Bill Thomson, Harold Jeffreys and Dark Stuff

[oldman]

Is it by now conclusively established that the universe is made mostly of exotic dark matter and dark energy? I need reassuring that there is no escape from this still startling conclusion.

The following cautionary tale will explain why I have the temerity to ask such a question:

In the late 19th century geologists had become convinced (correctly as it turned out) that the Earth is a great deal older (several Gyr) than the 6000 years that the Biblical calculator Bishop Ussher had estimated. Geologists had a great deal of evidence to support their conclusion.

Then along came Bill Thomson (a prestigous physicist, aka Lord Kelvin) who proved, using impressive mathematics not understood by most geologists, that the Sun couldn't possibly be older than about 0.1 Gyr, and also that the Earth was not much older than that!

Of course the discovery of radioactivity and fusion has long since resolved this conflict in favour of geology.

Then in the early 20th century the prestigous mathematician/physicist Harold Jeffreys proved, again with impressive reasoning not familiar to the geological community, that the continental drift ideas of the geologists Wegener and Du Toit were nonsense.

Of course the validation of plate tectonics by J.T. Wilson has since reversed this verdict in favour of the early proponents of continental drift.

These examples show that even the most prestigous scientific folk are sometimes deceived by sly nature.

The most prestigous scientific folk since WW II have been nuclear physicists, for very good reasons. And it is they who have explained to cosmologists, a different breed of scientists, how the observed elemental abundances of the lighter elements, placed in the context of their own area of expertise, nuclear physics, put an upper bound on the amount of baryonic matter in the universe. The fact that this bound is far below the critical density of stuff required to explain our flat universe has since made cosmologists jump through many hoops about exotic unobserved forms of dark matter/energy.

Is it not possible that somehere in the setting of this upper bound sly nature has deceived those prestigous nuclear physicists? This is a cynical possibility suggested by an ignorant someone who is neither a mathematician, nuclear physicist, cosmologist or geologist.

I don't suppose that nature would sink so low as to deceive folk clever enough to invent the bomb ... but I'd be grateful if contributers to this forum could reassure me that there is in fact plenty of independent evidence of nature's good character.

 

[marcus]

Is it by now conclusively established that the universe is made mostly of exotic dark matter and dark energy?

No.

It has not been conclusively established.

 

[Garth]

Only when the DM particle(s), DE itself and the Higgs Boson/Inflaton are discovered in the laboratory (LHC?), their properties measured and found to be concordant with cosmological constraints will we know what we are talking about, and then we will be able to say the existence of these entities has been "conclusively established".

Until then cross checking the observations in as many ways as possible is the only means of verifying their existence. However these checks are always 'theory dependent' and may lead to degeneracies in the interpretation of the data.

One possible way of escaping from the '4%' baryonic matter restriction is the strictly linearly expanding universe, but you have to explain how to deliver such an expansion see A case for nucleosynthesis in slowly evolving models .

One theory that might have delivered this expansion has just died a death with the GP-B geodetic result!

Garth

 

[marcus]

If you would put the question in a more easy-going fashion, like Is there a current consensus that... then you could get a more reassuring answer.

Is there a current consensus among professional cosmologists that, if one goes according to classical 1915 General Relativity, most of the world is dark stuff?
Yes but.

the history of science is full of cases where current consensus changes. this current consensus could merely be an artifact of the coincidence that nobody has yet thought of a satisfactory modification of classical Relativity.

To get rid of the dark stuff, one would need to do find the right way to modify GR.
Indeed there are scores of people working on doing just that! Do you want a list of URL? Perhaps you already know the writings of some of them*.
These people are just as qualified and reputable as the majority. The only difference is they are in the minority, so they are not part of the consensus.


*people trying out ways to obviate the need for DE or DM or both:
Bekenstein, Moffat, Wiltshire, many others I would have to do a search to get names

 

[Garth]

*people trying out ways to obviate the need for DE or DM or both: Bekenstein, Moffat, Wiltshire, many others I would have to do a search to get names

You could start with Alternative theories being tested by Gravity Probe B.

Garth

 

[marcus]

You could start with Alternative theories being tested by Gravity Probe B.

