Since the Big Bang, the universe has been emitting light. Even today, these photons speed through our universe, allowing scientists to see stars that have long ago vanished. Now, researchers have examined extragalactic background light (EBL) and have measured its evolution over the past five billion years for the very first time.

An accurate measurement of the EBL is fundamental to cosmology. The bath of ancient and young photons suffusing the universe today, the EBL can help tell scientists about the early origins of our universe. After all, ever since the Big Bang, photons--from ultraviolet to far infrared wavelengths--created by stars and galaxies still speed through our universe.

Yet actually measuring the EBL is no easy task. Earth is inside a very bright galaxy with billions of stars and glowing gas. In fact, sunlight scattered by all of the dust in the plane of Earth's orbit creates the zodiacal light radiating across the optical spectrum down to long-wavelength infrared. This means that ground-based and space-based telescopes have so far been unable to reliably measure EBL directly.

That's why astrophysicists developed an indirect method. They measure the EBL through measuring the attenuation of very high energy gamma rays from distant blazars. Blazars are supermassive black holes in the center of galaxies with brilliant jets directly pointed at us like a flashlight beam. Not all of these high-energy gamma rays emitted by a blazar make it to Earth; instead, some strike EBL photons on the way. When a high-energy gamma ray photon hits a much lower energy EBL photon, both are annihilated and produce an electron and a positron. By measuring how much gamma rays of different energies are attenuated or weakened from blazars at different distances from Earth indirectly allows researchers to measure the EBL.

In this latest study, though, researchers were able to measure the evolution of the EBL over the past five billion years. In order to accomplish this, researchers compared Fermi findings to intensity of X-rays from the same blazars measured by X-ray satellites and lower-energy radiation measured by other spacecraft and ground-based observatories. From these measurements, the scientists calculated the blazars' original emitted gamma-ray brightnesses at different energies.

They weren't done yet, though. The researchers then compared these calculations with direct measurements from special ground-based telescopes of the actual gamma-ray flux received at Earth from the same blazars. This, in turn, allowed the researchers to quantify the evolution of the EBL, essentially measuring how the EBL changed over time as the universe aged.

So what does this mean? The latest results confirm that the kinds of galaxies observed today are responsible for most of the EBL over time. It also sets limits on possible contributions from many galaxies too faint to have been included in galaxy surveys. In addition, it allows researchers to learn a little bit more about the origins of our universe and the Big Bang.

The findings are published in The Astrophysical Journal.