Undergrad Questions regarding dark matter dynamics

[nikkkom]

When they only have identified about 2 percent of the matter in the galaxy, finding another 2 percent is a "large change". As I pointed out, they were comparing the amount of known stellar mass to the amount of mass they found, and it was quite comparable to the known stellar mass of the galaxy.

You are backtracking now. You did not say "comparable", you said:
"we 'discovered' more ordinary baryonic matter *surrounding* every galaxy that exist inside of the stars themselves".

10 billion solar masses of gas claimed in the paper is not more than all stellar mass in Milky Way. It's much less.
There are different estimates of the latter, but they are all above 40 billion solar.
Example: https://arxiv.org/pdf/1102.4340.pdf estimates total stellar mass in MW to be 64+-6 billion solar.

 

[MichaelMo]

In addition to rotation curves, Bullet cluster, and nucleosynthesis, there's also the power spectrum of baryon acoustic oscillations, which so far I haven't seen mentioned in this thread - despite being the least burdened with uncertainties.
This means there are four independent data points, all suggesting the same conclusion, which is what makes DM such a strong hypothesis.

Let's take a close look at those four data points:

We have a Bullet Cluster study that is known to be *riddled* with baryonic mass underestimation problems.

We have galaxy mass estimation/rotation models that also suffer from the same mass halo underestimation problems as the Bullet Cluster study.

We have a potential *falsification* mechanism for LCMD theory with respect to nucleosynthesis requirements of exotic forms of matter for which we have no laboratory evidence whatsoever, even after spending billions of dollars looking for it.

Finally we have BAO estimates which also do not work *without* the need for exotic forms of matter and energy so we have two potential falsification methods for one specific cosmology theory, specifically LCDM. Both the nucleosythesis and BAO numbers seem to work out to the same ratio of exotic vs. baryonic matter which could be used as possibly *two* different remaining data points of evidence, but they could both be falsified in the absence of exotic forms of matter and we have no evidence from the lab that exotic forms of matter exist in nature.

I'd grant you two independent data points in 2017, but not 4. :)

 

[MichaelMo]

You are backtracking now.

Well, I've already admitted that my use of the term "more" was potentially a tad misleading, and that word was worth backtracking from, but you still seem to be using a single number related to their findings, whereas they actually suggested that your 10 billion solar mass estimate was just a low end starting point and the actual number could be significantly higher:

http://www.urban-astronomer.com/news-and-updates/milky-way-surrounded-by-hot-gas/

Astronomers estimate the temperature of this hot gas at between 1 and 2.5 million Kelvins, or several hundred times hotter than the surface of the Sun. It is also huge, containing a mass of gas of at least 10 billion Suns, and possibly as much as 60 billion Suns.

Emphasis mine.

You did not say "comparable", you said:
"we 'discovered' more ordinary baryonic matter *surrounding* every galaxy that exist inside of the stars themselves".

10 billion solar masses of gas claimed in the paper is not more than all stellar mass in Milky Way. It's much less.
There are different estimates of the latter, but they are all above 40 billion solar.
Example: https://arxiv.org/pdf/1102.4340.pdf estimates total stellar mass in MW to be 64+-6 billion solar.

Their top end figure is significantly higher than 40 billion solar masses (it's 60), so in theory at least my use of the term "more" could be correct, but admittedly it's an "optimistic" (and subjective) assessment on my part. If your 70 solar mass number is correct, their 60 billion solar mass top end is still very comparable to all the baryonic mass that we had found prior to 2012.

 

[nikkkom]

Their top end figure is significantly higher than 40 billion solar masses (it's 60), so in theory at least my use of the term "more" could be correct, but admittedly it's an "optimistic" (and subjective) assessment on my part. If your 70 solar mass number is correct, their 60 billion solar mass top end is still very comparable to all the baryonic mass that we had found prior to 2012.