Garth

Personally I would not want to get into that can of worms. What it looks like to me is that these days one can not SWING A CAT without hitting one of the people in this minority who are trying modifications of gravity to dispense with dark energy

I just went to arxiv and yelled modified gravity and got, e.g., this

http://arxiv.org/abs/0704.2406
Dynamics of the universe in the modified unimodular theory of gravity
Authors: Robert D. Bock
(Submitted on 18 Apr 2007)

Abstract: The equations that govern the dynamics of the universe in the modified unimodular theory of gravity are derived. We find a mechanism for inflation in the early universe without postulating a false vacuum state during the first $10^{-35}$ seconds after the Big Bang. In addition, we find a natural explanation for the acceleration of the universe without resorting to dark energy.
==================

http://arxiv.org/abs/0704.2520
Dark energy from modified F(R)-scalar-Gauss-Bonnet gravity
Authors: Shin'ichi Nojiri, Sergei D. Odintsov, Petr V. Tretyakov
(Submitted on 19 Apr 2007 (v1), last revised 20 Apr 2007 (this version, v2))

Abstract: The modified F(R)-scalar-Gauss-Bonnet gravity is proposed as dark energy model. The reconstruction program for such theory is developed. It is explicitly demonstrated that the known classical universe expansion history (deceleration epoch, transition to acceleration and effective quintessence, phantom or cosmological constant era) may naturally occur in such unified theory for some (reconstructed) classes of scalar potentials. Gauss-Bonnet assisted dark energy is also proposed. The possibility of cosmic acceleration is studied there.
=========

the search produced 595 papers, of which 12 were quite recent---posted April 2007.

Of those 12 that appeared this month, I chose two and pasted abstracts here as sample.

It is clear that there is a RUSH going on to try to modify the law of gravity (classic GR) so that one will not need Dark stuff. Many people are rushing into try their hand at this.

Obviously these people (although a minority) do not think that DE has been "conclusively" confirmed.

But it would be a waste of time and eyesight to try to keep abreast of this research IMHO

 

[Garth]

There is of course MOND as a way of also dispensing with exotic DM, although I think the case for DM is far more established.

If it does in fact exist the question then is; "What is it?"

Garth

 

[marcus]

"far more" established, unsure what it means, far more than minus infinity is still minus infinity
because don't think DE established at all (just a working consensus)
I withhold judgment---remain skeptical of Darks

Last year the famous NASA press conference where a guy I actually know Maxim was one of the presenters, advertised as the final blow to MOND---those bullet galaxy collision weak lensing findings.

but after that the MOND people found reasons to say those observations were compatible with MOND----there was that paper with the catchy title "Can MOND take a bullet?" (and survive, that is.)

Situation too complicated. Too much work to assemble all relevant sources.
MOND people are NOT looking only at DM, they also look at DE, some indication that a mond-like modification might eliminate need for both kinds of fudge.

I just happened to have these two links (don't presume to say they are especially relevant but I'll put'm down anyway)
http://arxiv.org/abs/astro-ph/0601581
http://arxiv.org/abs/gr-qc/0512109
another recent sample. I give the abstract for this one just to illustrate:
http://arxiv.org/abs/astro-ph/0605322
Dark energy, MOND and sub-millimeter tests of gravity
I. Navarro, K. Van Acoleyen
(Submitted on 12 May 2006)

"We consider modifications of General Relativity obtained by adding the logarithm of some curvature invariants to the Einstein-Hilbert action. These non-linear actions can explain the late-time acceleration of the universe giving an expansion history that differs from that of a pure cosmological constant. We show that they also modify the Newtonian potential below a fixed acceleration scale given by the late-time Hubble constant times the speed of light. This is exactly what is required in MOND, a phenomenological modification of the Newtonian potential that is capable of explaining galactic rotation curves without the need to introduce dark matter. We show that this kind of modification also predicts short distance deviations of Newton's law at the sub-mm scale and an anomalous shift in the precession of the Moon's orbit around the Earth, both effects of a size that is less than an order of magnitude below current bounds."

 

[Parlyne]

For completeness, it's probably worth noting that some people have suggested that it might be possible to get accelerated expansion (or, at least, the appearance of acceleration) straight out of GR with no dark energy. The idea is that variations in the density of matter in the universe can actually cause significant effects in terms of the expansion, due to the non-linearity of the GR field equations.

See, for example, http://arxiv.org/abs/astro-ph/0506534

Also, it's worth noting that, as easy as it seems to be to find modifications of gravity that might act like dark stuff, it tends to be even easier to find modifications of the standard model of particle physics that include massive particles that don't interact through the strong, weak, or electromagnetic forces - i.e. candidates for dark matter.

