Galaxy with no dark matter? (NGC1052-DF2)

[Paul Giandomenico]

It would make no sense at all to believe that gravity acts one way for normal matter and another way for dark matter.

Why? We don't even know if dark matter is matter at all? We assume it is because we see gravitational effects in space-time. But that only matter can cause gravitational effects in space-time is itself an assumption.

 

[PeterDonis]

that only matter can cause gravitational effects in space-time is itself an assumption.

Only in the sense that the Einstein Field Equation itself is an "assumption"; that is, we are "assuming" that General Relativity applies. Since GR has been experimentally confirmed to many decimal places this seems like a reasonable "assumption" to make; moreover, nobody has any other theory of gravity that makes any different "assumption" about what can cause gravitational effects, yet still makes correct predictions.

 

[JMz]

Why? We don't even know if dark matter is matter at all? We assume it is because we see gravitational effects in space-time. But that only matter can cause gravitational effects in space-time is itself an assumption.

We know that DM is matter because it clumps, at least around galaxy clusters, in a way that massless particles such as photons do not. In fact, it clumps more than neutrinos, as far as we can tell, and neutrinos are not massless, so we conclude that DM consists of particles even more "massive" than neutrinos [if we can use that word for neutrinos!].

Matter is certainly not the only thing that can cause gravitational effects: Any form of energy will do. Matter is the densest form of energy, but 1 joule of photons have the same effect as 1 joule of matter (times c^2).

This is not an assumption. Like any statement in natural science, this a provisional statement, subject to refutation based on observation. But it is already based on an enormous number of observations, so as conclusions go, it is exceptionally firm.

 

[Paul Giandomenico]

Only in the sense that the Einstein Field Equation itself is an "assumption"; that is, we are "assuming" that General Relativity applies. Since GR has been experimentally confirmed to many decimal places this seems like a reasonable "assumption" to make; moreover, nobody has any other theory of gravity that makes any different "assumption" about what can cause gravitational effects, yet still makes correct predictions.

Labeling an unknown gravitational phenomena a form of matter, tends to narrow one's thinking about the problem, and put it in a box. Does it not? We haven't had to contend with an potential alternate theory of gravity until the confirmation that the effects of "dark matter" exist.

 

[Orodruin]

so we conclude that DM consists of particles even more "massive" than neutrinos

This is not necessarily true. The main issue is how DM behaves and for a DM candidate that is thermally produced in the early Universe, you typically need it to be heavier than neutrinos to constitute cold dark matter. However, thermal production at a relatively late stage is not the only possibility. A very popular DM candidate these days is axion DM, which is very appealing from many perspectives. Axions typically have very (very!) light masses and the corresponding DM halos are not built from particles as much as from coherent states, i.e., essentially classical fields.

Labeling an unknown gravitational phenomena a form of matter, tends to narrow one's thinking about the problem, and put it in a box. Does it not? We haven't had to contend with an potential alternate theory of gravity until the confirmation that the effects of "dark matter" exist.

You are missing large pieces of evidence for the DM component actually behaving like matter. All of the gravitational effects that we observe of DM have exactly the same equation of state parameter as ordinary matter, i.e., pressureless, with an energy density that scales as $a^{-3}$. If it behaved in any other way it would not be called dark matter.

 

[Paul Giandomenico]

We know that DM is matter because it clumps, at least around galaxy clusters, in a way that massless particles such as photons do not. In fact, it clumps more than neutrinos, as far as we can tell, and neutrinos are not massless, so we conclude that DM consists of particles even more "massive" than neutrinos [if we can use that word for neutrinos!].

Matter is certainly not the only thing that can cause gravitational effects: Any form of energy will do. Matter is the densest form of energy, but 1 joule of photons have the same effect as 1 joule of matter (times c^2).

This is not an assumption. Like any statement in natural science, this a provisional statement, subject to refutation based on observation. But it is already based on an enormous number of observations, so as conclusions go, it is exceptionally firm.

I'm not sure we are clear that DM clumps around galaxy clusters. It would make more sense that matter (galaxy clusters) tends to clump around regions of space-time where "dark matter" is more prevalent?

 

[Orodruin]

I'm not sure we are clear that DM clumps around galaxy clusters. It would make more sense that matter (galaxy clusters) tends to clump around regions of space-time where "dark matter" is more prevalent?