Wrong. 64+-6 billion solar masses is not the estimate of all baryonic mass in Milky Way. It's estimate of _mass of stars only_. I'm sure you know that we _know_ (for at least a century) that stars are not the only baryonic mass in MW.
Total MW mass estimates are 500-800 billion.

 

[nikkkom]

requirements of exotic forms of matter for which we have no laboratory evidence whatsoever, even after spending billions of dollars looking for it.

Those very same experiments also fail to detect one form of matter - neutrinos - which we are 100.00% sure exist. Therefore, non-observation (so far) of dark matter is not a strong argument against it.
In fact, many theories posit that DM is nothing else than new types of neutrino (say, right-handed neutrinos with Majorana masses).

 

[MichaelMo]

Wrong. 64+-6 billion solar masses is not the estimate of all baryonic mass in Milky Way. It's estimate of _mass of stars only_. I'm sure you know that we _know_ (for at least a century) that stars are not the only baryonic mass in MW.
Total MW mass estimates are 500-800 billion.

Let's be specific. I was specifically comparing the amount of baryonic mass that they found in 2012 with the baryonic mass they'd discovered prior to 2012. If we use a 600 billion solar mass total, and divide that number by 6 because "dark matter" is presumed to be five times more abundant than baryonic mass, that's around 100 billion solar masses of baryonic mass total that is predicted to exist in our galaxy in LCDM theory. Of that total, only between 40 and 60 billion solar masses are concentrated in stars, and the rest is typically described as the "missing baryon" problem. Both of the "halo" papers were specifically describing that "missing baryon" mass, and it's presumed to be about half of the total baryonic mass. We're talking about comparing stellar baryonic mass, to a "plasma halo" mass that contains somewhere between 10 and 60 billion solar masses.

My original statement may have been a little "optimistic" by my use of the term "more", but either way, the authors did suggest that the they'd found the missing baryonic mass that we haven't accounted for yet.

Now of course there is not only a "hot plasma" halo that's been discovered since 2012, there's also a "neutral hydrogen" gas halo that's also been discovered and expected to also hold a tremendous amount of mass.

If anything, there isn't a "missing baryon problem" anymore, there's potentially an *excess baryon problem* when we add in both halo masses. Mind you that's all in addition to all the satellite galaxies that we keep discovering around our galaxy every year.

http://www.blastr.com/2017-2-23/astronomers-discover-new-satellite-milky-way

 

[MichaelMo]

Those very same experiments also fail to detect one form of matter - neutrinos - which we are 100.00% sure exist. Therefore, non-observation (so far) of dark matter is not a strong argument against it.
In fact, many theories posit that DM is nothing else than new types of neutrino (say, right-handed neutrinos with Majorana masses).

There are however other experiments which were/are specifically designed upon the mathematical predictions of neutrino theory rather than WIMP or Axion theory which do detect neutrinos. There really isn't any known evidence to support other types of neutrinos either based on neutrino detector data.

http://www.latimes.com/science/scie...ube-sterile-neutrino-20160809-snap-story.html

Neutrinos would also tend to be "hot dark matter", as opposed to "cold dark matter", and I have no idea how that might effect either the nucleosynthesis predictions of LCMD, or the BAO predictions.

 

[Adrian59]

In any conversation about dark matter, it is only a matter of time before the Bullet Cluster makes an appearance. However, reading some recent reports it is far from clear that the Bullet Cluster is a good piece of evidence for or against dark matter. A couple of early papers from 2010 and 2011 suggested that the in-fall velocity was too high to support ΛCDM. A more recent paper by Craig Lage and Glennys R. Farrar published 25 February 2015 in Journal of Cosmology and Astroparticle Physics, concluded “due to the paucity of examples of clusters with such a high mass in simulations, these features of the main cluster cannot presently be used to test ΛCDM.”

 

[Adrian59]

It appears that you don't know how historically dark matter theory came to be.

At first, astronomers and cosmologists did assume that baryonic matter is all that there is.

A few observations (in 1930-40) which claimed to maybe detect discrepancies, were ignored - which is ok, since there are _always_ some observations which find "something strange", but these may well be instrument errors or mistaken interpretation or logic of their authors.

