Blazars, some of the most powerful phenomena in the universe, are gigantic black holes at the centers of galaxies that are gorging on matter and shooting out jets of particles and light. This light is so bright astronomers can spot blazars from billions of light-years across the universe.

Recently, scientists used blazars to see another type of light that would otherwise be impossible to detect. The extragalactic background light (EBL) is a radiation field pervading space that includes the light emitted by all the stars in all the galaxies over the entire history of the universe. Despite its plentitude, this light is hard to see because it is diffuse throughout the sky and is outshone by brighter foreground sources. Some of the researchers who finally detected the EBL in 2013 after roughly a decade of searching describe how they did it with the help of blazars in Scientific American’s June feature article “All the Light There Ever Was,” by Alberto Domínguez, Joel R. Primack and Trudy E. Bell.

Credit: NASA's Goddard Space Flight Center

Blazars emit copious amounts of gamma rays, which are very high-energy photons. As the gamma rays fly through space they sometimes collide with lower-energy EBL photons, and the two can destroy one another. Thus, the farther blazars’ light travels, the more it will tend to get dimmed through interactions with the background light. The researchers figured out how to calculate how bright the gamma rays should have been when originally emitted by blazars, and then compared those figures with measurements of various blazars’ actual brightness as seen from Earth to determine how much light had been lost to the EBL, which in turn gave them the first measurement of the EBL itself.