First Glimpse of Moving Starlight

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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.

Gamma Ray Attenuation

Attenuation simply means that between us and the blazar, photons have been absorbed, meaning the light appears less intense than it should. The precise reason for this lies in the rather bizarre realm of quantum physics.

The universe at the quantum level, is a very strange place indeed. Einstein’s theory of relativity famously states that E = mc² — or in other words, mass and energy are two sides of the same coin. In deep space, photons are so numerous that they may collide with each other. When they do, if the combined energy of the two photons is high enough, they can spontaneously create matter (this is studied in a field known as two photon physics).

When photons create matter this way, they produce a pair of matter and anti-matter particles which then proceed to go their separate ways. Only photons with specific energies can interact in this way — so by looking at blazars and measuring which gamma ray wavelengths are attenuated, Dominguez and his colleagues could work backwards and find out which photons they were interacting with.

This gives an indirect measure of which photons were a part of the EBL that the blazar light traveled through. Because light only travels at a finite speed, looking at more distant blazars allows the EBL to be measured further away.

For a long time, this gamma ray attenuation had only ever been predicted. Until late last year, when observations taken with the Fermi gamma ray observatory confirmed that gamma rays from distant blazars are indeed absorbed before they reach us.

The Cosmic Gamma Ray Horizon

In order to actually measure the gamma ray attenuation, Dominguez and company first had to look at the blazars at lower energies, using a variety of different telescopes. Looking at x rays and other lower energy photons, they managed to calculate how bright the blazars should appear at gamma ray energies.

They then used several more telescopes to directly observe how bright the blazars are in gamma rays. The difference between the predicted and observed brightnesses gave the attenuation and, in turn, the EBL which would have caused that attenuation.

This research gives the first ever significant detection of a region of space known as the Cosmic Gamma Ray Horizon — the distance at which roughly one third of gamma rays at a certain specific energy have been absorbed.

By looking at the EBL over the past 5 billion years, cosmologists can learn about how galaxies change as the Universe ages. Whether or not ancient galaxies work the same way as modern ones is very important to our understanding of the Universe.

As it happens, the kinds of galaxies observed in the Universe today are responsible for most of the extragalactic background light over all time. Because there’s still a lot out there in the Universe which we don’t fully understand, it also sets a limit on any other light sources which we may not yet know about.

Observations like these do wonders for our understanding of the Universe and the way in which it works. It’s exciting to know that no matter how much we learn, there’s seemingly always more to be discovered out there. And perhaps most of all, we can learn all of this from the humble photons which are all around us.

Though it’s not exactly light reading, the research paper for this work is available to read online at arXiv, in case you’d like to know more about the technical details.

Image: Artists impression of a jet of radiation being shone outwards by a galaxy’s supermassive black hole. When these galactic jets are pointing directly towards Earth, the galaxy is observed as an extremely bright “blazar.” Credit: NASA/JPL

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