Dark Matter Real: Studies Confirm, Modifying Gravity Can't Work

[sector99]

Thanks. This raises a query about DM density waves. I'm guessing that since DM somehow can't be "compressed" there can't be any DM density waves thus the spiral appearance (which doesn't arise from kinematic rotation-including especially the barred spirals) is due solely to baryonic matter?

 

[mfb]

Dark matter can be compressed easily. What would stop it? It is collisionless (at least to a good approximation).

 

[Ken G]

Dark matter would be hard to compress externally by some kind of wall, but there aren't walls like that in any astrophysical context (other than perhaps magnetic fields, but those aren't active in, say, spiral density waves either). Collisionless does not mean low pressure, so can have high pressure and exhibit the same resistance to compression as any high pressure gas. Indeed, spiral density waves don't separate the stars from the gas, even though the former is collisionless and the latter is collisional. So something a little different than a gravitationally induced density wave must going on in the Bullet Cluster.

Gas pressure is the momentum flux density of the particles, so only requires velocities that are high and isotropic. In short, the ideal gas law. It's true that collisions are useful for maintaining the isotropic velocities (that's really all collisions do in regard to gas pressure), but there must be some other way that dark matter does it because dark matter is normally assumed to be at a temperature. I think it must be its history of obeying the cosmological principle which must ultimately be the source of the isotropy of its velocities. Honestly I am not sure why dark matter is always assumed to be thermalized, and even often treated as isothermal. Clearly the dark matter particles interacting in the Bullet Cluster has velocities that "remember" which cluster they came from, so are not thermalized, yet the Milky Way dark matter is generally assumed to be thermalized. It must have to do with the history.

The confusion about pressure and compressibility in collisionless gases probably traces to the fact that introductory sources often confuse pressure with forces on boundaries next to the gas, which requires collisions with the boundary (though still not between the particles themselves). But the force from/on a wall is actually nothing more than the action/reaction involved in the mundane "normal force" seen in so many other contexts, so is not pressure any more than standing on a bathroom scale is gravity, but it is a good way to measure pressure of gas that isn't dark matter. I don't know why so many sources like to think of gas pressure as a force on/from a wall, as it is more usefully and more flexibly thought of as a force on the gas itself, stemming from the gas itself, stemming from the momentum flux density within the gas. And even though the fluid approximation is made much easier to assume by collisions, it is used for dark matter also. But yes, the Bullet Cluster dark matter is more easily understood not in a fluid picture!

 

[sector99]

DM not being "compressed" or, collected by galactic center gravitation was what I meant to say.

On the characteristic of being collisionless–does this feature reduce exotic bosonic/hadronic matter as a DM candidate?

 

[jocarren]

I have a methodological objection about DM. Such objection doesn't mean that i reject the DM hypothesis nor all of the observations that support it, but to express concern about the following:

1.- The difficulty for detection.
2.- Reluctancy to postulate new physics.

Since the existence of DM is inferred from it's gravitational effect on galactic motion and there is no other interaction (internal nor external) it is pretty much defined by this two aspects.

This raises the question: how can DM be observed if not by it's defining qualities?

Up until this point, most of the observation efforts are focused on mapping it's gravitational interaction (mainly through lensing). The remaining effort is focused on finding correlations between expected behavior of particles both hypothetical and within the standard model, with no success as far as I know.

Problem is, as difficult as proving it's existence is, maybe it's even more difficult to disprove it. Since the only thing that can be observed is the very thing that defines the subject of observation: Gravity and non interaction.

The way i see it, the unfalsifiability issue is not inherent to the claim that something is causing the phenomena observed, but emerges from an excessively broad definition of DM. Efforts should be made not only to observe it, but to narrow it's definition so the verification can be methodologically simpler.

About the reluctancy towards new physics, i don't mean to say that GR is wrong and the subsequent cosmological models are too, everything seems to point out that they are correct (and i personally subscribe to that notion), but this sort of discrepancy between the observed phenomena and the proven theory claims for a paradigm shift or at the very least (which i believe is the case here) a fresh interpretation of mainstream physics.

But, as i said before, this is not about disproving DM, it's about the concern that a methodological issue can undermine it's verification.

 

[Orodruin]

I have a methodological objection about DM. Such objection doesn't mean that i reject the DM hypothesis nor all of the observations that support it, but to express concern about the following:

1.- The difficulty for detection.
2.- Reluctancy to postulate new physics.