 

[Garth]

The case for Dark Matter is established on the nearly flat rotation curves of spiral galaxies, the virial mass of galactic clusters, the gravitational lensing of distant quasars by galactic clusters such as the Bullet cluster, and the temperature distribution of hot gas in galaxies and clusters of galaxies. Dark matter is vital in large structure formation and galaxy evolution, and has measurable effects on the anisotropy of the cosmic microwave background.

These different types of evidence suggest that there is far more matter in galaxies, galactic clusters and the universe as a whole than that which interacts with electromagnetic radiation: the remainder is called the "dark matter component".

The question is: "What is it?"

The matter component that does interact with electromagnetic radiation, stars, gas and dust, comprises only 0.003 of the critical density. The matter component comprises 0.27 of the critical density, and the maximum baryon density that standard BBN can produce is 0.04 of the critical density.

Hence 0.037 of the critical density is dark baryonic matter and, according to standard model BBN, 0.23 of the critical density is exotic non-baryonic dark matter. It is this that needs to be identified in the laboratory, alongside the Higgs Boson/Inflaton and DE to confirm that model.

We may not know what the 0.23 dark matter is, but there are multiple reasons to believe that it is there in some form or other.

Garth

 

[Wallace]

I agree that DM and DE are not conclusively proven, though I would point out the same could be argued about electrons (or protons or many other things) but that comes done to semantic objections I have to the use of the word 'proof' in empirical science.

DM and DE I would think are the most likely answers to the questions they were postulated to solve that we currently have, but not as likely to be the correct answer as electrons etc are to be for the reasons they were postulated.

Pedantry aside, have a read of http://arxiv.org/abs/astro-ph/0608407" does a far better job than I could.

This result is very compelling!

 

[marcus]

Pedantry aside, have a read of http://arxiv.org/abs/astro-ph/0608407" does a far better job than I could.

This result is very compelling!

That is the paper by my dive-buddy Maxim Markevitch!
I mentioned that earlier. "Bullet cluster galaxy merger" weak lensing observations. Big NASA ballyhoo!

It was the most convincing demonstration to date that you could not explain effects just as well or better with MOND.

Yes, and I remember reading Sean Carroll's popularization on his blog, when it came out.

However Wallace I've seen several papers since then suggesting that competent MOND people have answers, have shown MOND more compatible with those observations than people indicated in the initial reaction.

So I think it is wiser to reserve judgment---"the wheel's still in spin."

 

[Wallace]

Ahh yes, sorry I missed your earlier reference to those papers.

I have my suspicions about the MONDish theory and the bullet cluster papers though. They are hard to read and it is not clear qualitatively how you get the effects seen in the bullet cluster with MOND. I'm always suspicious of an argument that can't be summarized clearly for even a technical audience.

 

[marcus]

I think it is highly respectable for someone to now consider the case settled and believe that MOND is dead as a substitute for dark matter. (as long as they respect my disbelief)
What we have here is perhaps a difference in temperament leading to an acceptable range of attitudes.

My attitude is to be skeptical both of DM and DE and also (paradoxically enough) of MOND and other modified gravity.

I am refusing to consider anything as convincingly established for the time being.

I would be delighted (as would you I assume) if a "dark matter particle" were found or some extension of the standard model provided a clear and probable favorite candidate. But I would also be delighted if some (simple elegant) technical improvement of the law of gravity provided explanations of DE and DM effects.

I am happy to see research proceeding in both directions and I don't want to favor one or the other.

What I appreciate in you and some others is an undogmatic quality. I am interested in what you Wallace think especially, in fact, because you seem to leave me room have a different attitude.

(and oh yeah, most MOND strikes me as ugly, overcomplicated, and hard to followthru and be sure about---long way to go to make it elegant---but I am glad they are trying)

 

[Wallace]

The thing about MOND is that it initially made sense and seemed symmetrical to DM to explain galaxy rotation and cluster dispersion velocities. So we found that things were moving too fast for the amount of stuff we could see to hold it is using standard gravity. The two reasonable answers were then that either gravity works differently to the way we think (MOND) or there is more stuff there than we can see (DM). DM for various reasons seemed more likely but either possibility is most certainly possible and the answer to the problem given by each theory was logical and understandable.