Then Vera Rubin in 1970s worked on galaxy rotation curves and found that galaxies seem to be heavier than they should be. Her work was high-quality and was checked by other independent measurements, but still, science did not jump on dark matter bandwagon overnight. The status shifted to "hmmm, there is indeed something fishy here! Let's look at it more carefully!"

The entire 1980s were spent doing more observations, looking at several disjoing pieces of evidence, and all of them pointed quite consistently to the conclusion that baryonic mass alone is far from being enough to explain them.

Since you don't remember this long and convoluted process of history, you seem to assume everybody just happily fudges their models and observations to satisfy their preconceived notion that "dark matter exists"?

I think you answered a different question. My interpretation is that Michael was suggesting care was needed in using CDM to answer problems about Deuterium abundance when CDM is still hypothetical until a definite candidate for CDM is discovered.

 

[nikkkom]

There are however other experiments which were/are specifically designed upon the mathematical predictions of neutrino theory rather than WIMP or Axion theory which do detect neutrinos.

My argument is, DM detectors are not detecting not only unknown DM particles, they also (predictably) fail to detect one type of _known_ ones. This invalidates argument "DM detectors don't see anything, thus existence of DM particles is becoming more questionable."

There really isn't any known evidence to support other types of neutrinos either based on neutrino detector data.

No wonder. If "usual" (straightforward extension of SM) right-handed neutrinos do exist, they are not interacting even via weak force, and interaction probability for them with anything is much smaller still than for left-handed ones. Current DM detectors _should not_ see them.

Neutrinos would also tend to be "hot dark matter", as opposed to "cold dark matter"

Only light ones.

 

[nikkkom]

Let's be specific. I was specifically comparing the amount of baryonic mass that they found in 2012 with the baryonic mass they'd discovered prior to 2012. If we use a 600 billion solar mass total, and divide that number by 6 because "dark matter" is presumed to be five times more abundant than baryonic mass, that's around 100 billion solar masses of baryonic mass total that is predicted to exist in our galaxy in LCDM theory. Of that total, only between 40 and 60 billion solar masses are concentrated in stars, and the rest is typically described as the "missing baryon" problem.

No. The rest is gas and dust. Astronomers were always aware that gas is a large fraction of MW mass.

 

[nikkkom]

Mind you that's all in addition to all the satellite galaxies that we keep discovering around our galaxy every year.

http://www.blastr.com/2017-2-23/astronomers-discover-new-satellite-milky-way

It's hard to corral all mass estimates for them, but I found a few for well-known (presumably more easily detectable, larger) satellites:
Leo I: 20+-10 million solar
Leo II: 27+-5M
Draco Dwarf: 86M

Color me not impressed by their masses...

 

[nikkkom]

I think you answered a different question. My interpretation is that Michael was suggesting care was needed in using CDM to answer problems about Deuterium abundance when CDM is still hypothetical until a definite candidate for CDM is discovered.

And my answer is that "dark matter" was not invented on a whim. Astronomers did consider other alternatives. And they will continue to do so. However, at the moment no unexpected huge new amounts of baryon matter are found. (New gas is found, and old estimates are being refined, but the changes are not huge). Large discrepancy still remains, so DM theory is still viable.

 

[nikkkom]

Look at this "keV sterile neutrino DM" review paper: https://arxiv.org/pdf/1602.04816.pdf

257 pages, 992 references to other papers.

Does it look likely to you that all these people somehow missed such a mundane possibility that no DM is necessary, all mass is in fact baryonic (gas / dust / rogue planets / etc)?

 

[Adrian59]

And my answer is that "dark matter" was not invented on a whim. Astronomers did consider other alternatives. And they will continue to do so. However, at the moment no unexpected huge new amounts of baryon matter are found. (New gas is found, and old estimates are being refined, but the changes are not huge). Large discrepancy still remains, so DM theory is still viable.

