I Galaxy with no dark matter? (NGC1052-DF2)

[kimbyd]

Well it falls to Occam's razor. The best theory is the one that predicts the most with minimum amount of assumptions. Modified Gravity don't give a toss about this effects in order to produce the Tully Fischer relation, as gravitational effects are always tied to visible mass. DM is akin to epycyclic models, always in need of more ad-hoc and unecessary tweaking in order to even be able to start to make a rigorous sense of it all.

Except feedback effects are not assumptions. They are a part of reality whether we include them in our models or not. Failing to include them doesn't mean that you've got a simpler theory: it means you're ignoring pieces of the puzzle.

The right way to deal with this is not to say, "Well, MOND doesn't need this!" but rather, "Time to work out the consequences of this feedback effect in the MOND model, to make sure it makes sense."

 

[mfb]

We know antimatter falls down.

Atoms are about 0.03% electrons, 1% quark masses and 99% QCD binding energy, the exact fractions depend on the atom. We know from comparisons of countless atoms that all atoms fall down at the same rate, this is only possible if all three components satisfy the weak equivalent principle.

Anti-atoms are about 0.03% positrons, 1% antiquark masses and 99% QCD binding energy. We already know the last part satisfies the weak equivalence principle. For antimatter to fall up, the other components would have to do something completely crazy, and no matter what they do different antiatoms would fall up at different rates. While this is not yet ruled out by experiment, it doesn't sound plausible at all.

To make antimatter fall up you would have to assign a "matterness" to binding energies. Not just QCD, but also QED which I neglected above. And that doesn't sound plausible either. Which fraction of the QCD binding energy in a pentaquark is matter? ;)

 

[kimbyd]

We know antimatter falls down.

This has yet to be experimentally demonstrated, though it is very true that there would be some pretty extreme theoretical challenges with explaining how anti-matter could possibly interact with gravity differently from normal matter while normal matter still obeys the equivalence principle.

 

[JMz]

The idea that antimatter should fall up seems like an idea based on nothing more than etymology. We call certain particles "anti" because people noted that they have opposite charge, charm, or whatever compared to particles we commonly encounter.

That doesn't prove that they don't have anti-mass as well, but the idea that they do (a) starts with much less plausibility, because they don't even behave that way for momentum, and (b) as @Jonathan Scott notes, photons, which are their own "anti"-particles, are observed to fall down in gravitational lenses.

 

[jerromyjon]

If for instance antimatter falls up consistently in the 3 CERN experiments, then GR is DEAD

What if antimatter and matter are more strongly attracted than either are to themselves? I only ask this naive question because my basic understanding is that matter and antimatter always seem to attract and annihilate so the premise in my mind is that there is something MORE powerful than typical physics involved.

 

[kimbyd]

The idea that antimatter should fall up seems like an idea based on nothing more than etymology. We call certain particles "anti" because people noted that they have opposite charge, charm, or whatever compared to particles we commonly encounter.

I honestly agree. It's highly speculative, and wouldn't match with existing theory in a wide variety of ways. However, it still has some potential value for two reasons:
1) It is a testable prediction. As I understand it, the current best method involves creating neutral anti-Hydrogen and using a photon trap to cool it and then observe its motion. This was attempted a few years ago, but the error bars were still too large (https://www.nature.com/articles/ncomms2787). A refinement of that experiment may produce a definitive answer (I believe they're currently trying for a 1% measurement error on the intertial properties of anti-Hydrogen).
2) However theoretically absurd it may be, there is always some possibility that our theories are incorrect, and so this kind of experiment is worth doing. Any discrepancy in the behavior of anti-matter compared to normal matter would provide tremendously valuable insights into how to improve our current theories.

 

[JMz]

If they are more strongly attracted to each other through electric charge, sure. But that's exactly how they are "anti's" (with respect to each other), and there's no new physics there: Opposite electric charges attract, always.

Being differently gravitationally attracted is the relevant concern here: If it were observed, then GR would be qualitatively wrong. Contrapositively, if GR, or anything like it, is right, then that doesn't happen. (Most of us would bet on the latter.)

 

[JMz]

I honestly agree. It's highly speculative, and wouldn't match with existing theory in a wide variety of ways. However, it still has some potential value for two reasons:
1) It is a testable prediction. As I understand it, the current best method involves creating neutral anti-Hydrogen and using a photon trap to cool it and then observe its motion. This was attempted a few years ago, but the error bars were still too large (https://www.nature.com/articles/ncomms2787). A refinement of that experiment may produce a definitive answer (I believe they're currently trying for a 1% measurement error on the intertial properties of anti-Hydrogen).
2) However theoretically absurd it may be, there is always some possibility that our theories are incorrect, and so this kind of experiment is worth doing. Any discrepancy in the behavior of anti-matter compared to normal matter would provide tremendously valuable insights into how to improve our current theories.