However the bullet cluster seems so much harder for MOND to explain. A valid theory of gravity (to me) must obey one simple condition, that of the isotropy of space. To be clear, in this context what I mean by this is that a law of gravity should tell you what acceleration you experience due to the presence of some mass given how far that mass is from you only, not the direction to that mass. I simply cannot see how any theory of gravity that satisfies this condition can be compatible with the bullet cluster result, even one made up entirely to explain the bullet cluster results without caring about the rest of the Universe.

I simply cannot see how a law of gravity that obeys the isotropy of space* could possibly make the gravitational potential of a body not lie where the body actually is!

I don't think the bullet cluster will be the last word on DM, for starters another system showing the same kind of result that was studied by a different research group, and preferably with a different telescope (the second is unlikely in the near future!) would be ideal to confirm the results, however if you take the observational result as unproblematic (i.e. they didn't stuff up the data analysis) then I really don't see how anything other than a DM type hypothesis can explain the results.

I would stress though that when I say I can't see how it could be done I don't mean that it can't be done at all! If anyone can suggest how I would love to hear it since I can't see a clear explanation in any bullet-cluster and MOND paper I've read. If you have to BTWBS and can't simply summarise the principle I will, as stated above, be very suspicious of the result!

* Note that what I mean by isotropy of space is the basic symmetry principle, not anything to do with the Cosmological principle of the isotropy of the the Universe

 

[oldman]

No.

It has not been conclusively established.

A succint and decisive reply. No pussyfooting ... I like that .. thanks.

To get rid of the dark stuff, one would need to do find the right way to modify GR.
Indeed there are scores of people working on doing just that! Do you want a list of URL? Perhaps you already know the writings of some of them*.

Thanks for the offer to educate me, but I find it difficult enough to appreciate the complex logic of mainstream cosmology without delving into alternatives to GR, which I understand only dimly, despite the help of experts like Pervect.

The main point of my post was to suggest that maybe it is the nuclear physics of element synthesis has somehow gone awry, rather than GR ... and that the angst I believe many feel about dark matter/energy might be misplaced. Perhaps I should have asked:

Has it been conclusively established that the physics which determined present light-element abundances, i.e. physics at temperatures of 10^9 K, is flawlessly understood? There may be more mysteries about physics in the simple state of the early universe other than the pedominance of matter over anti-matter. Recognition of this possibility could save lots of hoop-jumping of the kind you refer to:

What it looks like to me is that these days one can not SWING A CAT without hitting one of the people in this minority who are trying modifications of gravity to dispense with dark energy

I liked this also. Did you know that the cat referred to is not domestic, but the fearsome Cat O'Nine Tails? Lending bite to my definition of "space".

 

[oldman]

The case for Dark Matter is established ...there is far more matter in galaxies, galactic clusters and the universe as a whole than that which interacts with electromagnetic radiation]: the remainder is called the "dark matter component".

The question is: "What is it?"

... according to standard model BBN, 0.23 of the critical density is exotic non-baryonic dark matter ...

We may not know what the 0.23 dark matter is, but there are multiple reasons to believe that it is there in some form or other.

Garth

Thanks for your clear summary of the situation, which I agree with, except for a few niggles. It's true that "exotic non-baryonic dark matter" wouldn't "interact(s) with electromagnetic radiation", but what evidence (apart from elemental abundances, that is) is there that the "dark matter component" is indeed exotic?

I suspect that this evidence is the very impressive bullet-cluster observations, which are certainly evidence of huge quantities of dark matter that is not charged plasma. But does such dark matter have to be exotic stuff? This seems to me as fanciful as claiming it to be discarded dirty cell phones of an exotic civilisation, which I think would also fit the bill. Or would it?

 

[Garth]

But does such dark matter have to be exotic stuff? This seems to me as fanciful as claiming it to be discarded dirty cell phones of an exotic civilisation, which I think would also fit the bill. Or would it?

The only evidence that DM of 23% critical density is exotic non-baryonic 'stuff' is the limitation put on baryonic density by the standard BBN of 4% critical density.

If that density limit could be lifted to 20 -30% then the need to invent a new breed of 'stuff', or 'entity', to fit the data, or 'save the appearances', falls away. None interacting DM is also useful to begin the formation of large scale structure before the epoch of last scattering of the CMB.

Of course from the particle physics point of view candidates for such exotic particles abound.

The problem is none have yet been found.

