From the June 2015 issue

Web Extra: Gamma rays’ long road toward Fermi

Watch a gamma ray travel from its source in a faraway galaxy, through the extragalactic background, and finally to Earth, where the Fermi telescope can measure it.
By | Published: June 29, 2015
This NASA animation shows a gamma ray traveling from its blazar source, encountering the extragalactic background, and being absorbed by the Fermi telescope.
There are many ways to measure the extragalactic background (EBL), the universe’s glow from myriad sources. One of these measurement techniques uses the light from faraway blazars — active galaxies with supermassive black holes at their centers, blasting jets of high-speed particles and radiation out into space. Scientists study the gamma rays streaming from these high-powered jets. The gamma rays have a very long journey between their blazars and Earth, where astronomers can measure them. Along the way, some of these gamma rays will strike an intervening EBL photon and annihilate into a positron and an electron.
There is a special relationship between the energy of the gamma ray and the energy of the EBL photon. A higher-energy gamma ray annihilates only with a lower-energy EBL photon, and vice versa. Astronomers can therefore use the missing gamma rays to tell them about the EBL interference.

In this animation, watch a gamma ray complete its long journey from its blazar origins, through the EBL, to the Fermi telescope orbiting Earth that measures its final energy.

Video credit: NASA/Goddard Space Flight Center/Cruz deWilde