In 1992, astrophysicist Floyd Stecker of NASA Goddard and his colleagues suggested using this drop-off between the expected and observed gamma rays to measure how much extragalactic background light exists between the blazar and Earth.
But the telescopes also revealed one big problem: They couldn't probe very distant gamma-ray sources, because the extragalactic background light absorbs so many of their gamma rays en route to Earth.
Some physicists have proposed that gamma rays flooding from a blazar could turn into ALPs and travel through the universe unimpeded by the extragalactic background light.
15] times: The sun is a million billion times as bright as the extragalactic background light, as viewed from Earth.
Caption: 1,000 times: Zodiacal light, the visible glow created when sunlight hits dust in the solar system, is 1,000 times as bright as the extragalactic background light.
Caption: Gauging the glow: Astronomers have tracked gamma rays emanating from active galaxies called blazars (yellow ovals, right) to measure the extragalactic background light throughout the universe's history.
Caption: Observing the universe's first stars (illustrated here) remains difficult, but some of their photons traverse the cosmos as extragalactic background light.
Having a source at this distance will allow us to better understand how much background absorption there is and test the cosmological models that predict the extragalactic background light," he noted.
The extragalactic background light (EBL) is the diffuse radiation from all stars and galaxies, a dim but pervasive glow that fills the universe.