Garth

 

[Wallace]

I think that even ignoring BBN there are no plausible theories as to how there could be so much baryonic matter it a state that is so invisible. For instance I remember reading that while brown dwarves are so faint are hard to detect, if DM was entirely explained by brown dwarves the diffuse glow from them all would be very easily seen. There is just so much DM needed that if it really is just baryons, then the baryons making up the DM must be is such an exotic state that the discovery of such would be every bit as exotic as non-baryonic DM!

 

[Garth]

I think that even ignoring BBN there are no plausible theories as to how there could be so much baryonic matter it a state that is so invisible. For instance I remember reading that while brown dwarves are so faint are hard to detect, if DM was entirely explained by brown dwarves the diffuse glow from them all would be very easily seen. There is just so much DM needed that if it really is just baryons, then the baryons making up the DM must be is such an exotic state that the discovery of such would be every bit as exotic as non-baryonic DM!

Good point, but have you tried a combination of IMBHs and WHIM?

These might be the products of a PopIII epoch

Intergalactic cold H₂ is pretty difficult to observe as well.

Garth

 

[oldman]

If that density limit could be lifted to 20 -30% then the need to invent a new breed of 'stuff', or 'entity', to fit the data, or 'save the appearances', falls away.
Garth

Is there a good reason why it is impossible to even think of the limit being lifted to 100%, so as to go the whole hog and do away with the need for dark energy as well?

 

[Garth]

Is there a good reason why it is impossible to even think of the limit being lifted to 100%, so as to go the whole hog and do away with the need for dark energy as well?

Yes, you need an appropriate equation of state.

Whereas DM has basically zero pressure DE requires a pressure nearly equal to minus the density x c².

p = -$\rho$c² or nearly so.

This makes it pretty special stuff that is satisfied either by false vacuum energy or the cosmological constant, or some new 'stuff' called quintessence.

Quintessence can have an equation of state that evolves with time and its introduction increases the possibilities of resolving cosmological constraints, or it can be seen as just being highly speculative.

Until DE is discovered in a 'laboratory' experiment they are all just 'entities introduced to save the appearances' and it is right to be sceptical.

Garth

 

[Wallace]

Good point, but have you tried a combination of IMBHs and WHIM?

These might be the products of a PopIII epoch

Intergalactic cold H₂ is pretty difficult to observe as well.

Garth

I think IMBH's would have been seen with MACHO microlensing surveys if they were a significant contributor to DM. Nothing microlenses like a Black Hole! I'm unfamiliar with WHIM (do you mean WHIMPS?), if the later than of course it's not baryonic, if the former I apologies for my ignorance, could you explain what that stands for?

Cold H₂ would most definitely been seen if there was sufficient quantities to explain DM through Lyman $$\alpha$$ absorption lines in continuum sources such as QSO's.

Until DE is discovered in a 'laboratory' experiment they are all just 'entities introduced to save the appearances' and it is right to be skeptical.

I'm not so sure I agree. Of course a laboratory confirmation of DM and/or DE would be fantastic. However it may be the case that it is literally impossible to do this due to either the energy requirements or the lack of coupling with whatever DM an DE are (if they exist) and the stuff we can manipulate in a lab.

Do you think treating the Universe as our lab that we can observe will ever be sufficient enough? We don't have (and will never have) laboratory confirmation of many of the processes we observe and model in astronomy. Does this make it a lesser science? I would argue no, but I accept that I am somewhat biased

I would never claim that anything in empirical science is ever proven, but I do think that we could get to the stage where DM and/or DE micro physical theory is considered as sound as the standard model of PF is now entirely through cosmological observations and theoretical support of those observations. Of course we are a very long way from the at present, but as a somewhat philosophical question, do you think that cosmological observations alone will ever be convincing or will you always hold out for laboratory confirmation?

 

[hellfire]

I'm unfamiliar with WHIM

Stands for Warm-Hot Intergalactic Medium. The paper

Baryons in the Warm-Hot Intergalactic Medium, R. Davé et al.
http://arxiv.org/abs/astro-ph/0007217

pointed out that undetected baryons might be located in a WHIM phase. Since then, the Chandra has detected some WHIM clouds, IIRC.

 

[Garth]

I think IMBH's would have been seen with MACHO microlensing surveys if they were a significant contributor to DM. Nothing microlenses like a Black Hole! I'm unfamiliar with WHIM (do you mean WHIMPS?), if the later than of course it's not baryonic, if the former I apologies for my ignorance, could you explain what that stands for?

