Dark matter candidates, what chances would you give them?

[EL]

What chances would you give the different candidates to actually make up (the major constituent of) the dark matter? That is, if you were a bookmaker, what chances would you find appropriate?
I'd find it interesting to see what you all think.

At the moment I'll go with the following:

LSP (50%)
K-K DM (5%)
Axions (4%)
Sterile neutrinos (2%)
Mond (2%)
General relativity, more careful calculations (1%)
Misinterpreted data, no dark matter needed (1%)
Not yet suggested dark matter particle (15%)
Other not suggested reason (15%)
Other suggested reason (5%)

 

[Garth]

I guess I go with the "Other suggested reason (5%)", my suggestion being that all DM is baryonic produced in a Freely Coasting Model (FCM) as delivered by http://en.wikipedia.org/wiki/Self_creation_cosmology .

My suggestion to then explain where all this unseen baryonic matter resides is that roughly half of it is WHIM and roughly half IMBH's.

However I wouldn't want to put a percentage on it. The question is how well do these alternatives match up with the observed cosmological constraints. The GP-B experiment will sort out a few alternatives in about a year's time.

Garth

 

[EL]

However I wouldn't want to put a percentage on it

Sure, but what if you actually were a bookmaker? What odds would you decide? I mean you certainly would not put 100% on your own theory (couse in that case I would bet a lot of money you were wrong... )

 

[Garth]

Sure, but what if you actually were a bookmaker? What odds would you decide? I mean you certainly would not put 100% on your own theory (couse in that case I would bet a lot of money you were wrong... )

You might just lose it!

Seriously, while there are problems with the standard model, not least not being able to identify a Higgs Boson/Inflaton, DM particle or DE in the laboratory, other viable alternatives ought to be studied - just in case.

Garth

 

[yanniru]

DM Constituents

Since I am a layman, I have no right to do this.

Nonetheless, I put my money on a mixture of several components:

LSP
Axions
Mirror Matter

but hedge my bet with Bekenstein's MOND

 

[EL]

Seriously, while there are problems with the standard model, not least not being able to identify a Higgs Boson/Inflaton, DM particle or DE in the laboratory, other viable alternatives ought to be studied - just in case.

Sure, no doubt about that. Until we find the answer, all candidates not violating current constraints should be kept in mind. However, what I was looking for was people's personal trust in different candidates.
Even though it may not be "scientifically correct" to rank the candidates, I would like to see what people think, just for fun...

 

[Chronos]

I would like to place a side bet on WIMPS.

 

[EL]

I would like to place a side bet on WIMPS.

Ok, but what chances do you give them?

 

[Chronos]

Fairly good. It would explain why we have so much difficulty detecting then in particle colliders.

 

[EL]

You're all so scientifically moderate...
What should I do to get some numbers out?:tongue2:

 

[jimpy]

Dark Matter-fuzzy thinking,bad science

General Relativity was and remains an inspired 100 year old guess at the way things might be,only Albert Einstein(in later years dominated by divine convictions) really believed it to be the basis of the TOE.The nature of electrons,the existence of protons and the likelihood of a singular initial condition set were outside it's ambit.It presents a chillingly effective predictive algebra for narrow midz-one phyics,effectively it is local curve fitting.Go down to Planck radius or move moderately towards galactic size and the theory has no physics to accompany the expectations and predictions it makes.Dark matter is a conjectural condition,aberrations of incomplete older theories should not drive clear thinking.The candidate formally missing from your list does not involve more accurate partial theories it should be "Inadequacies of early incomplete or approximate theories to represent physical realities"and a logical response would be to see this rated at 70%

 

[Chronos]

Hi jimpy, welcome to PF. I think you would find almost everyone agrees current theories are incomplete, and the likely reason it has been vexingly difficult to unite GR and QM. On the other hand, both theories are amazingly predictive at macroscopic [GR] and plankian [QT] scales. So most people are fairly certain the correct unified theory will reduce to QM at Planckian scales and somehow emerge as GR at macroscopic scales. As in most human endeavors, time is the enemy. It is essentially irrelevant in quantum theory, yet indispensable in GR.