I still think you are missing a subtlety in Michael’s comment (no. 16) and your responses since prove that. I don’t think anyone, least of all myself, has said that dark matter hypothesis was a ‘whim”’. Your final comment is indeed correct that ‘DM theory is still viable’ though what you don’t say and is also correct is that DM theory is unproven ie it still is a possibility not a definite. So I stand by my comment (no. 40).

 

[Drakkith]

Your final comment is indeed correct that ‘DM theory is still viable’ though what you don’t say and is also correct is that DM theory is unproven ie it still is a possibility not a definite.

Sorry, but I fail to see how this is a valid criticism. Every theory in science is unproven and possibly wrong, so there's nothing unusual about not stating this outright.

 

[Adrian59]

Sorry, but I fail to see how this is a valid criticism. Every theory in science is unproven and possibly wrong, so there's nothing unusual about not stating this outright.

So you may have heard of Karl Popper as well. Though, there are different levels of certainty. This discussion goes back to using the deuterium anomaly as a robust argument for dark matter which as has been pointed out is questionable when dark matter is unproven. Quantum mechanics may yet be shown false but if the deuterium anomaly could be solved by a quantum mechanical argument this would be a more robust result since quantum mechanics is on a surer footing than dark matter.

 

[Drakkith]

So you may have heard of Karl Popper as well. Though, there are different levels of certainty. This discussion goes back to using the deuterium anomaly as a robust argument for dark matter which as has been pointed out is questionable when dark matter is unproven. Quantum mechanics may yet be shown false but if the deuterium anomaly could be solved by a quantum mechanical argument this would be a more robust result since quantum mechanics is on a surer footing than dark matter.

So what you mean in the part of your post that I quoted before is that because the uncertainty is larger in CDM than, say, Quantum Mechanics, we need to be careful when using it? If so, then forgive me but we already seem to be doing that.

 

[Drakkith]

Somewhere between the early 70's and 2006 the term however gradually "morphed" from being synonymous with "we don't know what that missing mass is made of", to being associated with an exotic type of matter. I don't have any doubt that there is evidence of 'missing mass' from galaxy mass estimation techniques, but I have no evidence to suggest that any of that missing mass is to be found in exotic types of matter, and in fact I have no laboratory evidence that exotic forms of matter even exist in nature.

Then the question is, why do so many cosmologists disagree with you?

 

[MichaelMo]

Then the question is, why do so many cosmologists disagree with you?

I doubt that they disagree about the fact that there is no controlled experimental evidence of exotic matter from lab experiments. I kinda doubt that they actually disagree about the fact that their bayonic mass estimates of galaxies in that 2006 bullet cluster study were flawed either. They might still hold out some "hope:" that exotic types of matter exists in nature, but after billions of dollars of lab tests, nothing from lab would require that exotic forms of matter *must* exist. What part do you think they actually disagree with?

 

[Drakkith]

What part do you think they actually disagree with?

For starters, I'd say they would disagree with your understanding of their position and the thinking that the LCDM model is in serious jeopardy.

As it stands, the nucleosynthesis argument seems more like a case of special pleading, only so that LCDM can be considered exempt by falsification by the non-existence of exotic forms of matter.

They'd probably disagree with this as well.

 

[MichaelMo]

For starters, I'd say they would disagree with your understanding of their position and the thinking that the LCDM model is in serious jeopardy.

I wasn't actually trying to describe the mainsttream position with respect to whether or not LCMD theory is in serious jeopardy, although I probably was wearing my own beliefs on my sleeve. I realize that I hold a minority viewpoint as it relates to cosmology theory, but like I said, I simply see no controlled experimental evidence that requires the introduction of exotic forms of matter, nor do I see any evidence from cosmological studies that would require the existence of exotic matter to explain.

They'd probably disagree with this as well.

Perhaps you could explain how one might go about falsifying the existence of exotic forms of matter? We have already spent billions of dollars on lab tests and we've found no hint of anything exotic beyond the standard particle physics model. Normally speaking the onus of responsibility falls to the ones that are making the claim because it's technically impossible to prove a negative. I cannot "prove" that invisible unicorns do not exist in nature, nor can I prove that exotic forms of matter and/or energy do not exist.