Since the existence of DM is inferred from it's gravitational effect on galactic motion and there is no other interaction (internal nor external) it is pretty much defined by this two aspects.

I believe you are, to a large degree, chasing after a red herring. If it was not difficult to detect dark matter, it would have been observed a long time ago and so it is rather a sign of the times we live in.

When it comes to reluctancy to postulate new physics in the dark matter community, it is simply untrue. If you had spent any significant amount of time reading research articles about dark matter and dark matter models, you would be aware of how eager dark matter theorists are to dream up new ways of postulating physics beyond the standard model that could explain the dark matter observations.

This raises the question: how can DM be observed if not by it's defining qualities?

This depends on what dark matter actually is, which we have a large number of theories about. For most of those theories, there are accompanying ways of looking for dark matter in other ways. Some are being actively pursued by the experimental community.

Up until this point, most of the observation efforts are focused on mapping it's gravitational interaction (mainly through lensing). The remaining effort is focused on finding correlations between expected behavior of particles both hypothetical and within the standard model, with no success as far as I know.

Problem is, as difficult as proving it's existence is, maybe it's even more difficult to disprove it. Since the only thing that can be observed is the very thing that defines the subject of observation: Gravity and non interaction.

There is absolutely no question about the fact that something is going on with respect to gravitational phenomena. This something is generally well described by cold dark matter. If it looks like a duck and quacks like a duck ... You might as well call it a duck.

The way i see it, the unfalsifiability issue is not inherent to the claim that something is causing the phenomena observed, but emerges from an excessively broad definition of DM. Efforts should be made not only to observe it, but to narrow it's definition so the verification can be methodologically simpler.

Again, this just shows that you are not familiar with how front-line research is being done within the dark matter community. Dark matter is not a single thing, it is rather a phenomenon that can possibly be explained by a particle with some particular properties. The point is not narrowing the definition of this phenomenon, it is to investigate models where the phenomenon arises and consider what other effects would arise from those models. This is precisely what is being done.

About the reluctancy towards new physics, i don't mean to say that GR is wrong and the subsequent cosmological models are too, everything seems to point out that they are correct (and i personally subscribe to that notion), but this sort of discrepancy between the observed phenomena and the proven theory claims for a paradigm shift or at the very least (which i believe is the case here) a fresh interpretation of mainstream physics.

Again, saying that the dark matter community is reluctant towards new physics could not be further from the truth.

 

[Ken G]

We should probably accept it as natural when a new theory exhibits growing pains. We were spoiled with general relativity, which went from formulative to well-verified in just a few decades. Dark matter has already taken much longer than that, and will likely exhibit further growing pains going forward-- it's not a cause for any panic. It hasn't even generated a Nobel prize yet, because the progress has been slow and rather incremental. Dark energy did generate a Nobel prize, but clearly has more growing pains to go through as well. We simply need to press forward on efforts to hone and test these theories, keeping open all alternatives but also investing in making gains in whatever area seems to be the most productive. At present, that's WIMP type theories.

 

[jocarren]

I believe you are, to a large degree, chasing after a red herring. If it was not difficult to detect dark matter, it would have been observed a long time ago and so it is rather a sign of the times we live in.

When it comes to reluctancy to postulate new physics in the dark matter community, it is simply untrue. If you had spent any significant amount of time reading research articles about dark matter and dark matter models, you would be aware of how eager dark matter theorists are to dream up new ways of postulating physics beyond the standard model that could explain the dark matter observations.

That is tranquilizing.

This depends on what dark matter actually is, which we have a large number of theories about. For most of those theories, there are accompanying ways of looking for dark matter in other ways. Some are being actively pursued by the experimental community.

There is absolutely no question about the fact that something is going on with respect to gravitational phenomena. This something is generally well described by cold dark matter. If it looks like a duck and quacks like a duck ... You might as well call it a duck.

It could be a goose (i think DM it's a lousy name)

Again, this just shows that you are not familiar with how front-line research is being done within the dark matter community. Dark matter is not a single thing, it is rather a phenomenon that can possibly be explained by a particle with some particular properties. The point is not narrowing the definition of this phenomenon, it is to investigate models where the phenomenon arises and consider what other effects would arise from those models. This is precisely what is being done.

Narrowing down the definition it's important in the "non-interaction" part, if the definition of something is "you can't observe it", experimental work becomes increasingly hard.

Again, saying that the dark matter community is reluctant towards new physics could not be further from the truth.

