (it will be published later this year).
"We know from direct observations of galaxies that less than a percent of all the stars in the present-day universe formed in the first billion years after the big bang."
(Related: "Gamma-Ray Burst Caused Mass Extinction?")
Setting the Stage
The 400-million-year pitch-black period following the big bang remains something of a scientific black box. Learning more precisely what led to the formation of stars—and therefore light—will be the job of several sophisticated and immense telescopes expected to be in place by the end of the decade, including NASA's James Webb Space Telescope.
The Fermi results, then, are "setting the stage" for what's to come, University of Texas astronomer Volker Bromm said in a statement.
"In simple terms, Fermi is providing us with a shadow image of the first stars," he said, "whereas Webb will directly detect them."