How Dark could Dark Matter get?

[.Scott]

TL;DR Summary

In theory, how thoroughly can Dark Matter conceal its particle nature?

Let's say that it turns out that Dark Matter is realllllly dark. And (except for gravity) none of our experiments actually result in any Matter/DM interactions at all. Not now, not for the next couple of centuries.

Just based on gravity observations, are there things that we will eventually be able to deduce about Dark Matter.
For example, limiting the question to those DM particles that are prevalent in our galaxy:
1) How many different DM particles are there?
2) Are any DM fermions?
3) Are any DM bosons.
4) What is the mass or range of masses of these DM particles.

 

[phinds]

what research have you done on this? There are known limits on some types of particles. Have you tried to look for that information?

 

[.Scott]

what research have you done on this? There are known limits on some types of particles. Have you tried to look for that information?

I read numerous DM articles every month.
So far, no DM particle has been found - and it's not clear what the prognosis is.

One recent article suggested that a galactic DM fermion cloud could be distinguished from a DM boson cloud - and that the DM particle mass might also affect its galactic distribution. That was not the main thrust of the article and I don't want to pull that article into this thread (though I did cite it in another thread). My interest is in a broader question than that.

Apart from any current experiment, and apart from the current technology limits to DM experiments, can we create a list of observables that are destined to reflect the nature of DM particles. Things that basically can't fail?

For example, as a DM particle is swallowed by a BH, would it somehow unavoidably reveal its particle mass?
That kind of thing.

 

[PeterDonis]

One recent article

Please give a reference.

 

[.Scott]

Please give a reference.

The group publishing this article claims that the paths of stars orbiting Sgr A* might reflect the nature of DM.
For the purpose of this thread, I am not interested in the status of Sgr A* as a black hole. But I am interested in the validity of their arguments.

So, to keep this thread on target, whether or not there is sufficient data (ie, single observed track of star G2), do their arguments hold water? Given enough G2-like observations, could we discriminate between a BH and some sort of DM phenomenon? And if we could, would we be destined to get a measure of the DM particle mass and boson/fermion status?

Or, could it be that no matter what the DM particle characteristics, we can never be certain that G2-like observations would disclose those characteristics?

And G-2 aside, if we were able to measure and monitor the fine structure of mass within our galaxy, would we unavoidably collect data that would indicate the nature of DM particles?

And failing that, might there be some other measurement which "couldn't fail".

 

[PeterDonis]

For the purpose of this thread, I am not interested in the status of Sgr A* as a black hole. But I am interested in the validity of their arguments.

I don't think you can separate the two. Their argument is that their preferred model fits the data better than the standard black hole model. Such an argument is purely statistical; they are not arguing on physical grounds that their preferred model makes more sense. So there's nothing to grab on to, so to speak, other than their claimed comparison of their model with the standard black hole model. They are not considering either one in isolation from the other.

Given enough G2-like observations, could we discriminate between a BH and some sort of DM phenomenon?

It's impossible to answer such questions in general. You have to look at a particular proposed DM model and see what it predicts.

 

[PeterDonis]

their preferred model

Which appears to be described in this paper by Ruffini et al:

https://arxiv.org/abs/1409.7365

 

[PeterDonis]

Which appears to be described in this paper by Ruffini et al

From the paper, an object composed of their fermionic "dark matter" with roughly the same mass as is estimated in the standard black hole model for the BH Sagittarius A, is much larger than the hole's estimated size: about $10^{-2}$ parsecs in radius as compared with about $10^{-7}$. I'm not sure how this can be reconciled with the recent published papers showing imaging of Sag A and confirming its size to be roughly that estimated by the standard black hole model.

 

[.Scott]

As a "Devil's Advocate", I assert that the particle state of DM is "metaphysical". That it is not only unknown, but unknowable.
Is there any currently known argument that can refute this?

 

[PeterDonis]

I assert that the particle state of DM is "metaphysical".

Metaphysics is off topic in this forum.

Is there any currently known argument that can refute this?

Of course not. If it ever gets refuted, it will be done by discovering what DM is made of. Or the DM hypothesis itself will get abandoned because some other hypothesis makes better predictions, which won't exactly "refute" the proposition you describe, but will make it pointless.

 

[DrChinese]

As a "Devil's Advocate", I assert that the particle state of DM is "metaphysical". That it is not only unknown, but unknowable.
Is there any currently known argument that can refute this?

I assume you know there really is no one "standard model" of dark matter at this time, much less one that fits into quantum field theory. Dark matter is a "solution" to some cosmological problems, however it creates about as many problems as it solves. Personally, I think there is likely something that corresponds to dark matter; but it is still pretty early in this area to be holding firm opinions.

 

[.Scott]

Is it possible for a particle to exist in this universe without having a spin value - even a zero spin value?
Is it possible for mass to distribute itself throughout our galaxy without being quantum in nature? ... and having a rest mass?

My presumption was that , even outside the standard model, a DM particle would have a rest mass and a spin value. Was I being overly presumptuous? Or are these attributes tied to the Standard Model? And anything that is not part of the Standard Model can be anything?

Actually, I can tell that the DM particle has a non-zero rest mass - otherwise it would not accumulate near galaxies.

