
Top Dark Matter Candidate Loses Ground to Tiniest Competitor - theafh
https://www.quantamagazine.org/why-dark-matter-might-be-axions-20191127/
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ailideex
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...](https://en.wikipedia.org/wiki/Weakly_interacting_massive_particles)

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

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ISL
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).

[https://en.wikipedia.org/wiki/Primakoff_effect](https://en.wikipedia.org/wiki/Primakoff_effect)

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](https://en.wikipedia.org/wiki/Grand_unification_energy)

~~~
ailideex
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.

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petjuh
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.

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ailideex
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...](https://phys.org/news/2019-11-evidence-anisotropy-cosmic.html)

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

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

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

Reads like Star Wars :-)

~~~
iameli
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.