Structure formation typically starts with the coalescence of dark matter structures that act as gravitational potential wells for galaxy formation. However, I don't think the order of things is what @JMz considered the important part of his post.

 

[Paul Giandomenico]

This is not necessarily true. The main issue is how DM behaves and for a DM candidate that is thermally produced in the early Universe, you typically need it to be heavier than neutrinos to constitute cold dark matter. However, thermal production at a relatively late stage is not the only possibility. A very popular DM candidate these days is axion DM, which is very appealing from many perspectives. Axions typically have very (very!) light masses and the corresponding DM halos are not built from particles as much as from coherent states, i.e., essentially classical fields.You are missing large pieces of evidence for the DM component actually behaving like matter. All of the gravitational effects that we observe of DM have exactly the same equation of state parameter as ordinary matter, i.e., pressureless, with an energy density that scales as $a^{-3}$. If it behaved in any other way it would not be called dark matter.

But isn't it true we only are seeing gravitational effects on space-time and not DM actually behaving like matter? I would agree the first assumption to make is that it is an effect caused by a form of matter we can't interact with. But that should not be the end point. Dark Matter may turn out to be a feature of space-time rather than having an effect on it.

 

[Orodruin]

But isn't it true we only are seeing gravitational effects on space-time and not DM actually behaving like matter?

What do you think "behaves like matter" means in this context? Essentially your statement in this context reads "isn't it true that we see X and not X?"

 

[PeterDonis]

Labeling an unknown gravitational phenomena a form of matter, tends to narrow one's thinking about the problem, and put it in a box. Does it not?

No. As @Orodruin has pointed out, calling it "dark matter" is just a way of describing its equation of state. And that is an observable, not an assumption. In other words, "dark matter" is just shorthand for "something that has a matter equation of state, and doesn't interact electromagnetically, but we don't know its microscopic composition". In other words, it makes no assumptions about what it is, it just describes the properties we have so far observed it to have.

 

[PeterDonis]

isn't it true we only are seeing gravitational effects on space-time and not DM actually behaving like matter?

No. We see that it has the matter equation of state; its density varies like the inverse cube of the scale factor.

Dark Matter may turn out to be a feature of space-time

No, it can't, because a feature of spacetime would have to have a density that is constant; it could not vary. We already have a name for this: "dark energy" (or "cosmological constant"). And we already have separate observations that tell us what the density of dark energy is, separate from the density of dark matter.

 

[kimbyd]

Why? We don't even know if dark matter is matter at all? We assume it is because we see gravitational effects in space-time. But that only matter can cause gravitational effects in space-time is itself an assumption.

No, it isn't. In General Relativity, energy, momentum, pressure, and twisting forces all act as sources for gravity. For most familiar matter, mass is always the dominant component because mass energy is so huge. For extremely dense objects or relativistic particles, the other components become significant.

The problem is that dark matter clusters. If dark matter is composed of any kind of particle, then those particles can't move too fast or else they won't cluster (this is why the known neutrinos can't make up dark matter: they move far too quickly due to their small masses). This means that the two primary competing theories to explain dark matter are:
1) A weakly-interacting particle with non-zero mass and low temperature. This can be achieved either through a neutrino-like particle which is very massive (typical estimates are around dozens to hundreds of times the mass of a proton per particle), or through the particles having some specific mechanism to achieve low temperatures despite having low masses (axions fall into this category).
2) Modified gravity. The proposal here is that if gravity behaves differently at very large distances compared to terrestrial experiments, then that might explain the discrepancies.

Currently, modified gravity theories appear to be incapable of fitting observational data without introducing some form of dark matter. Thus, dark matter appears to be the most likely solution to the puzzle.

 

[Orodruin]

or through the particles having some specific mechanism to achieve low temperatures despite having low masses (axions fall into this category).

I think it is worth thinking of axions as a different type of dark matter altogether as it is essentially a classical field and not really particle dark matter (note that this does not mean you cannot find axion particles! That DM would be a coherent state does not exclude single particle states). A good axion in cosmology review where this argument is made can be found here (it is pretty big so it might take some time to download).

