Bill Thomson, Harold Jeffreys and Dark Stuff

[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[/color].
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[/color].
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.[/size]

 

[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'?

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.
==============
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 :-)

 

[Nereid]

Sorry if my curiosity came across wrongly marcus; I had (and have) no intention of calling anyone to account, for anything.

A bit of background: one of the things I'm quite interested in now is how best to convey what astronomy (and cosmology) as a science actually is, given that, for example, we seem to have no opportunity (in any of our lifetimes) of actually traveling to 3C 273 (say) and conducting in situ experiments.

Showing that all (astronomical) observations contain (physics) theories - there are no 'pure', theory-free observations - is (or should be) relatively easy and straight-forward. Getting acceptance of the idea of a cosmological principle is also relatively easy (though working through a particular cosmological principle may be difficult).

Then you can start on some easy rungs of a ladder - an H emission spectrum, for example, or trigonometric parallax.

But before too long you get to things that no one has ever seen in any earthly lab, nor (likely) ever will. Some, such as forbidden transitions that occur in physical conditions unattainable in our labs, likely cause few qualms; others, such as neutron stars and black holes, are steps too far for some folk.

And then there's (non-baryonic) dark matter, and 'dark energy'.

About the only thing I've found - so far - which gets good traction is a path through particle physics ... an astonishing universe opened up this last century or so, as particle accelerators reached higher and higher energies. Some of these wonders could also be seen, clearly or with difficulty, in painstaking studies of cosmic rays; some were found first in cosmic rays, some not.

But UHECRs clearly show the universe has particle accelerators which make the LHC look utterly trivial. If we look at how much 'new physics' there is in the several decades of energy between the proton's mass and the top quark (say), and compare that with the number of decades of energy between the top quark and the highest energy UHECR we've detected to date, is it so certain that there will be no 'new physics' there? And if the possibility of some 'new physics' is (grudgingly) admitted, can the possibility of 'new mass' (i.e. non-baryonic dark matter) be also admitted?

 

[oldman]

To an outsider, it's interesting to see how this thread has developed into a discussion about the nature of dark matter/energy, rather than questioning accepted wisdom about "the limitation put on baryonic density by the standard BBN of 4% critical density" -- Garth, post # 18.

It seems as if the standard BBN must be accepted as dogma, unlike other relevant aspects of theory, like the laws of gravity of Newton ("... gravity works differently to the way we think (MOND)" -- Wallace, 15) and Einstein ("one would need to do find the right way to modify GR" -- Marcus,4).

Even "modifications of the standard model of particle physics (may provide) ...candidates for dark matter"-- Parlyne, 9. Or expansion in the standard model of cosmology might be "strictly linear" and remove the dark matter problem -- Garth, 3. And, if the dark energy problem "could be resolved by a quantum physics of spacetime that explains the 73 percent, then it may also turn out to explain the 23 percent" of dark matter --- Marcus, 31.

Cosmologists certainly are free-thinkers, and yet:

the large quantities dark matter known to exist for "multiple reasons -- Garth, 10, if baryons, cannot it seems be imagined to "hide themselves as well as DM does" -- Wallace, 29 and it is only "very exteme distributions of (baryonic matter that) can amount to enough mass to account for the estimated DM in rich clusters -- Nereid, 30.

Of course the snippets I have extracted from posts may not correctly reflect the attitudes of their authors, but isn't it time to revisit carefully the wisdom that has been received from prestigious nuclear physicists, or at least talk about it with less reverence?

 

[marcus]

...

It seems as if the standard BBN must be accepted as dogma, unlike other relevant aspects of theory,..

I would interpret the acronym alternatively as "Big Bounce Nucleosynthesis".

I think the standard BBN story can be questioned but I don't see how it could qualitatively change the picture----could it make an order of magnitude change in the 4 percent?

In case other folks would like a simple online discussion of BBN which compares it with CMB there is this
http://astro.uchicago.edu/~tyler/omegab.html
It is a bit out of date, I believe the apparent conflict he mentions has been resolved.

It is by Prof. Craig Tyler (PhD 2002 U Chicago) who teaches physics at Fort Lewis College in Colorado. He's not eminent or authoritative, but I like his easy-to-read style with lots of pictures. If someone has a better webpage on the BBN, or finds serious fault with Tyler's please let me know.
http://faculty.fortlewis.edu/tyler%5Fc/

So oldman, Tyler seems to think that the BBN story is not Gospel but can be challenged and tinkered---there is room for some controversy about it. But so much is known about nuclear reactions and cross-sections etc that I find it hard to imagine that revision would change the estimated abundances by more than few percent. What am I missing?

 

[Wallace]

If you took BBN away from the Pantheon of cosmological evidence I doubt the standard model would change much, if at all. There are independent measures of elemental abundances that are concordant with BBN (if not as accurate, in terms of constraining abundances) and as mentioned, the lack of Baryons is apparent in a plethora of observational methods.