Hypothetically speaking, it should be possible to falsify CMD theory, and LCDM theory along with it, but I haven't a clue how we might do that in the lab in any way that we have not already tried. What is the scientific value of consistent NULL results from billions of dollars worth of "tests" of various models of CDM?

 

[MichaelMo]

Sorry, but I fail to see how this is a valid criticism. Every theory in science is unproven and possibly wrong, so there's nothing unusual about not stating this outright.

https://arxiv.org/abs/astro-ph/0608407

I think there is something seriously wrong when one tries to claim that they have found 'proof' (rather than ordinary evidence) to support dark matter theory as was the case with that bullet cluster study. The "proof" which they claimed to find in 2006 was entirely (and subjectively) dependent upon the accuracy of their mass estimation techniques, and many studies since 2006 have called their baryonic mass estimation methods into question.

Even the terms they used to describe the evidence were not even "scientific" terms since theories are never actually 'proven', they are simply supported by evidence, or they are not. Only laws can be said to be 'proven' to some extent, but even laws can be overturned by new evidence. The terms which are used by the mainstream do seem to conflict with the concept of conservative science at times. 2006 was such a time and such an instance. In 2017 those boastful claims of 'proof' of dark matter from 2006 look pretty questionable. Did they actually 'prove' that dark matter exists, or did they "prove" that their baryonic mass estimation techniques were flawed and they needed to be fixed?

 

[Drakkith]

I realize that I hold a minority viewpoint as it relates to cosmology theory, but like I said, I simply see no controlled experimental evidence that requires the introduction of exotic forms of matter, nor do I see any evidence from cosmological studies that would require the existence of exotic matter to explain.

Nothing requires dark matter. There are potentially several other possibilities other than dark matter. The problem is that nothing so far works as well as dark matter. Modified laws of gravity are even more problematic and require alteration of fundamental laws of nature.

Perhaps you could explain how one might go about falsifying the existence of exotic forms of matter?

Hypothesize a particular form of exotic matter and then make models based on its properties. If observations don't match the model then that's a serious blow to that particular type of exotic matter, potentially falsifying it. This is especially true if observations can be adequately explained by other models that don't use that particular type of exotic matter. If your model based on exotic matter can't adequately explain the observations and another model can, that's essentially a death-blow to that model.

Currently, models using dark matter agree with observations in many areas, though not all and not as accurately as we'd like. On top of that we don't yet have a better way of explaining our observations.

We have already spent billions of dollars on lab tests and we've found no hint of anything exotic beyond the standard particle physics model. Normally speaking the onus of responsibility falls to the ones that are making the claim because it's technically impossible to prove a negative. I cannot "prove" that invisible unicorns do not exist in nature, nor can I prove that exotic forms of matter and/or energy do not exist.

Indeed. Detecting dark matter candidates in our test labs would support dark matter models enormously. But the fact that we haven't found any yet isn't nearly as detrimental as you think given that dark matter isn't thought to strongly interact with normal matter.

 

[Drakkith]

I think there is something seriously wrong when one tries to claim that they have found 'proof' (rather than ordinary evidence) to support dark matter theory as was the case with that bullet cluster study.

I agree that using the word "proof" is inappropriate. Unfortunately not everyone thinks this way. Though I certainly wouldn't consider it seriously wrong, just a minor mistake at worst.

Even the terms they used to describe the evidence were not even "scientific" terms since theories are never actually 'proven', they are simply supported by evidence, or they are not.

The fact that someone incorrectly uses "proof" rather than "evidence" has little bearing on the validity of the paper or the model/theory.

The terms which are used by the mainstream do seem to conflict with the concept of conservative science at times.

...because someone uses "proof" instead of "evidence"?