One paper on the amount of observed WHIM is: http://lanl.arxiv.org/abs/astro-ph/0501126"

$\Omega_{b}^{WHIM}$ (≥ 7 × 10¹⁴) = (2.4^+1.9−1.1) × 10^−[O/H]−1 %, consistent with both model predictions and the actual number of missing baryons.

So [O/H] is needed; now in Table 1 they state at:
z = 0.011 [O/H] > -1.47 and at
z = 0.027 [O/H] > -1.32,
so the upper limit is:
$\Omega_b$^WHIM > 4.3 × 10^0.47 % = 12.6%
and the lower limit:
$\Omega_b$^WHIM > 1.3 × 10^0.32 % = 2.7%.

Which is indeed consistent with the standard model of about $\Omega_b$ = 0.04, but it could be as high as $\Omega_{WHIM} = 0.126$!A population of IMBHs of around (10² - 10⁴)M_Solar may comprise some of the DM and I wonder whether these may have already been detected but mis-identified as MACHO's POINT-AGAPE Pixel Lensing Survey of M31 : Evidence for a MACHO contribution to Galactic Halos.

Note the mass of the lensing object depends on its relative distance to the object being lensed.

Cold H₂ would most definitely been seen if there was sufficient quantities to explain DM through Lyman $$\alpha$$ absorption lines in continuum sources such as QSO's.

But dense Lyman $\alpha$ forests are seen everywhere in quasar spectra!

Garth

 

[Wallace]

But dense Lyman $\alpha$ forests are seen everywhere in quasar spectra!

Garth

Exactly, but the amount seen falls far short of what would be needed for DM. If cold H₂ explained DM then we would see a lot more than we do. The point is that we can most definitely see it so it cannot be the missing DM.

 

[Nereid]

http://www.bautforum.com/showthread.php?t=42223" addresses many of the points raised in this thread, albeit not very technically (and doesn't get to DM in galaxies, but since such DM is a pretty trivial fraction of that in clusters, its absence has little bearing on a cosmological scale).

I am refusing to consider anything as convincingly established for the time being.

So what sorts of things would get you to think "convincingly established" marcus?

I'm quite curious, because there are lots of things in the astronomical universe (or perhaps one should say the astronomers' universe) that we can't convincingly establish, in one way or another, in Earthly labs - black holes, neutron stars, white dwarf stars, things to produce TeV gammas and UHECRs, metallic hydrogen, ...

 

[Garth]

http://www.bautforum.com/showthread.php?t=42223" addresses many of the points raised in this thread, albeit not very technically (and doesn't get to DM in galaxies, but since such DM is a pretty trivial fraction of that in clusters, its absence has little bearing on a cosmological scale).

I agree that all this is evidence of mass that cannot/has not yet been/ observed i.e. Dark Matter, but if there was a theoretical higher upper limit (say 27%) on the baryonic density from BBN would there be any other evidence that some, or most, of this DM is essentially non-baryonic in nature?

Garth

 

[Wallace]

Yes, as explained in previous posts we can't explain how that much baryonic matter could exist and not be seen. There is no state of that matter that would be sufficiently unobservable.

Black holes would microlens like crazy and we would see many more microlensing events than we do. Cold molecular hydrogen would be seen in absorption spectra in far bigger quantities than it is. Ionized hydrogen would give of far stronger recombination lines than are seen. Brown dwarves would give of a warm glow if they were in such vast quantities.

We simply do not know how baryons could hide themselves as well as DM does.

 

[Nereid]

I agree that all this is evidence of mass that cannot/has not yet been/ observed i.e. Dark Matter, but if there was a theoretical higher upper limit (say 27%) on the baryonic density from BBN would there be any other evidence that some, or most, of this DM is essentially non-baryonic in nature?

Garth

I think it's a question of how much consistency you are willing to accept (or demand).

For example, you may be able to construct a model with DM in the ICM (intra-cluster medium) of rich clusters being objects of characteristic size ~1 m, composed of highly refractory elements and/or compounds (I'm making this up, obviously), distributed throughout the ICM in such a way that it would also fit all relevant direct observational results (e.g. essentially none of it is being blowtorched by the jets from cluster AGNs). But I think most, if not all, such models would be hard to connect with the bigger picture - galaxy evolution, observed (nuclide) composition of the ICM, observed (nuclide) composition of high energy cosmic rays, ...