Again, good to know.

 

[Ken G]

The "W" in WIMP is "weakly", not "non." Most people regard neutrinos as dark matter, for example, and it was difficult, but not impossible, to verify them.

 

[sector99]

["Again, saying that the dark matter community is reluctant towards new physics could not be further from the truth."]

Well said. The effort may well trend towards a hybrid solution–given the extreme complexity of SDSS large-scale mapping. Starting with the question: Do DM constituents have to be matter?

As a total newbie, I stumbled upon Fahr/Heyl and their recent work involving abundant invisible possible candidates including photons plus vacuum energy & mass density under expansion scenarios:

The idea that CMB photons retain their 3000K creation energy until detection caught me by surprise–as did the theoretical standing wave mode involved in redshift (5,7 & 8 above).

 

[PeterDonis]

Reluctancy to postulate new physics.

I don't understand this objection: postulating dark matter is postulating new physics--it is postulating the existence of particles (more precisely, quantum fields) other than those that appear in the Standard Model of particle physics.

It is true that postulating dark matter is not postulating new gravitational physics--the dark matter hypothesis assumes that our best current theory of gravity, General Relativity, is correct, by contrast with MOND and similar hypotheses, which do not. But this is not a matter of postulating new physics vs. not postulating new physics; it's only a matter of where one prefers to put the new physics--new particles/fields vs. a new theory of gravity.

 

[Orodruin]

i think DM it's a lousy name

That's your prerogative. It does not change anything of substance whether you like the name or not.

Narrowing down the definition it's important in the "non-interaction" part, if the definition of something is "you can't observe it", experimental work becomes increasingly hard.

It is not part of the definition that it does not interact. You have simply somehow gotten the wrong ideas into your mind.

 

[jocarren]

I apologize, i don't mean to cuestion the work of physicist on the subject, just to place my concerns (which can be failures on my behalf).
What I mean by "a lousy name" is that is misleading: about "dark", it's low (or no) interaction can be better described as "translucid", and "matter" (which is the good part of the name, in my opinion) is not completely accurate because there is no certainty that the effect observed is indeed matter (although is the most plausible explanation).

 

[Ken G]

I tell students that the term "dark" means like the "dark ages," more along the lines that we are "in the dark" about it-- just like "dark energy." But the term still causes confusion, and there are always students who take the sucker answer on a multiple choice test that dark matter is what you see when you are actually looking at dust lanes in a galaxy. So yes, a better name would have been possible, but what else is new in astronomy? (Planetary nebulae, white dwarfs vs. main-sequence dwarfs, Big Bang, stars look brighter as their "magnitude" goes down, OBAFGKM, the list goes on and on.) A bad name can be used as a teaching moment, in the process of understanding its flaws and the history of why it came to be called that.

 

[phinds]

As for DM and DE: Are they like oil and water?

No. Oil and water are both liquids. DM and DE are more like fish and bicycles. They have absolutely nothing to do with each other.

 

[ohwilleke]

No. Oil and water are both liquids. DM and DE are more like fish and bicycles. They have absolutely nothing to do with each other.

Maybe, maybe not. There are many extant theories that attempt to explain both in an integrated fashion.

For example, Chaplygin gas models such as those discussed at https://arxiv.org/abs/1904.07510, or "Dark energy and dark matter unification from dynamical space time: observational constraints and cosmological implications" https://arxiv.org/abs/1904.05762, to name a couple of papers just in the last month.

There have also been modified gravity proposals to integrate dark matter and dark energy. Again, looking just at examples of papers released this month, these include f(T) gravity https://arxiv.org/abs/1904.09897 and (less recently) conformal gravity https://arxiv.org/abs/1208.4972.

One motivation for a unified explanation is the "cosmic coincidence problem" (i.e. that the total amount of dark matter and the total amount of dark energy in the universe are of the same order of magnitude). See, discussing this "problem" (scare quotes in the original) https://arxiv.org/abs/1410.2509.

 

[ohwilleke]

It is not part of the definition that it does not interact. You have simply somehow gotten the wrong ideas into your mind.

The lambdaCDM definition includes "almost collisionless" which isn't inconsistent with not interacting at all non-gravitationally, but also doesn't require that it not interact non-gravitationally. (Obviously, it must interact gravitationally since that is how we observe it.) Generally "almost collisionless" is defined operationally as not significantly more strongly interacting with ordinary matter than neutrinos.