Without BBN I think the form of the standard model would be the same. We may feel a little less certain of it, due to the absence of an additional concordant result, but the basic model I think would be the same.

We do live in interesting times for Cosmology as noted, which is fantastic.

 

[oldman]

But so much is known about nuclear reactions and cross-sections etc that I find it hard to imagine that revision would change the estimated abundances by more than few percent. What am I missing?

First, thanks for the Tyler reference, Marcus. He writes clearly and simply, which is very helpful to someone like myself who has trouble with stuff like manifolds, fibre bundles and sheaves.

Second, I agree that it is a stretch to imagine a serious error in the BBN scenario --- it looks so simple and well-founded. I'd also be surprised if it were wrong. But:

What I find amazing, and may be what you're missing, is that the cognoscenti (such as the folk who are presently posting in Wallace's poll) seem quite prepared to consider scenarios that require considerable modifications to very serious parts of physics; stuff like GR, or sometimes even Newton's approximation to the way gravity behaves (the Mad Mond Mob!). And they often debate the nature of unobserved stuff, rather like the explorers of old who worried about sea serpents and the edge of the world.

Yet, you (and perhaps they) seem to find it "hard to imagine" that there could be any serious flaws in the BBN story.

From my outsider's perspective, it seems that the current consensus about cosmology, LCDM, is a stretch too very far. I'm surprised that those involved with it aren't busiliy turning over all the simple stones, especially the ones cemented down that look like foundation stones! They're the ones most likely to be built on sand.

 

[Nereid]

First, thanks for the Tyler reference, Marcus. He writes clearly and simply, which is very helpful to someone like myself who has trouble with stuff like manifolds, fibre bundles and sheaves.

Second, I agree that it is a stretch to imagine a serious error in the BBN scenario --- it looks so simple and well-founded. I'd also be surprised if it were wrong. But:

What I find amazing, and may be what you're missing, is that the cognoscenti (such as the folk who are presently posting in Wallace's poll) seem quite prepared to consider scenarios that require considerable modifications to very serious parts of physics; stuff like GR, or sometimes even Newton's approximation to the way gravity behaves (the Mad Mond Mob!). And they often debate the nature of unobserved stuff, rather like the explorers of old who worried about sea serpents and the edge of the world.

Yet, you (and perhaps they) seem to find it "hard to imagine" that there could be any serious flaws in the BBN story.

From my outsider's perspective, it seems that the current consensus about cosmology, LCDM, is a stretch too very far. I'm surprised that those involved with it aren't busiliy turning over all the simple stones, especially the ones cemented down that look like foundation stones! They're the ones most likely to be built on sand.

I think you'll find that 'all the simple stones' have been turned over, dozens and dozens of times ... I find it can be hard to convey just how astronishingly strong all the interconnections are, how the stones have been weighed, measured, analysed, etc sixty-six ways to Sunday, by different teams, using different methods, ...

Sure there are some anomalies and outliers - maybe a quick list would be fun! - but if you hang out with the most vociferous and most knowledgeable critics for a while, you will quickly see just how little is in their closet*.

*And no, it's not because, pace the signatories to that open letter, there is a strong bias against allocating resources to investigating alternatives, or anomalies and outliers (I'm not talking about string theory/LQG/twistors here, just observational cosmology and astrophysics).[/size]

 

[oldman]

...'all the simple stones' have been turned over, dozens and dozens of times ... I find it can be hard to convey just how astronishingly strong all the interconnections are, how the stones have been weighed, measured, analysed, etc sixty-six ways to Sunday, by different teams, using different methods, ...

And no, it's not because, pace the signatories to that open letter, there is a strong bias against allocating resources to investigating alternatives, or anomalies and outliers (I'm not talking about string theory/LQG/twistors here, just observational cosmology and astrophysics

You sound like a Believer, Nereid, possibly even a proselytizer! I'm afraid I'm so far "outside" that I haven't even heard of "that open letter". It sounds interestingly scandalous. I also accept that cosmology and astrophysics are far from the desert of string theory, sustained as they are by the life-giving water of observation.

But I agree with Marcus, when he said:

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.

I also suspect that you put it a bit strongly when you talked of stone-turning activities above. What about the simple stone of BBN? Has it been turned over by different teams of cosmologists? Or nuclear physicists?

I also am still unclear how exactly the estimate of baryonic density obtained by analysing the WMAP results distinguishes between baryonic and "exotic" matter, both of which must help to shape the power spectrum.

 

[Wallace]

I also am still unclear how exactly the estimate of baryonic density obtained by analysing the WMAP results distinguishes between baryonic and "exotic" matter, both of which must help to shape the power spectrum.

They do but in quite different ways. Baryons are coupled to radiation in the early universe (essentially meaning that they interact, i.e. photons bounce of the ionized electrons and protons) whereas by definition dark matter is not coupled to the radiation, so does not scatter photons. The dark matter does influence the perturbation spectrum through gravity however.