Even mild constraints from nuclide composition - ~90+% be H or He, say - severely limit how much of the DM can be baryonic ... neither H nor He can be solid (or liquid) and still be in equilibrium with the ICM (CMB, x-ray flux, etc), except if in gravitationally bound objects ("Neptunes to brown dwarfs"). And if in such forms, then I think you'll find only pathological distributions would be adequate to both escape detection and add up to a sufficiently large fraction of the estimated DM.

For other readers: it's important to remember that various amount of baryonic DM have been observed in the ICM - planetary nebulae, red giant stars, cold dust, hot plasma, ... and from these observations, more can be inferred: cold grit to rocks, non-giant stars (down to red dwarfs), rogue planets, brown dwarfs, white dwarfs, ... However, only very exteme distributions of any such of these can amount to enough mass to account for the estimated DM in rich clusters.

Oh, and for the marcuses reading this: MOND and similar alternative gravitation theories have similar, if somewhat milder, problems.

 

[marcus]

I have no desire to convince anyone to doubt that DM is material.
I merely state my own private skepticism: that it might be material, or on the other hand it might be the effect of a modified law of gravity.
Withholding judgment hardly needs to be justified but Nereid has asked me to explain so I'll say a couple of words.

The main thing is the estimated 73 percent dark energy. (You may insist that this is a different business but I'm not convinced it is.)

I think there is a chance that quantum gravity may explain Lambda.
Several quantum gravity approaches aim at a new understanding of space time and matter (to quote the title of Oriti's forthcoming book at CUP). Several approaches present the possibility of long-distance effects. If one understands better what spacetime is, or what more fundamental degrees of freedom underlie it, then one may arrive at an explanation for the observed accelerated expansion.

there are several ways this could happen and i want to keep my mind open to several possibilities.

About dark matter, it is possible that it is just a footnote to what I've been talking about. If one is going to have a quantum physics of spacetime that explains the 73 percent, then it may also turn out to explain the 23 percent.
To repeat: a major revision of the law of gravity (which encapsulates our understanding of spacetime, its geometry, its interaction with matter) may explain DE and if it does so then we might just get DM from it as a package deal.

I don't know any place in science where the unknown and currently uncomprehended is such a big fraction of the picture---so personally I wish to keep my mental options open, so to speak.

 

[Garth]

I don't know any place in science where the unknown and currently uncomprehended is such a big fraction of the picture---so personally I wish to keep my mental options open, so to speak.

It is always wise to keep an open mind, especially in a subject that relies so much on 'remote sensing' as cosmology does.

As far as the remote sensing of the Earth, or Mars, is concerned the crucial thing is to calibrate the results with 'Ground Truth', i.e. the verification of the interpretation of data obtained at a distance by a satellite with observations made on the ground.

The equivalent of 'ground truth' in cosmology is the controlled testing of GR and fundamental physics by laboratory experiments. The present continuing analysis of the GP-B experiment is the latest chapter in this process.

We live in exciting times.

 

[Nereid]

I have no desire to convince anyone to doubt that DM is material.
I merely state my own private skepticism: that it might be material, or on the other hand it might be the effect of a modified law of gravity.
Withholding judgment hardly needs to be justified but Nereid has asked me to explain so I'll say a couple of words.

The part that I'm curious about is not the scepticism, private or otherwise, but the general question of how one establishes some reality about parts of the universe about which our only physical inputs are photons*.

Dark matter is a good case study - how long do we wait for a modified theory of gravity to make the non-baryonic part go away, before it becomes subject to only normal scepticism? Or will it always remain in special (astronomical) class of its own, somewhere beyond black holes and diffuse interstellar bands?

(Of course, if we find DM particles, in anyone of the Earthly DM detectors and DM telescopes, then the world changes; ditto if there is good observational results pointing to non-baryonic DM being black holes of one sort or another).

The main thing is the estimated 73 percent dark energy. (You may insist that this is a different business but I'm not convinced it is.)

Indeed - it's just this 'what would convince you' that I'm curious about: will they remain always 'the same business' for you, simply because they are both tied to 'gravity'?