 

[ohwilleke]

I don't understand this objection: postulating dark matter is postulating new physics--it is postulating the existence of particles (more precisely, quantum fields) other than those that appear in the Standard Model of particle physics.

It is true that postulating dark matter is not postulating new gravitational physics--the dark matter hypothesis assumes that our best current theory of gravity, General Relativity, is correct, by contrast with MOND and similar hypotheses, which do not. But this is not a matter of postulating new physics vs. not postulating new physics; it's only a matter of where one prefers to put the new physics--new particles/fields vs. a new theory of gravity.

Just to complicate matters further, many of the leading dark matter particle theories (e.g. self-interaction dark matter or SIDM) theories, introduce not only "new physics" particle/quantum field, but also introduce a "new physics" fifth force with its own (usually a Yukawa force with a light but massive carrier boson).

 

[ohwilleke]

Most people regard neutrinos as dark matter

I'm not convinced that this is accurate.

 

[ohwilleke]

At present, that's WIMP type theories.

WIMP theories are among the least viable of dark matter particle theories outstanding and have been ruled out over very wide areas of parameter space. The originally most popular WIMP theories, those in which the lightest supersymmetric particle serves as the dark matter candidate, are virtually entirely ruled out.

Some of the most promising dark matter particle theories these days are those with axion-like dark matter particles, and those with self-interacting dark matter particles with this fifth dark matter force mediated by a massive carrier boson.

 

[Orodruin]

I'm not convinced that this is accurate.

I do not think it is controversial to claim that neutrinos are hot dark matter. The problem is that their abundance can only account for between 0.1% and 0.3% of the energy budget as compared to the ca 1/4 of the budget that would be required to be the main dark matter component.

 

[timmdeeg]

The other problem is that neutrinos moving almost with light speed are to fast to form a “cloude” around a galaxy.

 

[Orodruin]

The other problem is that neutrinos moving almost with light speed are to fast to form a “cloude” around a galaxy.

As I said, neutrinos are hot dark matter, not cold dark matter. This does not mean that the neutrino background cannot cluster today. Whether they do that or not (and to what degree) depends on the neutrino masses.

 

[Ken G]

The originally most popular WIMP theories, those in which the lightest supersymmetric particle serves as the dark matter candidate, are virtually entirely ruled out.

Ruling out supersymmetric versions of WIMP theories is hardly the same thing as ruling out all WIMP theories.

Some of the most promising dark matter particle theories these days are those with axion-like dark matter particles, and those with self-interacting dark matter particles with this fifth dark matter force mediated by a massive carrier boson.

And the particles you are describing are weakly interacting, are they not? That's the "WI". They are nonrelativistic, are they not? That means they are massive, the "M". They are particles, are they not? That's the "P". I'm not sure what other distinction you are making, perhaps you are saying the term has fallen out of favor, but it's still WIMPs.

 

[sector99]

No. Oil and water are both liquids. DM and DE are more like fish and bicycles. They have absolutely nothing to do with each other.

The SDSS (and its supercomputer facimilies) reveal spatial voids and enveloping baryonic aggregations.

It appears that baryons/DM have more affinity for themselves than they do for whatever is filling the voids. Ergo my query.

 

[FourEyedRaven]

Two recent studies have found galaxies with little or no apparent dark matter, indicating modifying gravity can't work.
https://iopscience.iop.org/article/10.3847/2041-8213/ab0e8chttps://iopscience.iop.org/article/10.3847/2041-8213/ab0d92/meta

This PBS Space Time video gave a good explanation, at least for amateurs like me.

 

[sector99]

FYI

https://phys.org/news/2019-04-hubble-universe-faster.html

 

[Bandersnatch]

@sector99 FYI I'm pretty sure most posting here (and certainly Orodruin) are aware of the discrepancy referred to above, since it's nothing new and the thread level is 'A'. Some might even be aware of the avenues currently explored to address it.
What is objectionable is saying 'X isn't known, Y isn't known - therefore X is Y'. At least not without good reason.

 

[PeterDonis]

Thread closed for moderation.

Edit: Some off topic posts have been deleted. Thread reopened.

 

[PeterDonis]

It appears that baryons/DM have more affinity for themselves than they do for whatever is filling the voids

There isn't anything filling the voids; that's why they're called "voids". The "affinity" you refer to is called "gravity"; basically you're saying that baryons and DM are sources of gravity. This is hardly news.