Dark matter, however, is very much still alive these days. The fact it has not been detected in the lab is not a valid objection. Consider how long it took to validate the atom conjecture in human labs. If dark matter were a mathematical artifact introduced by a flawed theory of gravity, it would have a decidedly systematic effect on observation. But this is not observed. The evidence indicates non-baryonic matter [CDM] is just as clumpy and chaotically distributed throughout the universe as is baryonic matter. The only thing they appear to have in common is gravitational affinity - i.e., they tend to be drawn to one another. Find a big clump of baryonic matter, and it is almost a cinch you will find evidence it is embedded in an even bigger clump of CDM. The only variable is how much CDM appears to be hanging out in the hood. Oddly enough, this is frequently used to criticize the CDM conjecture - just add the right amount of CDM and all gravitational anomalies magically disappear. But isn't that exactly what you would expect if CDM really does exist - a variable amount at different locations? I would find it truly bizarre [and unbelievable] if every galaxy had the same proportion of CDM v baryonic matter. Pardon my rambling, but this is an interesting issue with many side bars to consider. Driving a stake through the heart of dark matter is simply not doable given current observational evidence - which is abundant and strikes from many directions. Science is hard. Ideas are more easily embraced than abandoned - as demonstrated by history.

Footnote - you may find this interesting:

Dark matter: A phenomenological existence proof
http://www.arxiv.org/abs/astro-ph/0601489

 

[EL]

The candidate formally missing from your list does not involve more accurate partial theories it should be "Inadequacies of early incomplete or approximate theories to represent physical realities" and a logical response would be to see this rated at 70%

Hi jimpy.
My list wasn't supposed to be exhausting, so everyone should feel free to come up with their own suggestions. It just reflects my current personal guess. Anyway, what you are looking for I would place under "Mond", something I have given a 2% chance to explain the dark matter problem.
Trying to build Mond theories at the scales were we observe the dark matter problem without being in conflict with current measurements has turned out to be pretty hard, and that's why I find other candidates much more likely to make up the dark matter. Of course GR has it's range of validity, but "at the scale of the dark matter problem" I find it probable to be trustful.
Btw, how would you divide your other 30%?

 

[Labguy]

I would find it truly bizarre [and unbelievable] if every galaxy had the same proportion of CDM v baryonic matter.

I would too. Any random early formation on a large scale would be very unlikely to "parcel-out" equal proportions of matter, dark or otherwise, to what we can see today as different galaxies/clusters/superclusters. Small scale H, He and Li seem to fit initial BB conditions though and there is a fair bit of observational evidence to make that era "mainstream" today. I do see, though, a lot of mass-relation studies going on today but have to think that those that seem to indicate a constant proportion would be just as prevalent as those that don't show the same relation/proportions.

I would like to place a side bet on WIMPS.

I would go with that also. No confirmed detection yet so there is also no known limit on an upper mass and/or lifespan. If there is no limit on the mass of virtual particles from the vacuum fluctuation as per Heisenberg (there isn't a limit) then WIMPS could be very massive and often replaced with similar mass after kicking the bucket. It all isn't just going to be electrons and positrons or quark-antiquark pairs.

Also, place emphasis on the "I" in WIMP for "Interacting". If they interact (and exist) then would we always have to think "non-baryonic"? I'm not a fan of weird and mysterious matter lurking around in a universe blasted out of baryonic matter. Mysterious energy maybe, but not matter..

 

[EL]

Also, place emphasis on the "I" in WIMP for "Interacting". If they interact (and exist) then would we always have to think "non-baryonic"? I'm not a fan of weird and mysterious matter lurking around in a universe blasted out of baryonic matter. Mysterious energy maybe, but not matter..