 

[Paul Giandomenico]

No. We see that it has the matter equation of state; its density varies like the inverse cube of the scale factor.
No, it can't, because a feature of spacetime would have to have a density that is constant; it could not vary. We already have a name for this: "dark energy" (or "cosmological constant"). And we already have separate observations that tell us what the density of dark energy is, separate from the density of dark matter.

Its clear that space time is not constant density, hence the variance of gravitational effects that have resulted in galaxy clusters.

No. As @Orodruin has pointed out, calling it "dark matter" is just a way of describing its equation of state. And that is an observable, not an assumption. In other words, "dark matter" is just shorthand for "something that has a matter equation of state, and doesn't interact electromagnetically, but we don't know its microscopic composition". In other words, it makes no assumptions about what it is, it just describes the properties we have so far observed it to have.

No, it isn't. In General Relativity, energy, momentum, pressure, and twisting forces all act as sources for gravity. For most familiar matter, mass is always the dominant component because mass energy is so huge. For extremely dense objects or relativistic particles, the other components become significant.

The problem is that dark matter clusters. If dark matter is composed of any kind of particle, then those particles can't move too fast or else they won't cluster (this is why the known neutrinos can't make up dark matter: they move far too quickly due to their small masses). This means that the two primary competing theories to explain dark matter are:
1) A weakly-interacting particle with non-zero mass and low temperature. This can be achieved either through a neutrino-like particle which is very massive (typical estimates are around dozens to hundreds of times the mass of a proton per particle), or through the particles having some specific mechanism to achieve low temperatures despite having low masses (axions fall into this category).
2) Modified gravity. The proposal here is that if gravity behaves differently at very large distances compared to terrestrial experiments, then that might explain the discrepancies.

Currently, modified gravity theories appear to be incapable of fitting observational data without introducing some form of dark matter. Thus, dark matter appears to be the most likely solution to the puzzle.

Yes I am aware of the multiple ways gravitational effects can manifest itself, but what are referring to is how these gravitational effects result in the observable universe, so not sure how twisting forces are revenant here in regards to forming galaxy clusters. It relates to how space time reacts to massive objects, and how matter and energy react to the "bending" of space time. Dark matter may not be a particle at all.

 

[Bandersnatch]

Its clear that space time is not constant density,

Please, define in mathematical terms what it means for space-time to have density.
I.e., typically, density is taken to mean the amount of some quantity contained in a unit of space. What does it mean if you say there's a quantity of space-time per unit of space?

 

[Paul Giandomenico]

What do you think "behaves like matter" means in this context? Essentially your statement in this context reads "isn't it true that we see X and not X?"

We see X. Where X = gravitational effects on space-time in turn effecting matter and light. We know that matter can have this effect on space-time. But am I wrong or we really don't really understand what makes up space-time, and why massive objects cause it to bend?

 

[Orodruin]

We see X. Where X = gravitational effects on space-time in turn effecting matter and light. We know that matter can have this effect on space-time. But am I wrong or we really don't really understand what makes up space-time, and why massive objects cause it to bend?

No. You are wrong in your nomenclature. What "matter" means in this context is just "something that have these effects on spacetime".

 

[Paul Giandomenico]

Please, define in mathematical terms what it means for space-time to have density.
I.e., typically, density is taken to mean the amount of some quantity contained in a unit of space. What does it mean if you say there's a quantity of space-time per unit of space?

That is a good question. The energy density of space-time, is always measured to be the same locally, only marginally less that matter. This constant density results in the constant speed of light locally.However this density is relatively different depending on the local curvature of space-time due to gravity.

 

[Orodruin]

Dark matter may not be a particle at all.

This does not mean that it is not matter. See earlier posts on axions.

 

[Bandersnatch]

That is a good question.

Then please, answer it. It's the term you're using to build an argument. Please, indicate what observations make it clear that it's not constant?

The energy density of space-time, is always measured to be the same locally

You've now introduced an additional new term: energy density of space time. Please define it. What measurements, which you mention, show it to be always the same locally?

 

[PeterDonis]

Its clear that space time is not constant density, hence the variance of gravitational effects that have resulted in galaxy clusters.