I think there is a chance that quantum gravity may explain Lambda.
Several quantum gravity approaches aim at a new understanding of space time and matter (to quote the title of Oriti's forthcoming book at CUP). Several approaches present the possibility of long-distance effects. If one understands better what spacetime is, or what more fundamental degrees of freedom underlie it, then one may arrive at an explanation for the observed accelerated expansion.

there are several ways this could happen and i want to keep my mind open to several possibilities.

As I said in Wallace's DE poll thread, I think there's some semantic confusion around the term 'dark energy' ... I also think that there's a vigorous effort to develop extensions and alternatives to GR, one motivation of which is to find a different explanation of the astronomical observations.

But still the question remains: 'dark energy', in one reading, only exists because of GR. In this sense, how different is DE from black holes?

About dark matter, it is possible that it is just a footnote to what I've been talking about. If one is going to have a quantum physics of spacetime that explains the 73 percent, then it may also turn out to explain the 23 percent.
To repeat: a major revision of the law of gravity (which encapsulates our understanding of spacetime, its geometry, its interaction with matter) may explain DE and if it does so then we might just get DM from it as a package deal.

Then again, it might not.

Or such a major revision may lead, via re-analyses and re-interpretation of the relevant astronomical results, to some new thing which is apparently just as mysterious and wonderous as DE.

And so on.

Of course various alternatives and possibilities should be investigated ...

I don't know any place in science where the unknown and currently uncomprehended is such a big fraction of the picture---so personally I wish to keep my mental options open, so to speak.

In terms of our daily lives, a very great deal of modern science has vast unknowns and currently uncomprendeds that have zero impact (unless you happen to be a scientist working in that particular field).

Why single out DE or DM? After all, to take just one example, how many decades are there from Planck lengths to the finest (size) resolution our best 'microscopes' can see? Yet that terra incognita must surely be, by some measure, a big fraction of the picture?

Just curious, nothing more.

*For some distance beyond our solar system, there are direct physical inputs in the form of neutral gas and interstellar dust; for some unknown distance beyond the solar system, there are also 'galactic' cosmic rays; for SN1987 there are also 19 neutrinos. At some time - hopefully very soon! - there will also be gravitational radiation; within the next decade or three, there may be some source direction info from UHECRs as well as detection (and maybe source directions too) of high energy neutrinos.

 

[marcus]

Indeed - it's just this 'what would convince you' that I'm curious about: will they remain always 'the same business' for you, simply because they are both tied to 'gravity'?

:rofl:
Of course not Nereid!

I would, of course, be absolutely delighted if a dark matter particle were found and dark matter were shown conclusively to be a form of material----that would could presumably fit into our scheme of matter (extensions of the standard model of particle physics.)

then it would definitely be a different business, wouldn't it?

I really think I should not be called to account for my skepticism, but you seem to think there is something wrong with it.

1. I do not consider the bullet cluster data conclusive. I require more such evidence and I need to see how the MOND people accommodate it in THEIR picture. (You probably realize that MOND accommodated the bullet cluster report in several ways and were not as put out by it as some of us expected.)

2. At present we are in a time of flux, with only 4 percent of the matter in the universe accounted for and research proceding actively on several fronts:
a. quantizing GR, so that quantum corrections appear (both UV and IR corrections being discussed)
b. modifying GR so as to obviate the need for either or both DE and DM.

With so much in doubt, so much new information coming in, and unexpected theoretical developments also appearing, I prefer to take a "wait and see" stance about Dark Matter.

I don't worry about you. We don't have to agree. I am quite happy if you are convinced that it is certain to be a particle which will be discovered in due time, etc., or whatever you believe.

I really don't think it is appropriate for someone to insist that I account for my lack of conviction or my mental reservations on this issue.

 

[marcus]

It is always wise to keep an open mind, especially in a subject that relies so much on 'remote sensing' as cosmology does.We live in exciting times.

Thank you Garth. I appreciate your expressing understanding for my attitude.
In fact we do live in exciting times. Cosmology has suddenly become an observational quantitative science (rather than merely a field of speculation)
and knowledge about the universe is exploding.
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EDIT to save a post I will reply here to Nereid #36.

Your post is a well written and reasoned short essay in itself, Nereid. And quite persuasive to boot. Thanks for taking the care you did with it.
With luck, lots of us will read it. Delightful if you are right and a DM particle can be detected. Presumably there would be swarms of them, bathing the earth. So if they are there perhaps we will learn to detect them. Until then, I remain as usual your doubting marcus :-)