Could you please elaborate this, maybe I'm just getting you wrong. Why does non-baryonic (dark) matter sound mysterious to you? I mean, we have found plenty of it already...

 

[Labguy]

Could you please elaborate this, maybe I'm just getting you wrong. Why does non-baryonic (dark) matter sound mysterious to you? I mean, we have found plenty of it already...

We have seen evidence for the existence of "dark matter", but there are also other theories floating about about MOND, adjusted GR, etc. that may not require DM at all.

But, if we accept DM, what do we have to show (yet) that it is specifically non-baryonic?

 

[EL]

But, if we accept DM, what do we have to show (yet) that it is specifically non-baryonic?

E.g. constraints from the BB nucleosynthesis which only allows baryonic matter to make up a few percents of the total energy density in the universe.

 

[Labguy]

E.g. constraints from the BB nucleosynthesis which only allows baryonic matter to make up a few percents of the total energy density in the universe.

But maybe all the rest is energy. BB nucleosynthesis can be up to 10% baryonic matter (Ned Wright) but remember I first mentioned virtual particles from vacuum fluctuations. Also, all DM is most likely not to be of a single type; neutrinos can also contribute and they are "non-baryonic". With non-baryonic being defined as:

non-baryonic: not made up of neutrons, protons and electrons, and thus not like any of the known chemical elements. (anything made from atoms)

I guess that would even include WIMPS and many particle-antiparticle VP pairs. So, by that definition I could agree that the DM is likely to be non-baryonic (atoms/elements), but not likely to be a type of matter unknown (or mysterious) to us. Maybe "non-atomic" would be a better description, but many of the known sub-atomic particles, already existing and virtual, are "real" particles known to us and could be candidates for DM. It seems the only requirement for all candidates is that they are gravitationally affected, hence their probable detection.

 

[EL]

But maybe all the rest is energy. BB nucleosynthesis can be up to 10% baryonic matter (Ned Wright)

That's a much higher percent than what is commonly accepted. Do you have a link to the paper?

but remember I first mentioned virtual particles from vacuum fluctuations.

This is consideder as a potential source of the dark energy.

Also, all DM is most likely not to be of a single type; neutrinos can also contribute and they are "non-baryonic".

Sure they can, and for a while they were a hot candidate. However, we now know they can just make up some percent or so of the total energy density. I.e. they have practically been ruled out.

I guess that would even include WIMPS

Of course that includes WIMPS, which are the standard example of non-baryonic dark matter.

but not likely to be a type of matter unknown (or mysterious) to us.

So you can agree WIMPS is a good candidate, but you don't like unknown candidates? In, that case, which WIMPS are you speaking of?

 

[Garth]

Could you please elaborate this, maybe I'm just getting you wrong. Why does non-baryonic (dark) matter sound mysterious to you? I mean, we have found plenty of it already...

We have?

We have plenty of evidence of Dark Matter. The only reason that this is said to be non-baryonic is the limitation on baryonic density by the standard model BBN as you later said EL.

If we are prepared to invoke undiscovered species to make the model fit perhaps we ought also to be prepared to consider alternative BBN models such as the “Freely Coasting” model, which do not need to invoke such undiscovered species as it identifies DM as baryonic.

Garth

 

[Labguy]

That's a much higher percent than what is commonly accepted. Do you have a link to the paper?

Wright's FAQ:
http://www.astro.ucla.edu/~wright/cosmology_faq.html#DM

But the theory of Big Bang nucleosynthesis says that the density of ordinary matter (anything made from atoms) can be at most 10% of the critical density, so the majority of the Universe does not emit light, does not scatter light, does not absorb light, and is not even made out of atoms.

Sure they can, and for a while they were a hot candidate. However, we now know they can just make up some percent or so of the total energy density. I.e. they have practically been ruled out.

Same Guy:

This "non-baryonic" dark matter can be neutrinos, if they have small masses instead of being massless, or it can be WIMPs

and we now know that they do have mass.

