NASA's Fermi Gamma Ray Telescope Has Completed its Primary Mission of Measuring the "Fog" of Ancient Starlight

Light measured across the visible Universe; the green dots are blazers (see below) 

Per NASA (on Thursday, November 1):

Astronomers using data from NASA's Fermi Gamma-ray Space Telescope have made the most accurate measurement of starlight in the universe and used it to establish the total amount of light from all of the stars that have ever shone, accomplishing a primary mission goal.

"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," said lead scientist Marco Ajello ... .

NASA used the Fermi Telescope to measure "blazers," which are galaxies with giant black holes at their center shooting out high energy jets perpendicular to the plane of the galaxy (see diagram, below). The jets are rich in gamma rays -- the most energetic source of light -- though their strength attenuates as they interact with other light. So, measuring the attenuation of the gamma rays from blazers at different distances (and, hence, ages) from Earth gives us an accurate measurement of the total amount of light that the Universe has ever generated -- the thickness of the background "fog" of ancient light that permeates space.