I would not question the value of running the experiments. However, I might make the same remark about measuring the gravitational attraction of a mountain range: a reassuring demonstration, and one that might teach us much about how to conduct experiments of that nature, teaching that might be valuable in other experiments. But this is as nearly settled science as we can expect, not a major uncertainty that keeps people from knowing how to make further progress in gravitation.

The odds are overwhelmingly against this observation -- it seems to me -- but that's not a reason not to look.

 

[kimbyd]

I would not question the value of running the experiments. However, I might make the same remark about measuring the gravitational attraction of a mountain range: a reassuring demonstration, and one that might teach us much about how to conduct experiments of that nature, teaching that might be valuable in other experiments. But this is as nearly settled science as we can expect, not a major uncertainty that keeps people from knowing how to make further progress in gravitation.

The odds are overwhelmingly against this observation -- it seems to me -- but that's not a reason not to look.

Right, if there's a discrepancy at all it will most likely not be of the simple form "anti-matter falls up". That's why it's so valuable that they're trying to get 1% measurement error on the equivalence principle applied to anti-Hydrogen: even a small deviation from equivalence would be a truly dramatic finding.

 

[jerromyjon]

But that's exactly how they are "anti's" (with respect to each other), and there's no new physics there: Opposite electric charges attract, always.

Yes, I understand that. Electrons are repelled from protons and positrons are repelled from antiprotons. But when we are referring to say hydrogen and antihydrogen, there would be no net charge imbalance and therefore no attraction or repulsion due to charge. So what would make all the pieces annihilate unless electrons only attract positrons and not repel antiprotons? Or is there a sequence where fermions and bosons interact in succession?

 

[kimbyd]

Yes, I understand that. Electrons are repelled from protons and positrons are repelled from antiprotons. But when we are referring to say hydrogen and antihydrogen, there would be no net charge imbalance and therefore no attraction or repulsion due to charge. So what would make all the pieces annihilate unless electrons only attract positrons and not repel antiprotons? Or is there a sequence where fermions and bosons interact in succession?

If you mix a neutral Hydrogen and anti-Hydrogen gas, they'll still annihilate. They'll just do so more slowly. As they don't have a long-range attraction, you'll have to wait until two atoms randomly get close enough that their electron/positron shells start attracting one another. Once the electron/positron annihilate, there will be a bare proton/anti-proton which will attract one another strongly, and they'll annihilate pretty rapidly.

What happens next depends upon whether the energy from that annihilation escapes the gas or not. If the energy escapes the gas, then the remaining interactions will remain pretty slow. However, if it ionizes the gas, then that may result in an increase in reaction rates (but this may also cause the gas to disperse, making it less dense and slowing the reaction back down).

 

[jerromyjon]

If you mix a neutral Hydrogen and anti-Hydrogen gas, they'll still annihilate. They'll just do so more slowly.

What if they are larger atoms? Then there is no "outward dissipation" of the particles and they all annihilate quickly and completely? What if this galaxy is a rare mix of nearly equal proportions and that is why there is no "dark matter component" to be observed? I mean in that case GR is perfectly fine if annihilation and symmetry don't conflict with it, right? We'd be looking at sides of a coin in most galaxies and the rare spread of proportions... like this, where no dark matter or cosmological expansion is neccesary to model it...

 

[mfb]

What if they are larger atoms? Then there is no "outward dissipation" of the particles and they all annihilate quickly and completely? What if this galaxy is a rare mix of nearly equal proportions and that is why there is no "dark matter component" to be observed? I mean in that case GR is perfectly fine if annihilation and symmetry don't conflict with it, right? We'd be looking at sides of a coin in most galaxies and the rare spread of proportions... like this, where no dark matter or cosmological expansion is neccesary to model it...

Whatever you put together, as long as there is both matter and antimatter in it you will get annihilation. If it is dense enough to form a galaxy, this galaxy (which has no plausible mechanism of forming in the first place) will be immediately obvious to us due to its gamma ray emission.

> What if this galaxy is a rare mix of nearly equal proportions and that is why there is no "dark matter component" to be observed?

Why exactly would you expect such a galaxy, which cannot even form, to have no dark matter?

 

[Mike Johnson]

<Moderator's note: Post approved as alternative to another thread. @Mike Johnson: Please read this discussion first to find out, whether your question has already been answered.>

Could dark matter fill 'empty' space, strongly interact with visible matter and be displaced by visible matter?