To get a feel for different influence of Baryons and DM try having a look at the nice animations on Wayne Hu's site http://background.uchicago.edu/~whu/metaanim.html" .

For a more in depth look, I have recently started using an excellent CMB code that has a nice GUI frontend called 'CMBeasy'. You can get it for free at "www.cmbeasy.org"[/URL], though you need Linux or Mac OSX to run it. With this you can change parameters (such as energy densities) yourself through the GUI and see instantly (well after a few seconds of calculation at least) how that change affects the CMB spectrum.

 

[Nereid]

You sound like a Believer, Nereid, possibly even a proselytizer! I'm afraid I'm so far "outside" that I haven't even heard of "that open letter". It sounds interestingly scandalous. I also accept that cosmology and astrophysics are far from the desert of string theory, sustained as they are by the life-giving water of observation.

So why not be prepared to wrestle with the gigabytes of high quality data pouring from the dozens of independent sources, examining such a vast expanse of the EM spectrum?

Is it that much easier to whip out a 'Believer' branding iron than to take the trouble to have a (deep) look for yourself?

Or does there lurk, in your heart, some vestige of Popperian naive falsificationism?

But I agree with Marcus, when he said:

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.

I also suspect that you put it a bit strongly when you talked of stone-turning activities above. What about the simple stone of BBN? Has it been turned over by different teams of cosmologists? Or nuclear physicists?

Well, one of my personal pet peeves is the conflating of dark matter with dark energy - I think the observational basis for each is quite different, in just so many ways ... yet so many seem to so blithely dump them into the same bucket.

Oh, and isn't this thread about 'Dark Stuff'? Would you be kind enough to explain how either is related to BBN?

I also am still unclear how exactly the estimate of baryonic density obtained by analysing the WMAP results distinguishes between baryonic and "exotic" matter, both of which must help to shape the power spectrum.

Wallace already answered this; I'll simply add that the CMB isn't the only thing with cosmological significance which astronomers observe.

 

[oldman]

So why not be prepared to wrestle with the gigabytes of high quality data pouring from the dozens of independent sources, examining such a vast expanse of the EM spectrum?

Broadband is not available in my backwater!

Is it that much easier to whip out a 'Believer' branding iron than to take the trouble to have a (deep) look for yourself?

If I've caused offence by wrongly labelling your approach to matters cosmological, Nereid, it's a pity. But you do sound quite disapproving of folk who don't accept what you tell them.

Or does there lurk, in your heart, some vestige of Popperian naive falsificationism?

Here I can strongly recommend Susan Haack's Defending Science to your attention. I'm situated, as she subtitles her book, firmly Between Scientism and Cynicism. Perhaps you're more inclined to the first of these positions. But then it takes all sorts to make a world!

Well, one of my personal pet peeves is the conflating of dark matter with dark energy - I think the observational basis for each is quite different, in just so many ways ... yet so many seem to so blithely dump them into the same bucket.

Perhaps because they are both invented constructs (see below) which share the attribute of being unobserved. But I agree that they are uneasy bedfellows.

Oh, and isn't this thread about 'Dark Stuff'? Would you be kind enough to explain how either is related to BBN?

I'm happy to oblige. This is the way I see it.

BBN sets an upper limit to the baryonic density , which is in conflict with the amount of dark matter inferred from straightforward observations, starting with those of Zwicky and then Vera Rubin (virial equilibrium of galaxy clusters, galaxy rotation curves) and continuing with the role dark matter must play in structure formation. This is why "exotic" dark matter was invented in the first place ---to resolve this conflict --- so BBN has lotsto do with dark matter.

BBN is not directly connected with dark energy, which was invented to resolve another conflict --- between the amount of dark matter for which there is sound observational evidence, such as that mentioned above, and the subsequent conclusion (WMAP) that the universe's overall geometry is very close to Euclidean.

But (I believe) that BBN, by setting cosmologists on the slippery path of convenient invention (exotic but still unobserved matter), is ultimately responsible for the later convenient invention of lots more invisible "dark stuff" (quintessence, dark energy, cosmological constant; call it what you will). Which lands cosmology in the unhappy state that Marcus described; or, as a cynic might say, a subject that is nowadays "all hat and no cattle".

I'll stay with Susan Haack's position: betwixt and between.

 

[oldman]

They do but in quite different ways...

Thanks for this help, and for the URL's. My downloads are tediously slow, but I'll try the animations. I've also been slow in appreciating just how the wealth of information in the WMAP power spectrum has been teased out of the raw data. The reference Marcus supplied

...a simple online discussion of BBN which compares it with CMB is ...http://astro.uchicago.edu/~tyler/omegab.html...

also helped.

I note that in this reference Tyler says that 10 parameters are used in fitting the data. Are these parameters set by observation/theory that is part of the LCDM model; i.e. externally obtained, as it were, or are they just varied by curve fitting as in the 'CMBeasy' software that you mentioned? Or are both methods combined?