Scientists Measure All the Light in the Universe Going Back 5 Billion Years

Extragalactic background light, also known as EBL, is a term that refers to the ancient and young particles of light saturating space and a crucial data point in understanding those secrets in relation to the nature and evolution of the universe that remain locked.

However, because the solar system in which Earth is found is especially bright, accurately measuring the EBL is difficult in a similar manner to that of detecting faces in a crowd from a bright stage. For this reason, telescopes have not yet been able to accurately detect the total EBL and scientists have been left empty handed.

That is, until now.

By measuring the absorption of very high-energy gamma rays by supermassive black holes located in the centers of galaxies, called blazars, researchers have been able to measure the evolution of the EBL of the last 5 billion years indirectly, according to a new study.

The journey from the blazars to Earth is billions of light-years long and fraught with danger and many don’t make it all the way. Rather, they are caught somewhere in between when a high-energy gamma ray photon comes into contact with its counterpart, a much lower energy EBL photon, and both are annihilated.

Ultimately, different energies of the highest-energy gamma rays are waylaid by different energies of EBL photons, which in turn opens an opportunistic window for scientists who can then measure to what degree gamma rays of different energies are absorbed or weakened from blazars at different distances from Earth. In so doing, the researchers are left with a measurement of how many EBL photons of different wavelengths exist along the line of sight from a single blazar to Earth over each distance.

And while this theory was first proved in a study released in fall 2012, the new study marks the first time scientists have measured the evolution of the EBL over a period of time.

The latest results, according to a press release, not only confirm that the kinds of galaxies observed today are responsible for most of the EBL over all of time, but it further sets limits on possible contributions from many galaxies too faint to have been included in the galaxy surveys as well as hypothetical additional sources, such as the decay of possible unknown elementary particles.

The findings are published in The Astrophysical Journal.