 

[.Scott]

Metaphysics is off topic in this forum.

But arguing that something (such as a DM particle) is physical (vs. metaphysical) is "on topic".
On the other hand, if I combine your statement with the observation that DM particle studies are "on topic", could I conclude (in humor) that DM particle properties must be physical?

Of course not. If it ever gets refuted, it will be done by discovering what DM is made of. Or the DM hypothesis itself will get abandoned because some other hypothesis makes better predictions, which won't exactly "refute" the proposition you describe, but will make it pointless.

I did not expect that answer. So we really don't know whether there is any particle basis for DM.

 

[PeterDonis]

arguing that something (such as a DM particle) is physical (vs. metaphysical) is "on topic".

No, it isn't. Arguing about particular proposed particle models of DM is on topic. But a bare claim that DM is "physical" is just as off topic as claiming it is "metaphysical".

I did not expect that answer. So we really don't know whether there is any particle basis for DM.

Why does that surprise you? Isn't it pretty much common knowledge that we don't know what DM is made of?

 

[PeterDonis]

Is it possible for a particle to exist in this universe without having a spin value - even a zero spin value?

This question is unanswerable. The best we can do is to say that none of our current models include any such thing.

Is it possible for mass to distribute itself throughout our galaxy without being quantum in nature? ... and having a rest mass?

What would that even mean?

My presumption was that , even outside the standard model, a DM particle would have a rest mass and a spin value.

What is this presumption based on?

anything that is not part of the Standard Model can be anything?

What does this even mean?

Basically you appear to be asking what would be allowed by laws of physics that are different from our best current ones. How is anyone supposed to answer that?

I can tell that the DM particle has a non-zero rest mass - otherwise it would not accumulate near galaxies.

We don't know for sure that DM does accumulate near galaxies. We have no way of knowing what the DM distribution is outside of galaxies since we can't observe it directly; we can only observe it indirectly by its gravitational effects, and the only objects we can see that would show such effects are galaxies.

 

[.Scott]

We don't know for sure that DM does accumulate near galaxies. We have no way of knowing what the DM distribution is outside of galaxies since we can't observe it directly; we can only observe it indirectly by its gravitational effects, and the only objects we can see that would show such effects are galaxies.

I was going to cite this article. But on reading it more closely, I see that it's making many presumptions about DM properties.

 

[romsofia]

As a "Devil's Advocate", I assert that the particle state of DM is "metaphysical". That it is not only unknown, but unknowable.
Is there any currently known argument that can refute this?

ok but that is basically everything in theoretical physics? The reason why people are searching for a particle aspect of DM is because our current generation of physicsist have fallen in love with particle based models thanks to QFT. Before that it was forces, everything HAD to be model by a force!

So, does DM HAVE to be based on a particle model? Of course not, and you can stand simply by the logic that we haven't detected gravitons, so why would i expect something that has only been shown to interact with the gravitational field to be particle based? You can't even show me evidence that the force carrier for the gravitational field is “real”.

But on the other hand, the predictions of QFT has a good track record as well! And based on that track record, you can assume certain things are true, because theyre bound to be true in their eyes. So, based of whatever assumptions they made, they form conclusions. And this HAS worked in the past, so... time will tell!

Last talk I attended on primordial black holes (a “candidate” for the particle nature of DM), the presenter believed that LISA (going up in 2023?) will be sensitive enough to detect them, and you can read more about it here: https://arxiv.org/pdf/1810.12218.pdf

But overall, no. You arent wrong to question the particle nature of DM because no one knows quite yet!

 

[Vanadium 50]

Is it possible for a particle to exist in this universe without having a spin value - even a zero spin value?

What does this even mean? If I apply the angular momentum operator to such a thing, what comes out?
I can believe you can have a particle without a well-defined spin value, but what does it mean to say it has no spin value, not even zero.

 

[.Scott]

What does this even mean? If I apply the angular momentum operator to such a thing, what comes out?
I can believe you can have a particle without a well-defined spin value, but what does it mean to say it has no spin value, not even zero.

OK.
Does that mean that (at least in theory) there will always be a way of determining the spin of a newly discovered particle?

 

[AdvaitDhingra]

Summary:: In theory, how thoroughly can Dark Matter conceal its particle nature?

Let's say that it turns out that Dark Matter is realllllly dark. And (except for gravity) none of our experiments actually result in any Matter/DM interactions at all. Not now, not for the next couple of centuries.

Just based on gravity observations, are there things that we will eventually be able to deduce about Dark Matter.
For example, limiting the question to those DM particles that are prevalent in our galaxy:
1) How many different DM particles are there?
2) Are any DM fermions?
3) Are any DM bosons.
4) What is the mass or range of masses of these DM particles.

We don't yet have a theory of quantum gravity, so determining it's subatomic structure is tricky.

 

[.Scott]

We don't yet have a theory of quantum gravity, so determining it's subatomic structure is tricky.

When I said "gravity observations", I was alluding to astronomical observations - the shape of galaxies and the velocities and paths of the objects in them.

 

[AdvaitDhingra]

When I said "gravity observations", I was alluding to astronomical observations - the shape of galaxies and the velocities and paths of the objects in them.

Ah, my bad.