All the light from every galaxy that has ever existed in the Universe has been measured for the first time. Doing so revealed what sort of galaxies modern or ancient - have been responsible for the production of most of that energy.
Nearly every photon of light ever produced is still around, permeating the Cosmos. These photons of light collectively make up the extragalactic background light (EBL). By compiling measurements made by several ground-based and space observatories including the Fermi Telescope, researchers were able to create the most in-depth study ever made of the evolution of the EBL over the last five billion years. Researchers were able to calculate that most of the light produced in the history of the Cosmos was created by galaxies much like we see today. Alberto Dominguez, a postdoctoral researcher from the University of California and six other authors announced the results of their study of the EBL May 24 in the Astrophysical Journal.
Measuring the EBL directly poses a challenging feat - we live surrounded by radiation of all forms which drowns out the faint glow of the extragalactic background light. This is similar to not being able to see the diffuse light of the Milky Way galaxy from the middle of a large city.
Blazars paved the way to being able to measure the wavelengths associated with the EBL. These celestial objects are super-massive black holes at the centers of ancient galaxies, producing massive jets of high-energy photons aimed in the direction of Earth.
When one of these highly-energetic photons collides with a lower-energy photon from the EBL, the two annihilate can each other, creating an electron and its anti-matter counterpart, a positron. Each photon is characterized by having a certain wavelength, and will only interact with photons of compatible energies. Therefore, by measuring how many photons were absorbed and their wavelengths, astronomers can calculate how many EBL photons are present, as well as their energy levels. Current technologies allow researchers to see blazers only out to a distance of about 5 billion light years.
"[T]here are blazars farther away, but we are not able to detect them because the high-energy gamma rays they are emitting are too attenuated by EBL when they get to us - so weakened that our instruments are not sensitive enough to detect them," said Dominguez.
This is the first significant detection of the gamma ray horizon for objects, the distance at which roughly 37% of the gamma rays they emit have been absorbed by the EBL. Some astronomers are comparing the potential scientific impact of this measurement of the EBL to the detection of the cosmic background radiation.
Cosmologists believe that if the could successfully measure the quantity and wavelengths of all the photons produced since the beginning of time, they could unlock secrets of the Universe, such as the shape of early galaxies, and uncover information about the beginning and end of the Cosmos.
