The review article is 120 pages long, but the abstract and introduction alone are very helpful in providing a grounding in what the weak gravity conjecture is, and why it matters.

The Weak Gravity Conjecture holds that in a theory of quantum gravity, any gauge force must mediate interactions stronger than gravity for some particles.

This statement has surprisingly deep and extensive connections to many different areas of physics and mathematics. Several variations on the basic conjecture have been proposed, including statements that are much stronger but are nonetheless satisfied by all known consistent quantum gravity theories.

We review these related conjectures and the evidence for their validity in the string theory landscape.

We also review a variety of arguments for these conjectures, which tend to fall into two categories: qualitative arguments which claim the conjecture is plausible based on general principles, and quantitative arguments for various special cases or analogues of the conjecture.

We also outline the implications of these conjectures for particle physics, cosmology, general relativity, and mathematics.

Finally, we highlight important directions for future research.

Daniel Harlow, Ben Heidenreich, Matthew Reece, Tom Rudelius, "The Weak Gravity Conjecture: A Review" arXiv:2201.08380 (January 20, 2022) (invited review, submitted to Reviews of Modern Physics).

## 12 comments:

The Weak Gravity Conjecture holds that in a theory of quantum gravity, any gauge force must mediate interactions stronger than gravity for some particles.

so there can't be a gauge force must mediate interactions weaker than gravity for some particles

but what If dark matter particles is mediate by gauge force interactions weaker than gravity by say 1% or more ?

The point of the hypothesis is that this is basically logically impossible if you make some very weak assumptions that are not inconsistent with observed facts.

One of the purposes of this hypothesis is to eliminate the need to test for very faint forces.

The point of the hypothesis is that this is basically logically impossible if you make some very weak assumptions that are not inconsistent with observed facts.

One of the purposes of this hypothesis is to eliminate the need to test for very faint forces.

do you find this plausible ?

dark matter is said to form clumps - dark matter could easily have dark matter particles is mediate by gauge force interactions weaker than gravity that causes clumping

"do you find this plausible?"

Yes. While it is only a hypothesis, there are some very good theoretical reasons to think that any other gauge coupling would have to be stronger, basically, because gravity couples to all forms of mass-energy and any weaker force would be completely overshadowed by gravity if it existed.

"dark matter is said to form clumps"

Actually, canonical dark matter is collisionless, or nearly so, which means that it is the least clumpy kind of stuff that there could be.

"dark matter could easily have dark matter particles is mediate by gauge force interactions weaker than gravity that causes clumping"

If it was weaker the gravity, then it wouldn't have any meaningful effect that would be discernible since the gravitational pull would predominate.

Yes. While it is only a hypothesis, there are some very good theoretical reasons to think that any other gauge coupling would have to be stronger, basically, because gravity couples to all forms of mass-energy and any weaker force would be completely overshadowed by gravity if it existed.If it was weaker the gravity, then it wouldn't have any meaningful effect that would be discernible since the gravitational pull would predominate.

if it is say 90% as strong as gravity it would be discernible for galaxy rotations curve and galaxy clusters as both newton and even mond is not enough

Dark matter self-interactions have been introduced because the inferred shape of the distributions of dark matter are inconsistent with collisionless dark matter, not because the magnitude of the gravitational interactions implied is wrong

Dark matter self-interactions have been introduced because the inferred shape of the distributions of dark matter are inconsistent with collisionless dark matter, not because the magnitude of the gravitational interactions implied is wrong

how strong are the Dark matter self-interactions?

they could be gauge force must mediate interactions weaker than gravity thus violating Weak Gravity Conjecture

To do what dark matter self-interactions are supposed to do, dark matter self-interaction need to be almost as strong as the electromagnetic interaction, but with a massive mediator (typically 100 MeV/c^2 or less) that limits its range greatly.

but it could be possible to propose a model of dark matter self-interaction with , dark matter self-interaction with infinite range, but weaker than gravity, or even limited range weaker than gravity then tweak it to fit observation.

or propose multiple types of dark matter, sterile neutrinos, axions, PBH, with one or more more type of dark matter with self-interaction weaker than gravity, and with some fine tuning make it fit observation

you can propose 2 types of dark matter self-interaction, one which need to be almost as strong as the electromagnetic interaction, but with a massive mediator (typically 100 MeV/c^2 or less) that limits its range greatly, and a second type with gauge interactions weaker than gravity and infinite range.

then fine tune it to fit observation.

there can be an indefinite number of dark matter particles and gauge interactions, at least 1 of which might be weaker than gravity, in the dark matter sector.

and maybe something weaker than gravity is needed to explain dark energy.

"but it could be possible to propose a model of dark matter self-interaction with , dark matter self-interaction with infinite range, but weaker than gravity, or even limited range weaker than gravity then tweak it to fit observation."

No. This wouldn't work.

why wouldn't it work?

particle dark matter can have any number of properties and gauge interactions.

if a certain type of dark matter particles has gauge interactions long distance but weaker than gravity, it would make itself known in the large scale structure of the universe.

ultimately the only criteria is, does it match observation?

to prevent the core-cusp problem in dark matter, weaker than gravity but also causes dark matter to be repulsive to baryonic matter, so there's no density peak, would avoid this problem. it would spread out the dark matter over a larger volume.

its weaker than gravity, so dark matter don't form a core cusp, but not strong enough to prevent the dark matter from building large scale structures via gravity.

See http://dispatchesfromturtleisland.blogspot.com/2022/02/simple-dark-matter-models-dont-work.html re the strength of SIDM 5th force required to match observations.

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