Their guide? All the light "from all of the stars that have ever shone"
If we're stardust, and if we're golden, we're now just a little bit closer to understanding what that means. This afternoon, NASA made an announcement: Astronomers, using data from the Fermi Gamma-ray Space Telescope, have developed a new way to understand the most ancient light of the universe.
And: all the light. Or, as NASA sums it up, appropriately pragmatically and appropriately poetically: "the total amount of light from all of the stars that have ever shone."
That light -- photons from primordial stars, formed some 400 million years after the big bang -- is still extant in the universe. It is more commonly known as extragalactic background light, or EBL -- which is an accumulation of all the radiation in the universe, but which is also, more awesomely, all the starlight and all the goldenness that that universe will ever know. (It is also, less awesomely, "a kind of cosmic fog.") One of Fermi's main missions has been to understand the EBL more clearly, to use its clues to create a skeletal guide to the stars of the early universe.
And today: Mission accomplished. Marco Ajello, a postdoctoral researcher at the Kavli Institute for Particle Astrophysics and Cosmology at Stanford and the Space Sciences Laboratory at Berkeley, has led a team that turned to gamma rays -- the most energetic form of light we know of -- to understand those ancient stars. "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 we can explore using gamma rays from distant sources," Ajello said.
Those distant sources are blazars -- compact quasars, or galactic nuclei -- that boast more than a billion times the energy of visible light. Ajello and his team, for this project, studied 150 of them. Blazars are powered by massive black holes that emit jets of energy. And those jets include gamma rays. Gamma rays were the keys to the EBL project: When those rays collide with ancient photons, they're converted into electrons and their antimatter (positrons). That collision effectively dims their light -- meaning that gamma rays, when they finally hit our Fermi telescope, have an energy that belies their path through the universe. Using measures of that energy, Ajello and his colleagues were able to determine the amount of photons between Earth and the blazars.