The entire cosmos is filled with light that has never been seen. Every star in every galaxy that’s ever existed in our universe has emitted starlight. Curious as it may sound, almost all of that light is still traveling through the cosmos. Yet remarkably enough, a group of researchers has managed to get a glimpse of this starlight as it’s still traveling.
This sea of photons, some newly created, some fantastically ancient, which surrounds everything is known as Extragalactic Background Light (or EBL for short). In a similar way to the Cosmic Microwave Background (CMB) — the leftover radiation from the big bang — measuring the EBL is rather important in cosmology.
Most recently, new research published by Alberto Dominguez, together with six co-authors, gives the best measurement ever made of this background light, showing how the EBL has varied over the past 5 billion years!
Measuring Light Before it Reaches You
Physicists consider light to be comprised of tiny, discreet packets of energy, referred to as photons. Photons that reach our eyes, we see. Whether emitted by shining stars, generated by computer screens, or reflected from surfaces, these photons allow us our view of the Universe. But unless they reach our eyes, we’d never even know they were there.
Consider that thought, and you might realize that our universe is actually filled with photons. Because space is mostly empty, most of the photons leaving a star like the Sun will never land on any surface. Only a small percentage illuminate planets like ours, and the number which reach planets around other stars is infinitesimal, compared to the total number of photons the Sun produces. Soon enough, these photons will actually leave the galaxy. Once they do, they become denizens of intergalactic space, with only a tiny chance of ever entering another galaxy.
Needless to say, seeing light while it’s still traveling is no easy task. In fact, it’s actually impossible to measure directly. In order to glimpse the EBL, Dominguez’s team needed to take some rather ingenious measures. They turned their attention to a type of galaxy known as a blazar. Blazars are distant galaxies whose central supermassive black holes are pointed directly towards Earth. This means that intensely bright light emitted by those black holes is easy to spot, even from halfway across the Universe.
By looking at gamma rays emitted by those blazars — or more specifically, the attenuation of certain energies of gamma ray — the scientistss managed to accurately gauge what photons were in the intergalactic space between us and the blazars.
Read the rest here First Glimpse of Moving Starlight