This is due to variations in the density of matter (and energy, pressure, etc.--all the things that go into the stress-energy tensor). It is not due to variations in "the density of spacetime". There is no such thing as "the density of spacetime" unless you want to use that term to describe the cosmological constant, but then, as I've already said, it must be constant if it's going to be a property of spacetime (as opposed to a property of matter, energy, pressure, etc.).

The energy density of space-time, is always measured to be the same locally, only marginally less that matter.

I have no idea what you are talking about here. Can you give an actual equation, and a reference for where you are getting it from?

 

[phinds]

We see X. Where X = gravitational effects on space-time in turn effecting matter and light. We know that matter can have this effect on space-time. But am I wrong or we really don't really understand what makes up space-time, and why massive objects cause it to bend?

Since you put "bending" in quotes in a previous post, I assume you understand the following, but just in case you don't, space-time does NOT bend / stretch, or do anything that matter does. We SAY that it bends because objects in space-time with no external force being applied to them follow geodesics, which are STRAIGHT lines in space-time but are "curved" only when looked at by improperly applying Euclidean Geometry to a domain where it is not valid but where you need instead Riemann Geometry (actually, I've been told it's "pseudo" Riemann Geometry but in any case it's not Euclidean and nothing bends/stretches/curves, etc).

 

[JMz]

This is not necessarily true. The main issue is how DM behaves and for a DM candidate that is thermally produced in the early Universe, you typically need it to be heavier than neutrinos to constitute cold dark matter. However, thermal production at a relatively late stage is not the only possibility. A very popular DM candidate these days is axion DM, which is very appealing from many perspectives. Axions typically have very (very!) light masses and the corresponding DM halos are not built from particles as much as from coherent states, i.e., essentially classical fields.

Very nice. This makes the unit of field construction some kind of clump of many axions. Presumably the effective number of degrees of freedom of such a field is far smaller than the number of axions comprising it, right? (My guess, from this reasoning, is that the d.f. ratio is much larger than the neutrino/axion mass ratio.)

 

[Orodruin]

where you need instead Riemann Geometry (actually, I've been told it's "pseudo" Riemann Geometry but in any case it's not Euclidean and nothing bends/stretches/curves, etc).

Just to make this clear. Riemann geometry describes a manifold that is equipped with a metric tensor, which by definition is positive definite. Euclidean geometry is a special case of Riemannian geometry so its really not excluding Euclidean to state Riemannian. A pseudo-Riemannian geometry involves a pseudo-metric, which instead of being positive definite has the requirement of being non-degenerate. If you want to split hairs further, Lorentzian geometry has a pseudo-metric with a 1+n or n+1 signature. Minkowski space is to Lorentzian geometry what Euclidean space is to Riemannian geometry.

 

[phinds]

Just to make this clear. Riemann geometry describes a manifold that is equipped with a metric tensor, which by definition is positive definite. Euclidean geometry is a special case of Riemannian geometry so its really not excluding Euclidean to state Riemannian. A pseudo-Riemannian geometry involves a pseudo-metric, which instead of being positive definite has the requirement of being non-degenerate. If you want to split hairs further, Lorentzian geometry has a pseudo-metric with a 1+n or n+1 signature. Minkowski space is to Lorentzian geometry what Euclidean space is to Riemannian geometry.

So do you think space-time really "bends"?

 

[Orodruin]

Very nice. This makes the unit of field construction some kind of clump of many axions. Presumably the effective number of degrees of freedom of such a field is far smaller than the number of axions comprising it, right? (My guess, from this reasoning, is that the d.f. ratio is much larger than the neutrino/axion mass ratio.)

Yes. As with any coherent state, the field expectation value satisfies the classical equations of motion and it does not contain a well-defined number of axions (it is not an eigenstate of the axion number operator). If I understand correctly, a dark matter halo in the axion DM models is essentially a soliton solution to the classical field equations, but I am not an expert in axion DM.

 

[Orodruin]

So do you think space-time really "bends"?

"Bend" is not a well defined term. Also, we all know that "really" is a bit subjective. Please define what you mean by "spacetime bends".

 

[JMz]

Labeling an unknown gravitational phenomena a form of matter, tends to narrow one's thinking about the problem, and put it in a box. Does it not? We haven't had to contend with an potential alternate theory of gravity until the confirmation that the effects of "dark matter" exist.

