Merged Dark Matter vs. Modified Newtonian Dynamics

[Reality Check]

Dark matter continues to evade detection. I've read a few articles lately suggesting theories like Modified Newtonian Dynamics are being taken more seriously. Anyone working in the field have any comments on where things stand/are headed?

Dark matter has been detected since the 1930's when it was first noticed that galaxies could not be bound in galaxy clusters given the amount of visible matter. What is still missing is the direct detection of any dark matter candidates that could be detected. The point is that it is possible for dark matter to be undetectable by experiments on Earth. There is no requirement for the universe to not contain "weird" stuff that cannot be detected directly.

MOND is still a minor theory. MOND works better than dark matter for galaxies but fails badly for larger scales. That has been its state for a few decades now. There were hints that modifying GR would work better (e.g. TeVeS) but that seems to have bigger problems.

 

[Reality Check]

Two reasons: he's an authority in the relevant field and seems to know what he's talking about; his conclusion matches up with my own intuition.

Two problems:

• Argument from authority (not necessarily bad).
• Cherry-picking 1 opinion that matches your own intuition.

What I see cited is an National Public Radio opinion article by an Adam Frank. I read an opinion with a rather dumb analogy to ghosts which misses out the best evidence for dark matter (colliding galaxy clusters, CMB data).

I can cite an authority whose opinion matches my knowledge of the evidence for and against dark matter - the many blog articles on dark matter at Starts With a Bang Ethan Siegal, an astrophysicist.

 

[JeanTate]

stuff
stəf/Submit
noun
1.
matter, material, articles, or activities of a specified or indeterminate kind that are being referred to, indicated, or implied.

I don't know why you have a problem with "stuff". It seems to fit what we're talking about quite well.

Of course.

Except that this is the Science (etc) section of ISF, and we're discussing something that concerns astronomy and HEP. Using vague, misleading, etc terms in such discussions frequently, and usually quickly, leads to confusion, misunderstanding, etc.

Energy can legitimately, in everyday English, be called "stuff", as can photons, even forces.

In my experience, being very clear in the meanings of key words you use in discussions in science greatly helps.

When I get a chance, I'll write a post about CDM, and the different perspectives on it, from astronomy and HEP.

 

[Fudbucker]

Dark matter has been detected since the 1930's when it was first noticed that galaxies could not be bound in galaxy clusters given the amount of visible matter. What is still missing is the direct detection of any dark matter candidates that could be detected. The point is that it is possible for dark matter to be undetectable by experiments on Earth. There is no requirement for the universe to not contain "weird" stuff that cannot be detected directly.

MOND is still a minor theory. MOND works better than dark matter for galaxies but fails badly for larger scales. That has been its state for a few decades now. There were hints that modifying GR would work better (e.g. TeVeS) but that seems to have bigger problems.

Nobody said there was. There's no requirement that the universe not be a simulation. That doesn't mean it is.

I'm not saying DM doesn't exist, or isn't currently the best answer. My question is a hypothetical: what are we eventually going to conclude if every attempt to directly observe DM fails? How long do you search? Is it scientific to posit the existence of undetectable things? How would you prove an undetectable thing exists? This is already causing some problems wrt the existence of other universes. Inflation theory seems to imply (or at least allow for the existence of) other universes. But how do you test to see if other universes actually exist, though? Is multiverse theory scientific?

 

[Fudbucker]

Two problems:

• Argument from authority (not necessarily bad).

• 
  
  We argue from authority all the time. It's only a fallacy in formal logic. In discussions like this, it's the rule.
  
  

  [*]Cherry-picking 1 opinion that matches your own intuition.
  
  What I see cited is an National Public Radio opinion article by an Adam Frank. I read an opinion with a rather dumb analogy to ghosts which misses out the best evidence for dark matter (colliding galaxy clusters, CMB data).
  
  I can cite an authority whose opinion matches my knowledge of the evidence for and against dark matter - the many blog articles on dark matter at Starts With a Bang Ethan Siegal, an astrophysicist.

  
  I like Ethan Siegel a lot, and already cited him. This is a great article: "http://scienceblogs.com/startswithabang/2012/12/07/yes-we-really-really-need-dark-matter/"
  
  BUT. Siegel is not addressing my point, and the NPR author is. My point isn't about what the best explanation currently is. Since I wrote the OP, I've clearly learned DM is still the leading candidate. My point is where things are possibly headed. What happens down the road if we keep failing to directly detect it?

 

[Reality Check]

We argue from authority all the time. It's only a fallacy in formal logic. In discussions like this, it's the rule.

Argument from authority is generally considered a fallacy in many areas. It is a always a fallacy when someone relies on the opinion of an authority. Especially when they cherry pick that opinion to agree with their intuition.
A more obvious example of a bad argument from authority is argument from non-authority, e.g. a biologist writing on cosmology. A less obvious example of a bad argument from authority is an argument from a biased authority, e.g. citing Jacob Bekenstein who works on relativistic MOND theories.

The answer to your question is simple: If we exhaust the possibilities of detectable dark matter candidates then we are left with non-detectable dark matter candidates.