 

[MichaelMo]

Nothing requires dark matter. There are potentially several other possibilities other than dark matter. The problem is that nothing so far works as well as dark matter. Modified laws of gravity are even more problematic and require alteration of fundamental laws of nature.

Well, MOND doesn't seem to explain the Bullet Cluster data, but ordinary plasma and dust would probably do the trick.

Hypothesize a particular form of exotic matter and then make models based on its properties. If observations don't match the model then that's a serious blow to that particular type of exotic matter, potentially falsifying it. This is especially true if observations can be adequately explained by other models that don't use that particular type of exotic matter. If your model based on exotic matter can't adequately explain the observations and another model can, that's essentially a death-blow to that model.

WIMP theory was typically associated (not necessarily exclusively) with SUSY theory which seems to have made a host of predictions that were falsified at LHC. That hasn't stopped anyone from funding WIMP theories in general however.

Currently, models using dark matter agree with observations in many areas, though not all and not as accurately as we'd like. On top of that we don't yet have a better way of explaining our observations.

I suppose we'll just have to agree to disagree on that point. :)

Indeed. Detecting dark matter candidates in our test labs would support dark matter models enormously. But the fact that we haven't found any yet isn't nearly as detrimental as you think given that dark matter isn't thought to strongly interact with normal matter.

I think that the next LUX experiments, and possibly the Xenon1T experiments get us uncomfortably close to neutrino interaction. Does the weakness of the interaction have any influence on nucleosynthesis predictions or power spectrum predictions?

 

[MichaelMo]

I agree that using the word "proof" is inappropriate. Unfortunately not everyone thinks this way. Though I certainly wouldn't consider it seriously wrong, just a minor mistake at worst.

The fact that someone incorrectly uses "proof" rather than "evidence" has little bearing on the validity of the paper or the model/theory.

I agree with you that it's a minor mistake in terms of the impact on "scientists", but a lot of folks in the general public might just hear the term "proof" and in their mind it's a "done deal".

...because someone uses "proof" instead of "evidence"?

I'm picky. :)

 

[Drakkith]

I suppose we'll just have to agree to disagree on that point. :)

Just as long as you realize you're disagreeing with most mainstream cosmologists, sure.

Does the weakness of the interaction have any influence on nucleosynthesis predictions or power spectrum predictions?

No idea, sorry.

 

[Adrian59]

Quote from Drakkith in entry no. 59: 'Hypothesize a particular form of exotic matter and then make models based on its properties. If observations don't match the model then that's a serious blow to that particular type of exotic matter, potentially falsifying it.'

This is a good point though some, Stacy McGaugh for example, question whether this is actually what is happening since early experiments have failed to find exotic dark matter particles the goal posts are moving. The graph of cross section versus mass of possible WIMPs is constantly being redrawn as experiments fail to find the WIMP at a particular energy. So with each failure the line is redrawn to a new mass / cross section and the hunt continues.

 

[Adrian59]

Hypothesize a particular form of exotic matter and then make models based on its properties. If observations don't match the model then that's a serious blow to that particular type of exotic matter, potentially falsifying it.

In addition to rotation curves, Bullet cluster, and nucleosynthesis, there's also the power spectrum of baryon acoustic oscillations, which so far I haven't seen mentioned in this thread - despite being the least burdened with uncertainties.
This means there are four independent data points, all suggesting the same conclusion, which is what makes DM such a strong hypothesis.

I have recently being trying to track down the original paper(s) relating to the interpretation of the CMBR anisotropy which is what I assume is synonymous with BAO. I have found papers by Hu and Dodelson from 2002, Hu and white from 1996 and Seljak from 1994 as well as a recent review by Tojerio 2006. The Hu and Dodelson seem the most comprehensive review and it was this paper that referenced Seljak (1994) for the relative heights of the peaks as determined by the ratio of baryonic matter to CDM. However, none of these papers appear to come clean as to how they directly derive the CMBR curves. It does appear that the theoretical derivation of the CMBR curves are very model dependent and as such it can be argued that the result is in the assumptions made for the model.