Extragalactic background light: The history book of the universe

Meeting #3 of the Columbia VERITAS journal club

The most famous light in the universe might be the cosmic microwave background, or CMB. A relic of the first million years of the universe, the CMB has provided evidence for the Big Bang theory and the cosmological standard model. Along with other types of background “radiation”, including the cosmic neutrino background, it’s been one of the most important tools for early-universe cosmology.

An all-sky map of the CMB, from WMAP’s nine-year dataset. Image credit: NASA/WMAP Science Team.

I’m not going to talk about the CMB, though. I’m going to talk about a more modern cousin of it — the extragalactic background light, or EBL. The reason why is that I spent most of my Wednesday trying to understand what the EBL is, how it’s produced, and why another student described it as “the history book of the universe”.

Keep reading if you want to know why a bunch of photons are so amazing (spoiler: it turns out that the extragalactic background light essentially absorbs other photons).

At Columbia, the VERITAS group has a weekly journal club. What this means is that each Wednesday afternoon, we put aside tea, croissants and coding and discuss a paper related to what we’re working on, chosen by popular vote. Two weeks ago, we read a paper on that blazar neutrino you might have heard about. This week, as you might have guessed, our paper was on the extragalactic background light.

A diagram of IceCube, the detector that discovered the first blazar neutrino. Image credit: IceCube Science Team — Francis Halzen, Department of Physics, University of Wisconsin, CC BY 3.0.

Journal clubs are actually a lot of fun — I’ve taken one for credit the past two spring semesters at Swarthmore. I like them because sometimes nobody sitting around the table fully understands what we’re discussing, even the graduate students. It’s a shared learning experience, and it provides an environment in which every feels comfortable asking basic questions.

This week, the paper we read was titled A GeV-TeV Measurement of the Extragalactic Background Light (Desai et al. 2019), and it was exactly what you might imagine — measuring the spectrum of the EBL by observing sources of high-energy photons, with energies of gigaelectron volts and teraelectron volts (GeV and TeV). Before I talk about the paper, though, let me tell you what the EBL actually is.

The observable universe is filled with hundreds of billions of galaxies and billions of billions of stars. All of that luminous matter (plus some active galactic nuclei) creates a lot of photons, which travel through intergalactic space and reach us. Some of those photons are absorbed by dust and gas and reradiated, also eventually reaching us. Put together, these vagabond photons make up the extragalactic background light. It’s as simple as that — wandering bundles of light.

Figure 2, Cooray 2016. The spectrum of the EBL, in the wavelength bands we care about, without GeV/TeV data.

The EBL extends across many wavelength bands, but what I care about for now is the portion of it from 0.1 microns to 1000 microns (a micron is a millionth of a meter). This includes infrared, optical and ultraviolet light — just what you’d expect to see from stars and cool dust. Now, these fairly normal photons do something really weird. When high-energy gamma rays travel through the EBL — the kind Desai et al. were interested in — they can actually be absorbed by the EBL photons. Isn’t that wild?

This absorption happens through something called pair production. It turns out that a gamma ray, along with an infrared/optical/ultraviolet photon, has enough energy to form an electron-positron pair. Essentially, two photons go in, and matter and antimatter come out. The EBL reduces the gamma ray signal just like a dense fog absorbs normal light.

Figure 1, Dwek & Krennrich 2012. Photon-photon interactions with the EBL forms electron-positron pairs.

We can study the EBL through blazars, active galactic nuclei with jets that are pointing right at us. Blazars are luminous and great sources of high-energy gamma rays. They’re also far enough away from us that light from them has plenty of time to interact with background radiation. We know what their spectra should look like, barring absorption by the EBL, so if we can observe the spectra of a bunch of blazars, we can figure out the spectrum of the extragalactic background light.

With me so far?

My very short summary might have made it seem like studying the EBL is easy, but the exact opposite is true. You need a lot of blazars — dozens or hundreds — to get a good idea of the EBL’s spectrum. Moreover, you need to ensure that the model of the EBL you chose works well. An incorrect choice of model can throw your entire results off — and we don’t have great ideas for what the right model of the EBL is!

Here’s where Desai et al.’s paper comes in. They decided to look at data from Fermi and ground-based imaging atmospheric Cherenkov telescopes (like VERITAS!) from observations of over 100 blazars, and fit four spectral models to the data. Then they averaged out the results to get a model-independent idea of the EBL’s spectrum from the infrared to the ultraviolet. This is an enormous portion of the spectrum, and with a large sample size, the results look pretty good.

Fig. 3, Desai et al. 2019. An extended plot of the spectral intensity of the EBL, taking all of the models (and previous data points) into account. The thickness of the two curves represents the 68% confidence interval of the fits.

So, why do we care? Well, we blazar people care because it affects our data! The absorption of gamma rays by the EBL does affect the signals we see. Aside from that, though, the EBL might let us look back into the universe and trace the evolution of starburst galaxies, young, gas-rich objects rapidly forming stars. These stars should contribute to the EBL, and if we can determine the EBL’s spectrum even more precisely, we might be able to better understand star formation across the universe further back in time.

That’s the extragalactic background light! Once we understand it well enough, it truly can act as a history book of the universe, because it contains traces of stars and galaxies throughout time. There’s information locked in there, waiting to be found. We just have to do a bit of digging to get to it.

You can read my introduction to my research this summer here. Yesterday, I wrote about my ongoing journey with Bash. Tomorrow’s blog post topic? We’ll see what comes up!