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Top Dark Matter Candidate Loses Ground to Tiniest Competitor (quantamagazine.org)
59 points by theafh 5 days ago | hide | past | web | favorite | 10 comments

So one of the main differences between Axions and WIMPs is that WIMPs:

> [Interacts] only through the weak nuclear force and gravity, or possibly other interactions with cross-sections no higher than the weak scale [1]

But Axions interact via the electromagnetic force which is stronger than the weak nuclear force [2]

[1] https://en.wikipedia.org/wiki/Weakly_interacting_massive_par...

[2] https://en.wikipedia.org/wiki/Axion

There are two points above that require some care:

First: If the axion exists, its electromagnetic coupling is extremely small.

The axion-photon conversion would be a Primakoff process, which can proceed through a loop. Loops suppress cross-sections drastically. The axion is not electrically charged -- we would know all about them by now (and they could not be the dark matter).


If the axion is the mechanism by which the CP problem is resolved and the dark matter is composed of axions, then there are predictions for the strength of the effective axion-photon coupling constant. What is special about the ADMX experiment, and why Quanta is writing about them, is that ADMX is beginning to explore that region in parameter-space in a definitive manner. If both of those conditions are met, there are now masses which the axion cannot have, or ADMX would have seen them.

Second: The weak force is not, in many senses, radically weaker than the electromagnetic force. We don't experience it in our daily lives because the force-mediators (the W and Z) are massive, while the photon is massless. Massive force-mediators severely restrict the distance over which a force can act. The coupling constants of the known forces "run" with energy, and appear to roughly match at the "GUT" scale: https://en.wikipedia.org/wiki/Grand_unification_energy

Thanks for the clarification and details - I was kind of unclear on why Axions are not considered to be weakly interacting and therefore not WIMPs and my comment is based on what I found in my brief search.

I recently read another article suggesting the universe expansion might be anisotropic. The most interesting part was mentioning that data from supernova standard candles had been normalized to fit with the assumption that expansion is isotropic.

Once they removed those adjustments to the data it turned out that expansion is a local phenomenon only for our galaxy and nearby galaxies.

Dark matter != dark energy - dark energy is what is proposed to be the cause of the expansion of the universe, dark matter is what binds galaxies together.

For others, the article mentioned is likely: https://phys.org/news/2019-11-evidence-anisotropy-cosmic.htm...

Ah, yes, sorry. I didn't realize this article talks about dark matter, not dark energy.

It's in the first three words of the headline though

Also worth noting is that it isn't necessarily that expansion anisotropic. It's the acceleration of expansion that is anisotropic.

> who proposed a way to restore balance to the strong force

Reads like Star Wars :-)

Yeah, this strikes a good balance between technical correctness and understandable analogy. I also liked:

> A neutron with lopsided charge would fail CP symmetry, because reflecting it would flip its electric field relative to its intrinsic angular momentum, an effect similar to looking in a mirror and seeing yourself wearing your sweater on your legs and your jeans on your torso.

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