So you can agree WIMPS is a good candidate, but you don't like unknown candidates? In, that case, which WIMPS are you speaking of?

Any that haven't been confirmed. Do you have have a list of confirmed WIMPS for me?

 

[SpaceTiger]

The only reason that this is said to be non-baryonic is the limitation on baryonic density by the standard model BBN as you later said EL.

WMAP also places a constraint on the baryonic density and gives similar numbers to those from nucleosynthesis.

 

[Gerinski]

If non-baryonic DM is attracted gravitationally to baryonic matter, why do we always think of it just hovering in deep space, surrounding the galactic halos or making up filaments around galaxy clusters?

Shouldn't it also have got merged with the ordinary stuff everywhere, even right here where we are?
If non-baryonic DM gets merged with ordinary matter, even if it does not interact chemically or electrically or nuclearly with it, wouldn't it stay attached to it by gravitational attraction, however weak this is?

So, shouldn't the Earth itself, or the Sun, or whatever ordinary body around us, contain also non-baryonic DM?

And even if that answer is no, IF non-baryonic DM gets merged with baryonic matter, which space among it would it occupy? would it maybe occupy intermolecular and/or interatomic space, making such baryonic body appear heavier (denser) than it actually is?

 

[Garth]

WMAP also places a constraint on the baryonic density and gives similar numbers to those from nucleosynthesis.

Thank you, a good point.

However the interpretaiton of the WMAP data is model dependent.

For example are the peaks consistent with a spatially flat universe, as normally thought, or with a conformally flat one? If conformally flat then the total density $\Omega_{total}$ need not be ~ 1 and if finite then that could explain the quadrupole deficiency. Such a change in model would alter the cosmological parameter fit.

Secondly the calculation of the baryon density is convoluted with the primordial Deuterium abundance. This is fitted to the present epoch observed abundance as an upper limit as Deuterium is fragile and can be destroyed but not easily nucleosynthesised. However spallation in shocks, perhaps in the formation and demise of PopIII stars, can also produce Deuterium (Deuterium Production by High Energy Particles - Richard I Epstein Ap.J. 212 595-601 1977) and if this is significant then the Deuterium primordial abundance has been over-estimated and the baryon abundance consequently out.

The WMAP data is certainly consistent with an $\Omega_b = 0.04$, however it might also be consistent with a different value as indeed claimed by Gehlaut et al. for the Freely Coasting model.

Garth

 

[EL]

We have?

Neutrinos, electrons, positrons, muons...

We have plenty of evidence of Dark Matter. The only reason that this is said to be non-baryonic is the limitation on baryonic density by the standard model BBN as you later said EL.

No, it's not the only reason (but even if it was it would still be a very good one). As Spacetiger mensionen the baryon fraction can be extracted from the WMAP data (if I remeber it correctly, it mainly depends on the hight of the second peak in the power spectrum).
Also, simulations show that a large fraction of the matter must be non-baryonic in order for structures to form fast enough, something a baryon dominated universe would not be able to do.

 

[EL]

and we now know that they (neutrinos) do have mass.

We have known that for a while now, and people have calculated what fraction they may make up, and we have concluded that neutrinos can only make up a few percent of the total energy density of the universe. Hence they can just make up a small part of the dark matter.

Any that haven't been confirmed. Do you have have a list of confirmed WIMPS for me?

Of course not. There are no confirmed WIMPS. And that is why I asked that question: Why are you willing to accept WIMPS as a good candidate, but not new species of non-baryonic matter? I mean, WIMPS are new species of non-baryonic matter...

 

[EL]

If non-baryonic DM is attracted gravitationally to baryonic matter, why do we always think of it just hovering in deep space, surrounding the galactic halos or making up filaments around galaxy clusters?

But we aren't...

Shouldn't it also have got merged with the ordinary stuff everywhere, even right here where we are?