 

[Reality Check]

what are we eventually going to conclude if every attempt to directly observe DM fails?

We have strong evidence dark matter that exists so we will conclude that DM cannot be directly detected.

How long do you search?

As long as we are curious - a billion years?

Is it scientific to posit the existence of undetectable things?

Yes. We have strong evidence that quarks exist and they are even more undetectable than dark matter! Ditto for dark energy.

How would you prove an undetectable thing exists?

From indirect evidence, e.g. dark matter, dark energy, quarks.

 

[JeanTate]

Nobody said there was. There's no requirement that the universe not be a simulation. That doesn't mean it is.

I'm not saying DM doesn't exist, or isn't currently the best answer. My question is a hypothetical: what are we eventually going to conclude if every attempt to directly observe DM fails?

Who can say?

Could anyone in Newton's day have said anything meaningful about quantum mechanics?

How long do you search?

As long as it takes, surely.

Is it scientific to posit the existence of undetectable things?

Depends on what you mean by "undetectable".

Black holes were, up until the LIGO result was published, just as undetectable as CDM. In things like the LHC, unstable neutral particles are frequently inferred from the presence of charged particles and/or photons into which they decay. And so on.

How would you prove an undetectable thing exists?

We're talking science, not mathematics; science doesn't do proof.

How about this: the existence of CDM is consistent with all relevant observational and experimental results, to date; and no theories/hypotheses/models/etc which seek to account for these observational and experimental results via something other than CDM is (consistent), also to date.

What else can you ask for, in science?

This is already causing some problems wrt the existence of other universes. Inflation theory seems to imply (or at least allow for the existence of) other universes. But how do you test to see if other universes actually exist, though? Is multiverse theory scientific?

Different discussion, as far as I know. Why do you think CDM has anything to do with multiple universes?

 

[JeanTate]

Two reasons: he's an authority in the relevant field

Are you sure?

His Rochester University page doesn't mention anything about CDM (his focus is "the hydrodynamic and magneto-hydrodynamic evolution of matter ejected from stars"), and a quick look at publications by "A. Frank", using ADS, didn't seem to turn up anything by him on CDM.

and seems to know what he's talking about

Again, are you sure?

His NPR piece, which you cited, isn't 100% clear, but it sure leaves one with the impression that he is unaware of the extensive work that's been done, over many years, on "other explanations for the bumps we know we're hearing in the night". Myself, I don't doubt that he knows about MOND (and many other such alternatives), but I am most unimpressed that he chose to downplay them. I am equally unimpressed by his creating a false dichotomy; an astrophysicist with as long a work/publication history as he has, and with an apparently big stake in science communication, can do much better.

; his conclusion matches up with my own intuition. When you search for something long enough, and don't find it, your confidence that it's there drops with each failed search attempt. Positing an unprovable explanation for an observation isn't intellectually satisfying. It's a dead end.

May I suggest that you have a mental construct of what science - and astronomy/astrophysics in particular - is that is, um, misaligned with reality?

 

[Perpetual Student]

It is not correct to say that dark matter is undetectable. By definition, dark matter interacts with matter gravitationally.
We may or may not be able to construct some small scale (laboratory) experiment to duplicate this interaction (it is possible that there is no other interaction with matter).
Unless some mathematically consistent and observationally confirmed modification of GR is developed, dark matter is the only alternative. Take it or leave it.

 

[JeanTate]

<snip>

When I get a chance, I'll write a post about CDM, and the different perspectives on it, from astronomy and HEP.

Here goes ...

Astronomy: starting in the 1930s, observations relevant to CDM of extragalactic objects, and of the CMB (starting much later), have been made. Those relevant to CDM are all made by detecting photons (though there's some data with implications for CDM that come from the non-detection of certain gravitational waves).

Astrophysics: analyses of these observations, using standard physics (mostly GR and relevant parts of atomic, nuclear, and particle physics), show very good consistency with CDM hypotheses. A general summary: there appears to be a form of matter, called cold dark matter, which comprises ~80% of the matter in the observable universe. CDM does not seem to interact with other matter except via gravity; it does not produce, or absorb, photons. It does not seem to be composed of baryons. The smallest clumps of CDM seem to be ~100 pc or so in size. While there certainly is a lot of mass in the form of baryons that is 'dark', this can comprise at most some ~10% of all dark matter.

Particle physics: the Standard Model (of particle physics; SM) is very successful, one reason why we can say with considerable confidence that whatever CDM is, it cannot be comprised of baryons. However, the SM has been tested 'only' up to ~10 TeV; cosmic rays with energies many OOM (orders of magnitude) greater have been observed, and they're still way short of the Planck regime (where, among other things, GR and QM are so wildly mutually inconsistent that nothing meaningful can be said). In the early universe, energies vastly exceeding those probed by the LHC were the norm. With the success of the LHC, high energy particle physics (HEP) is at a bit of a dead end; there are few pointers to what to do, experiment-wise, to study BSM (Beyond the SM) physics.