Could the reason for the mistaken notion the galaxy is missing dark matter is that the galaxy is so diffuse that it doesn't displace the dark matter outward and away from the galaxy to the degree that the dark matter is able to push back and cause the stars far away from the galactic center to speed up?

What if it's not that there is no dark matter connected to and neighboring the visible matter; it's that the galaxy is not well defined enough to displace the dark matter to such an extent that it forms a 'halo' around the galaxy?

Could a galaxy's halo be displaced dark matter and these types of galaxies are not coalesced enough to displace the dark matter into forming a halo?

 

[PeterDonis]

[Moderator's note: response provided since this poster's question was moved here from a separate thread.]

Could dark matter fill 'empty' space, strongly interact with visible matter and be displaced by visible matter?

No, because of the "strongly interact with visible matter" part. The whole point of dark matter is that it does not strongly interact with anything except through its gravity: no EM, no weak interaction, no strong interaction. If it did interact via any of those three mechanisms, we would have other ways of seeing that it was there besides its gravitational effect.

See post #8 for an example of a way dark matter and normal matter could be separated; note that it involves the normal matter interacting with other normal matter; it does not involve any interaction between normal matter and dark matter.

 

[ohwilleke]

Stacy McGaugh makes an important observation about a paper on NGC1052-DF2 (the "dark matterless galaxy"), in which van Dokkum et al. measure the rms velocity dispersion for DF2 to be 8.4 km/s, with a 90% confidence upper limit of 10 km/s.

It turns out that this is greatly influenced by a key methodological point that is doubtful:

On closer reading, I notice in the details of their methods section that the rms velocity dispersion is 14.3 km/s. It is only after the exclusion of one outlier that the velocity dispersion becomes unusually low. As a statistical exercise rejecting outliers is often OK, but with only 10 objects to start it is worrisome to throw any away. And the outlier is then unbound, making one wonder why it is there at all.

McGaugh also notes that:

I’ve seen plenty of cases where the velocity dispersion changes in important ways when more data are obtained, even starting from more than 10 tracers. Andromeda II comes to mind as an example. Indeed, several people have pointed out that if we did the same exercise with Fornax, using its globular clusters as the velocity tracers, we’d get a similar answer to what we find in DF2. But we also have measurements of many hundreds of stars in Fornax, so we know that answer is wrong. Perhaps the same thing is happening with DF2? The fact that DF2 is an outlier from everything else we know empirically suggests caution.

McGaugh (the leading authority on MOND) notes that the correctly calculated MOND prediction, including the External Field Effect is:

σ = 14 ± 4 km/s.

van Dokkum, et al., incorrectly calculated a MOND prediction of 20 km/s.

So, the evidence that this is really a no dark matter phenomena galaxy is not strong, and the evidence that it contradicts MOND is likewise weak.

[Moderator's note: This was posted in a separate thread while this thread was closed. Since the thread is reopened, this post and its responses have been moved here.]

 

[mfb]

σ = 14 ± 4 km/s.

That is not much better than no prediction at all (the 2 sigma range covers everything from 22 km/s to 6 km/s, a factor of nearly 4), and it is unclear how many of the assumptions that went into that number are included in the uncertainty.

van Dokkum, et al., incorrectly calculated a MOND prediction of 20 km/s.

I don't think it is fair to call it incorrectly. They used a different assumption (e.g. a larger separation from the other galaxy is sufficient).

The outlier has a very large uncertainty and a very large separation from the rest. Using an unweighted rms doesn't make sense. As far as I understand the original paper they don't remove it, they just assign a smaller weight to it according to the larger uncertainty. Which is the most reasonable thing to do.

 

[Vanadium 50]

That is not much better than no prediction at all

That is true. It is, however, key to understanding the dynamics of the galaxy. Ignore MOND. It's still important to the dynamics what the relationship between DF2 and NGC1052 is. And that's far from clear.

One way to look at this galaxy is that there is a discrepancy between the distance we infer from the dynamics, about 8 MPc, and the distance we infer from standard distance measures. The authors' favor the larger distance, which means the dynamics is odd, and it's interpreted as zero gas and zero dark matter (or worse, some gas and negative dark matter ). While I tend to agree with them, a closer distance would change their results (the authors say this as well) and would explain the anomalous brightness of the globulars. You would have to understand why DF2's light profile is 40-45% too smooth (the SBF distance measurement), but that's the only discrepant result left in this view.

 

[PeterDonis]

Moderator's note: A number of off topic posts have been deleted and the thread has been reopened. Please keep discussion within the bounds of peer-reviewed literature relevant to this particular galaxy.

 

[mitchell porter]

"Three papers today on the galaxy apparently lacking DM"

 

[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.