I think you are downplaying the huge variety of theories of gravity that have been proposed in the century+ since GR was proposed. Many of these generated creative thinking about new experiments that could distinguish them from GR. So far, no theory has done better than GR.

As recently as last year, a whole swath of alternative theories were ruled out by the single observation of the LIGO/Virgo observation of the kilonova. Why did they exist? Because people are contending with, and proposing, alternative theories of gravity, all the time.

 

[phinds]

"Bend" is not a well defined term.

I think it is quite well defined to say, for example, "a metal rod bends". It really bends Does space-time?

 

[Orodruin]

I think it is quite well defined to say, for example, "a metal rod bends". It really bends Does space-time?

Again, "bends" has no well defined meaning in the context. Please define what you mean by the word if you want to make a question.

 

[phinds]

Again, "bends" has no well defined meaning in the context. Please define what you mean by the word if you want to make a question.

Sorry, I meant to say in the previous post that I agree w/ you that "bends" is ill defined in the context of space-time, so I think it's a bit of a meaningless argument, I just don't like seeing people say "space-time bends" because that, to me, makes a false assumption that space-time is material that can be bent / stretched, etc and that is a misunderstanding that can lead to other misunderstandings.

 

[JMz]

Yes. As with any coherent state, the field expectation value satisfies the classical equations of motion and it does not contain a well-defined number of axions (it is not an eigenstate of the axion number operator). If I understand correctly, a dark matter halo in the axion DM models is essentially a soliton solution to the classical field equations, but I am not an expert in axion DM.

Thanks. Given how large the ratio must be, the "effective number in a clump", I would have been shocked if someone proposed an eigenstate that matched it, especially if it was supposed to be particularly stable against perturbations.

 

[Orodruin]

Sorry, I meant to say in the previous post that I agree w/ you that "bends" is ill defined in the context of space-time, so I think it's a bit of a meaningless argument, I just don't like seeing people say "space-time bends" because that, to me, makes a false assumption that space-time is material that can be bent / stretched, etc and that is a misunderstanding that can lead to other misunderstandings.

This likely comes from a misappropriation of "curved spacetime". People unfamiliar with nomenclature are likely to use them as essentially synonymous. If you use them as synonymous, then yes, spacetime is "bent" (i.e., "curved" in the well-defined mathematical sense of parallel transport around a loop not necessarily giving back the same vector) and the equations governing this curvature has the stress-energy tensor as its source term.

Thanks. Given how large the ratio must be, the "effective number in a clump", I would have been shocked if someone proposed an eigenstate that matched it, especially if it was supposed to be particularly stable against perturbations.

Even if it is not an eigenstate, as with any state you can of course compute the expectation value of the number operator ... It will be large.

 

[Grinkle]

This likely comes from a misappropriation of "curved spacetime".

I think it comes from a lack of math background.

I propose that a reasonable definition of bends is something that does not follow a geodesic. I speculate , as I do not have any formal mathematical understanding of geometries, that saying 'spacetime bends' is not a self-consistent statement, because its (spacetime's) shape is defined by some mathematical description which also defines consistent geodesics and it is therefore contradictory to say that spacetime bends. By definition, it can't. Its shape, whatever it is, defines straight. Any two points in a spacetime are connected by a geodesic.

If I am wrong that a geodesic is the shortest path length connecting two points for some given geometry, then the above make no sense. I can only hope it makes at least some sense otherwise.

edit:

In case that is too scattered to make sense of, an example of what I am thinking is 'curved Euclidean planes' - if a Euclidean plane is curved with respect to Euclidean geometry, its not Euclidean, its instead a plane in some other geometry.

 

[kimbyd]

Yes I am aware of the multiple ways gravitational effects can manifest itself, but what are referring to is how these gravitational effects result in the observable universe, so not sure how twisting forces are revenant here in regards to forming galaxy clusters. It relates to how space time reacts to massive objects, and how matter and energy react to the "bending" of space time. Dark matter may not be a particle at all.

The only other option besides a particle is modified gravity. And as I pointed out, modified gravity theories conceived to date do not fit with observation without at least some dark matter.

Edit: To clarify, based upon our understanding of quantum mechanics, everything in the universe is made out of fields, and fields can be quantized into particles.