Physics: GR and QM are both wildly, astonishingly successful; they can account for almost all observed phenomena (to date). Yet they are mutually incompatible, at a very deep level. Attempts to find a deeper theory, one which produces both GR and QM 'naturally', in the appropriate regimes, have not been very successful so far (not for want of trying). One problem: physical regimes where the incompatibility between GR and QM can be explored are waaaay beyond anything we can produce in labs here on Earth. And searching for astronomical answers - other parts/times in the universe where this incompatibility must occur - is ... challenging.

Alternatives: Perhaps the most exciting thing about CDM (and Dark Energy) is the possibility that it may be a footprint of gravity which is not well described by GR.

Questions?

 

[Belz...]

But imagine 100 years from now and we still haven't figured out what it is or directly detected it? That would be a huge problem, wouldn't you agree?

Science doesn't work on a timetable.

Sure, Dark Matter could turn out to be an error in our current theories. Correcting that error might make the problem go away, but currently they're still the best explanation for the discrepancies.

 

[Belz...]

We're talking science, not mathematics; science doesn't do proof.

I think we can simply carry on with conversations about science by understanding that we're using "proof" to mean "evidence" and "prove" to mean "provide conclusive evidence", no? Everybody knows what we mean by those words and so they maintain their use here.

 

[Belz...]

Alternatives: Perhaps the most exciting thing about CDM (and Dark Energy) is the possibility that it may be a footprint of gravity which is not well described by GR.

Questions?

About that last bit: could determining the nature of Dark Matter be a step towards reconciliating QM and GR?

 

[JeanTate]

We're talking science, not mathematics; science doesn't do proof.

I think we can simply carry on with conversations about science by understanding that we're using "proof" to mean "evidence" and "prove" to mean "provide conclusive evidence", no? Everybody knows what we mean by those words and so they maintain their use here.

In ISF SMMT threads where I'm pretty certain that's true, for all active participants, I agree.

However, my impression is that that's not the case in this thread. Fudbucker?

 

[JeanTate]

Alternatives: Perhaps the most exciting thing about CDM (and Dark Energy) is the possibility that it may be a footprint of gravity which is not well described by GR.

Questions?

About that last bit: could determining the nature of Dark Matter be a step towards reconciliating QM and GR?

Yes, it could.

And I think you can find the odd paper or two, in arXiv's gr-qc section (General Relativity and Quantum Cosmology), which explore some such ideas. Here, for example, is a recent one (perhaps not directly what you were thinking of): "Gravitational Waves as a New Probe of Bose-Einstein Condensate Dark Matter" (arXiv:1609.03939)

Myself, I'd rate them extremely speculative ...

 

[Fudbucker]

In ISF SMMT threads where I'm pretty certain that's true, for all active participants, I agree.

However, my impression is that that's not the case in this thread. Fudbucker?

You guys gave me a lot to think about and read up on. Thanks for the replies!

 

[Roboramma]

I'm not saying DM doesn't exist, or isn't currently the best answer. My question is a hypothetical: what are we eventually going to conclude if every attempt to directly observe DM fails? How long do you search? Is it scientific to posit the existence of undetectable things? How would you prove an undetectable thing exists? This is already causing some problems wrt the existence of other universes. Inflation theory seems to imply (or at least allow for the existence of) other universes. But how do you test to see if other universes actually exist, though? Is multiverse theory scientific?

Dark matter that only interacts with gravity isn't undetectable. It's detectable by it's gravitational influence.

 

[fuelair]

Despite what you might read, that's not how it works.

The various experiments aimed at detecting DM particles are based on certain assumptions about the properties of those particles, and they in turn are based on theories of particle physics.

The unsearched search space for such postulated particles is huge, and experiments so far have explored just a really tiny part of it.

The same is true for astronomical searches for such particles, e.g. via gamma ray lines.


Actually, as I said above, MOND doesn't work, because it is inconsistent with Relativity. Maybe you'd like to pick one of the many relativity-consistent models?

Also, to repeat, MOND-like models are inconsistent with, e.g., CMB observations. And there's no obvious fix for that.


This is a false dichotomy.

An alternative model/idea/whatever will become accepted once it does a good job of accounting for all relevant experimental and observational results. Irrespective of any failures to detect DM particles.
Again, that's not how it works.

Take the question "what powers the Sun?"

That question has been around since forever, certainly since the start of the scientific revolution. And no "mainstream theory" could provide an answer, until the relatively new nuclear physics was applied to it. So, for several centuries it was a mystery, and no Fudbucker ancestor could have predicted when it would be solved.

Oh, and DM can most certainly be detected!

If it couldn't, how would we know it exists?

Cue questions about existence and detection, especially in astronomy, and about direct vs indirect ...

And once something has been determined to exist research on it is the natural flow of science - until we (or some other intelligent life forms) figure it out. Which does not mean just accepting any random idea that sounds good, but actually finding and verifying the actual cause. And throwing out any/all of those that do not/cannot.

 

[fuelair]

Dark matter that only interacts with gravity isn't undetectable. It's detectable by it's gravitational influence.

aka, you can't see it but you know it is there and is a real thing.