Sure. The dark matter should be all around us, and there are plenty of experiments around the world trying to detect it.

So, shouldn't the Earth itself, or the Sun, or whatever ordinary body around us, contain also non-baryonic DM?

There should be an excess of DM around strong gravitational sources (e.g. the sun. or why not even better: the galactic centre). However a DM particle propagating in the direction towards a massive body, will certainly just pass through it, just like neutrinos do.

 

[Labguy]

Hence they can just make up a small part of the dark matter.

That was my point when I said DM won't turn out to be of just one type. Neutrinos, some WIMPS, a bit of undetected baryonic matter, a pinch of salt, some tabasco, etc.

Why are you willing to accept WIMPS as a good candidate, but not new species of non-baryonic matter?

What new species? I've already mentioned that I think some will be massive particles formed by vacuum fluctuations. If some other particles are found to be massive enough to count and interact (with gravity) then they would be WIMPS, no? Maybe in the future we'll have sub-classes of WIMPS. Everything new we may find will have to be named something.

 

[jimpy]

ole El,
I have a quotation from another time:
"-we have not to discover the properties of a thing which we have recognised in nature but to discover how to recognise in nature a thing whose properties we have assigned.This development seems inevitable;but it has grave drawbacks especially when theories have to be reconstructed.Fuller knowledge may show that there is nothing in nature having precisely the properties assigned;or it may turn out that there is nothing in nature having precisely the properties assigned;or it may turn out that the thing having these properties has entirely lost it's importance when the new theoretical standpoint is adopted..."
Mond theories are a convenient baggage receptacle,the wisdom of yesterday is often inconvenient when it gets in the way.
I only take issue with the 1-2% versus my 70%.The rest is fine

 

[Garth]

Neutrinos, electrons, positrons, muons...

Come on.. we are talking about $\Omega_{DM} \sim 0.23$ here!

No, it's not the only reason (but even if it was it would still be a very good one). As Spacetiger mensionen the baryon fraction can be extracted from the WMAP data (if I remeber it correctly, it mainly depends on the hight of the second peak in the power spectrum).

Well as I replied to SpaceTiger, the conclusions from WMAP are model dependent.

And as I said above the standard interpretation of that data would be more robust if it could be shown to be concordant with the deficient quadrupole.

Other models such as the FCM A Concordant “Freely Coasting” Cosmology also claim to be concordant with the WMAP data.

3 Summary

The main point we make in this article is that in spite of a significantly different evolution, the recombination history of a linearly coasting cosmology can be expected to give the location of the primary acoustic peaks in the same range of angles as that given in Standard Cosmology.

.....

[$\Omega_b \sim 0.2$]Thus linear coasting has the potential of relegating the need for any form of dark matter or dark energy (or for that matter, any physics not already tested in the laboratory) to the physics archives where they enjoy the same status as ether and phlogiston.

The message this article is to convey is that a universe that is born and evolves as a curvature dominated model has a tremendous concordance and there are sufficient grounds to explore models that support such a coasting.

([] my insertion for clarity)

In that conclusion the authors should have added the caveat that the missing laboratory tested physics in the model is a gravitational theory that delivers the strictly linearly expanding universe. http://en.wikipedia.org/wiki/Self_creation_cosmology at this very moment.

Also, simulations show that a large fraction of the matter must be non-baryonic in order for structures to form fast enough, something a baryon dominated universe would not be able to do.

The FCM universe expands more slowly than the standard [itex]\Lambda[/tex]CDM model so there is more than twice the time available for structures to form at the high (>1) z epochs.

Garth

 

[EL]

That was my point when I said DM won't turn out to be of just one type. Neutrinos, some WIMPS, a bit of undetected baryonic matter, a pinch of salt, some tabasco, etc. What new species? I've already mentioned that I think some will be massive particles formed by vacuum fluctuations. If some other particles are found to be massive enough to count and interact (with gravity) then they would be WIMPS, no? Maybe in the future we'll have sub-classes of WIMPS. Everything new we may find will have to be named something.

