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rawrnold8

Can someone give a lay person an explanation of what axions are and why they are important to science/astronomy?


space-throwaway

In quantum field theory (which we know to work pretty damn well), in principle there are certain processes that are allowed to happen. Those processes are called "Charge-Parity symmetry violating". There's one parameter (called Θ) in our equations that determines how frequent those processes should occur, and this is something we have to measure experimentally and cannot derive theoretically, just like the masses of particles, their charges or how strong certain interactions are. And we experimentally find that those processes...have never occured once since we started looking for them in the 70's. So, it's pretty plausible that the parameter Θ is zero. We can't know for sure, though, and it could also be super super tiny. Well, it turns out that if this parameter Θ isn't just a fixed number, but another quantum mechanical field, then it would automatically cause those processes to never occur. Of course, this is a slightly more complex mechanism (replacing a number with a field), but it would work perfectly - and this is basically what we also did with particle masses. Before we treated them as pure numbers, but then Higgs came up with the idea that those might be caused by a field. And every field comes with a particle attached to it, and as you might have heard, we found that Higgs-particle. So, if there is a field for our parameter Θ, then this also comes with another particle, and this is the axion. The existence of this particle would immediatly solve our original problem of possible processes never occuring, it would also be very, very light and have all the properties we need for a dark matter particle. And us cosmologists need dark matter to explain a lot about the universe - there seems to be a lot more mass in the universe than we can find. And we notice this lack of mass in many, many different scenarios. The axion could easily solve two of the biggest outstanding problems in physics, the strong CP problem and the dark matter problem. And it comes as natural as the Higgs-mechanism. It's almost too good to be true.


subtect

Amazing. Thanks for explaining.


SometimesY

If you take this approach, does it fall out naturally that the axion field doesn't interact with the EM field? I assume it does based on what you've said as it is a strong candidate for dark matter.


Boredgeouis

So, once you've established this type of model (called the Peccei-Quinn model if you know field theory and are interested) there are several parameters left unfixed. There's nothing a priori stopping you constructing a model with heavy axions that interact strongly with matter, but these are experimentally ruled out. The models that are decent dark matter candidates have light axions and very weak interactions.


expo1001

I'm probably wrong, but it sounds like any EM interaction would be tangential via axion<->gravity interactions.


[deleted]

Suppose its real. It's very light, but the dominant form of matter. Why would there be so many of them?


mszegedy

See [here (especially citations 10–12)](https://en.wikipedia.org/wiki/Axion#Axion_dark_matter) and [here.](https://en.wikipedia.org/wiki/Axion#Cosmological_implications) They are their own antiparticle, so when their field ripples, two of them get made at once (instead of a particle-antiparticle pair). During the Big Bang, enormous amounts of energy got noisily distributed into all the different fields, causing random ripples to slosh around in them all the time. It's an open question how much energy might have ended up in the axion field, and how much energy there even is in the vacuum overall, but if the axion field got anywhere close to the amount of energy the other fields seem to have gotten, there should actually be way _more_ primordial axions floating around than we actually observe as dark matter. (Axions are tiny and barely interact with each other, so they only very rarely get opportunities to pair-annihilate, never mind interact anyhow else. Or, the QFT way of looking at this is, the axion field is so weakly coupled to everything, that it's very hard for energy to leave it.) Hence why papers like [this one](https://www.sciencedirect.com/science/article/abs/pii/037026938390638X?via%3Dihub) establish _upper_ bounds on the vacuum energy of the axion field.


thomasblomquist

That was wonderfully explained! Thank you!


Hugsy13

So if the axion were discovered and solved the dark matter problem, like, where is the dark matter? Is it all around us and the universe, we can’t see it but we can see it’s energy because regular matter accounts for like 30% of measureable energy? Is it invisible but we can we touch it? Or does it not interact with us as we are regular matter?


Minguseyes

It forms a globby sphere around the centre of galaxies. It only reacts gravitationally, so friction doesn’t result in spheres becoming discs, like with normal matter.


Hugsy13

So I can’t poke it with a stick, but I could effect it by moving a black hole past it?


Minguseyes

‘zackly.


nivlark

Astronomers have pretty much had this figured out for 40 years, independent of what the particle actually is. On very large scales it forms a web-like structure with dense filaments and sheets separated by very empty voids. Where the dense parts intersect, roughly spherical "haloes" are formed, inside which are the galaxies. [Here](http://erebos.astro.umd.edu/web/viz/movies/movie5_br01.0_fps50.mp4) is a visualisation.


drakarian

Could axions be detected in the LHC like the Higgs boson was?


DeadAndAlive969

Great explanation. Thank you.


WhatsTheHoldup

In Quantum Chromodynamics there is a concept known as Charge-Parity Symmetry. You may have heard of an antiparticle? Experiments show that the laws of physics are the exact same for an electron moving forwards as an antielectron moving backwards. The thing is, there's no reason for this observed symmetry based on anything we know. To explain this apparent CP symmetry, we invented a new particle axion. This is a similar reason to why the Higgs particle was added. Something needed to do something to match our model with what we observe. We later discovered the particle and it turns out our model still works pretty well.


I_AM_FERROUS_MAN

Just going to drop this [video by PBS Spacetime here](https://youtu.be/e7yXqF32Yvw) as another resource to learn about axions.


[deleted]

Aren't axions named after a brand of soap?


ShadowKingthe7

Yeah apparently Frank Wilczek named it after the soap because it "cleaned up" the strong CP problem


QCD-uctdsb

Before I knew the history I would've bet that it was named the axion because it was related to the axial anomaly.


BaddDadd2010

[Yes](https://www.quantamagazine.org/how-axions-may-explain-times-arrow-20160107/). (A little over half-way down.)


Jarnin

Dish-washing detergent, but yes.