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The Universe gently glows in gamma radiation.
This is nothing to worry about; we’re not all about to release the Hulk. Well, to be clearer, there is no need to worry unless a) you are an astronomer, and / or 2) you find the Universe to be quite an interesting place and want to know more about it.
You’re in luck: I’m # 1 (and # 2), and if you’re # 2, read on.
The Universe shines with many different flavors of light. For example, cosmic background radiation is a glow all over the sky made up of microwaves (low-energy light similar to radio waves), which remains the energy of the Big Bang itself. There is also a diffuse glow of ultraviolet light from hot hydrogen gas floating around and between galaxies at the edge of the observable Universe.
There is also a diffuse glow all over the sky made of gamma rays. They are extremely energetic forms of light, just like the type of light we see (which we call visible light) except that every gamma ray photon, every particle of light, contains much more energy than a photon of visible light. A parcel More: Although there is no good, tightly and quickly defined lower limit to their energy, gamma rays typically have tens of thousands to billions of times the energy of visible light photons.
And the sky shines from them. It’s not a bright glow, so it’s not like these things are blazing – that would be bad, as they can destroy cells and DNA, leading to unfortunate issues like radiation poisoning and death. . But it is detectable in orbiting observatories sensitive to these high-energy photons, and the source of the glow is unclear.
A new article just published, however, may have shed some light on this problem. If so, this is very important – we like to know where the light is coming from, since this is how the Universe tells us what it does – but it will take a few words to explain it. These are still funny words.
Visible light can be produced by an object if you heat it (think of something like an oven element, which glows red or orange as it warms, or the Sun, which emits visible light because it is made hot gas). But gamma rays cannot be produced this way; if you heat something hot enough to produce a decent amount of gamma rays, it will decay. It’s embarassing.
Gamma rays are created in different ways, but they are not thermal (i.e. not related to the heating of a gas). Typically, in astronomy, they are made when subatomic particles such as protons, electrons, or atomic nuclei are accelerated to a ferociously high speed, just a mustache below the speed of light, usually by extremely strong magnetic fields. . We generically call these particles zippy cosmic rays.
These create gamma rays in several ways. A high energy cosmic ray can strike an atomic nucleus floating in space – what we call the interstellar medium – and create a new subatomic particle called a pawn. The pions are not stable and quickly decay in gamma rays.
The other way is that the cosmic ray hits a low energy photon, such as visible light or infrared photon. They can exchange energy, with the cosmic ray giving much of its energy to the photon, turning it into a gamma ray (this is called inverse Compton scattering and if you’re a super dork, click this link for an extremely nerdy science joke; be well warned, I say).
Hang in there, we’re almost there.
So where do cosmic rays get such high energies? One is in magnetic fields that coil in material encircling a supermassive black hole near the point of no return. These magnetic fields are powerful, and monstrous black holes, located in the center of galaxies, can emit a lot of cosmic rays, and therefore gamma rays. These kinds of galaxies are called blazars, which is just plain cool, and there are a lot of them in the Deep Universe.
In fact, these were thought to be the source of the gamma ray background noise, but it turns out they can’t be. They are small point sources, emission points of gamma rays in the sky, but they are not spread out enough to constitute the diffuse background. Think of it like being outside of a city when it’s foggy; from a distance you see a bunch of street lights which are inconspicuous sources of light, but the fog is blurry and unresolved. Floor lamps = blazars and fog = diffused background. They are different.
Scientists in the new work are instead looking to galaxies that make a lot of stars. If they make a lot of stars, they make massive stars too, and massive stars end up exploding into supernovae. The gaseous debris from the detonated star expands outward, creating extremely powerful shock waves. The gas contains very strong magnetic fields that can move around subatomic particles, accelerating them until they approach the speed of light, after which they escape and flee into space. These then produce gamma rays as described above.
The problem is, the exact mechanics weren’t fully understood… until recently. Some scientists have created physical models of how cosmic rays from supernovae produce gamma rays, and in a new paper a team of scientists have used these models to determine how many gamma rays star-forming galaxies emit and what are their energies. They tested their models against nearby galaxies (like Andromeda and NGC 253) as a proof of concept, and they worked! They therefore used observations of more distant galaxies to determine the emission of gamma rays from galaxies located about 8 to 10 billion light years away; we see galaxies this far from us when the Universe was reaching its peak in its ability to make stars, so most of the gamma rays should be coming from them.
What they found is that their model accurately matches both the brightness of the glow as well as its energy distribution (i.e. how many gamma rays there are from different energies; think of it as the colors of gamma rays). It’s pretty amazing! The models could have had any sort of results, but the fact that they matched the observed glow so well implies that they are doing something right.
So it seems that the glow can be fully explained by fertile galaxies producing baby stars when the Universe was around 4-6 billion years old, some of which exploded, produced cosmic rays which made gamma rays, then they have traveled this terrible distance to fall into the detectors of our gamma telescope.
And that tells us more about what the Universe is! These different diffuse glows are incredibly important; the cosmological diffuse background is considered evidence of the Big Bang model of the Universe, and the ultraviolet glow tells us that stars are born in tiny galaxies just after the Universe has cooled down enough to form stars first place. And now that gamma glow can be used to understand how stars formed and how galaxies behaved billions of years later.
I tend to think of fog as a hindrance to sight. But in fact it’s the fog itself that we need to see to be able to understand the big picture.
The Universe is an amazing place, but it is sometimes also downright poetic.
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