So maybe we're just talking around each other, but what I was reacting to was that I got the impression you didn't like the thought of non-baryonic dark matter:

If they interact (and exist) then would we always have to think "non-baryonic"? I'm not a fan of weird and mysterious matter lurking around in a universe blasted out of baryonic matter.

As we have argued, the known particle species will hardly make up any greater part of the matter density needed. This means that (if the dark matter problem is solved by particles, which I'm a great fan of) the DM has to be made up by some species we havn't found in the laboratory yet.
I can understand if you object to inwoking new particle species at all, but why would it be so strange if this new particle was non-baryonic?
I mean I can't see why non-baryonic particles are not in any way more "mysterious" that baryonic...

 

[EL]

I only take issue with the 1-2% versus my 70%.The rest is fine

Well, I based my 2% on the fact that despite many attempts, no one (at least that I have heard of) have managed to cook up a mond theory consistent with observational data. Mond isn't a hot candidate in the scientific society at the moment.

 

[EL]

Come on.. we are talking about $\Omega_{DM} \sim 0.23$ here!

But I wasn't talking about dark matter. I was just trying to make clear that non-baryonic matter is not in anyway stranger than baryonic, which I got the impression Labguy was saying.

Well as I replied to SpaceTiger, the conclusions from WMAP are model dependent.

Sure you are of course right about that, but I'm just not a fan of leaving mainstream models.

The FCM universe expands more slowly than the standard [itex]\Lambda[/tex]CDM model so there is more than twice the time available for structures to form at the high (>1) z epochs.

Interesting. Are there any simulations done? Will the FCM model account for the structures we observe today?

 

[Garth]

But I wasn't talking about dark matter. I was just trying to make clear that non-baryonic matter is not in anyway stranger than baryonic, which I got the impression Labguy was saying.

Okay, crossed posts, always a problem on PF!

Interesting. Are there any simulations done? Will the FCM model account for the structures we observe today?

I'm working on it! Actually I do not have the facilities to run such simulations, perhaps after GP-B comes up trumps somebody else will do it for me. [Anybody out there willing to have a go?]

All I have done are 'back of the envelope' Jeans mass and free fall time estimations.

Garth

 

[Labguy]

I can understand if you object to inwoking new particle species at all, but why would it be so strange if this new particle was non-baryonic?
I mean I can't see why non-baryonic particles are not in any way more "mysterious" that baryonic...

Yo!, Stop!, Halt! and Alto!:

That part is my fault and from your other posts just above it seems we do agree. With WIMPS, neutrinos, mesons and even virtual particles all being known or soon-to-be-known (?) items and "particles", I was thinking (dangerous for me) "non-particle" instead of "non-baryonic". There is nothing at all mysterious about these particles not fitting the definition of baryonic that I posted before.

I started out by voting/betting with Chronos on WIMPS as a large candidate and mentally ruling out such things as large masses of anti-matter or any possibility of any EM or gravitational waves (gravitons?) etc. as having a property fitting the apparent observations indicating that DM is out there in great abundance.

It is all from my own foul-up of not paying attention to the difference in the definitions between baryonic and particle; I was just thinking too fast and too wrong! Anyone reading my posts should look at the signature first and then read the post..

But, I will still give a fair percentage of the various DM possibilities to virtual particles of any mass being formed from vacuum fluctuations. In their short-lived existence they would have a property affected by gravity, exert a gravitational influence and blink out with an energy return. If they are a seething and self-replacing mass, that might explain why they don't have the other observational properties (non-interacting) of baryonic matter that all DM is also lacking.

Is it possible that galaxies/clusters/superclusters is where DM is most easily detected because more energy (gravitational, magnetic/EM and angular momentum) is available there and therefore would lead to higher virtual particle production than more "empty" space?? (rhetorical question).