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. The ALPs could then reconvert to an ordinary photon before reaching Earth, and so astronomers observing them would never know the change had happened.
Caption: [10.sup.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.
Coauthor David Williams, adjunct professor of physics at UC Santa Cruz, said the findings may indicate something new about the emission mechanisms of blazars, the extragalactic background light, or the propagation of gamma-ray photons over long distances.
The extragalactic background light (EBL) is the diffuse radiation from all stars and galaxies, a dim but pervasive glow that fills the universe.
By using HESS to measure the gamma-ray spectra emitted by relatively close blazars, they evaluated the effect of the interaction of highly energetic gamma rays with the diffuse extragalactic background light within a sphere of a three billion light year radius.
(2) The Imprint of the Extragalactic Background Light in the Gamma-Ray Spectra of Blazars.