What is the nature of Dark Matter?

[oldman]

Recently reported observations of the Bullet Cluster of galaxies (see http://www.nasa.gov/centers/marshall/news/news/releases/2006/06-096.html ) claim proof that what I'll call Exotic Dark Matter (EDM) exists. Previously the most direct evidence for lots of some sort of Dark Matter (DM) was, as far as I know, the observations of Zwicky, Vera Rubin and others on galaxy rotation curves, taken together with excessive galaxy velocities in virialised clusters.

I know that these observations show that there is too much DM (about 20% of amount needed to flatten the universe) to be accounted for by calculations of nucleosynthesis that match observed cosmic element abundances.

What I don't know is whether these calculations are the only justification for the very bold assumption that the extra DM is EDM of a kind otherwise unknown to physics. Is this indeed so?

A possibility that seems to have been rejected on this account is that the unobserved DM in the universe is planetary sytems, Pluto-like objects and stuff like the postulated Oort cloud on the fringes of our system, brown and black dwarfs and possibly shoals of isolated black holes -- i.e. that it is miscellaneous Big Lumps of Junk (BLJ) that we can't detect. Such BLJ would be unscattered as it moved through hot gas, like the recently observed DM in the Bullet Cluster.

I also don't know if there is any evidence (say from tiny deviations of Keplerian orbital speeds of outer planets) for the existence of either the Oort cloud or a halo of BLJ in deep space beyond the solar system.

Does anybody know for sure that there are not enough BLJ out there to account for the gravitationally observed DM?

 

[EL]

What I don't know is whether these calculations are the only justification for the very bold assumption that the extra DM is EDM of a kind otherwise unknown to physics. Is this indeed so?

Another justification comes from the CMB.
Structure formation needs nonbaryonic dark matter.
http://en.wikipedia.org/wiki/Dark_matter

 

[oldman]

Another justification comes from the CMB.
Structure formation needs nonbaryonic dark matter.
http://en.wikipedia.org/wiki/Dark_matter

Thanks for this very up-to-date reference, in which the first question I asked is answered as follows:

Ordinary baryonic matter had too high a temperature, and too much pressure left over from the big bang to collapse and form smaller structures, such as stars, via the Jeans instability. Dark matter acts as a compactor of structure...This bottom up model of structure formation requires something like cold dark matter to succeed These studies have been crucial in constructing the Lambda-CDM model which measures the cosmological parameters, including the fraction of the universe made up of baryons and dark matter

I interpret this as meaning that dark matter has again to be invoked to compensate for a deficiency in gravitational attraction, as in the earlier work I mentioned. But I'm not clear why in this case such extra matter has to be non-baryonic and exotic. I guess I don't know enough about how/if other details of the standard model prescribe the proportions and state of the baryonic matter involved. Have you a more detailed web-reference available, perhaps?

 

[EL]

Maybe this will help you:

http://en.wikipedia.org/wiki/Structure_formation

Dark matter turns out to play a key role in accelerating the process of structure formation precisely because it feels only the force of gravity. As a result, dark matter begins to collapse into a complex network of dark matter halos well before ordinary matter, which is impeded by pressure forces. Without (non-baryonic, my note) dark matter, the epoch of galaxy formation would occur substantially later in the universe than is observed.

Edit: Been looking some minutes for a detailed reference, without big succes. But I'm sure you can find one at www.arxiv.org

 

[Chronos]

Recently reported observations of the Bullet Cluster of galaxies (see http://www.nasa.gov/centers/marshall/news/news/releases/2006/06-096.html ) claim proof that what I'll call Exotic Dark Matter (EDM) exists. Previously the most direct evidence for lots of some sort of Dark Matter (DM) was, as far as I know, the observations of Zwicky, Vera Rubin and others on galaxy rotation curves, taken together with excessive galaxy velocities in virialised clusters.

I know that these observations show that there is too much DM (about 20% of amount needed to flatten the universe) to be accounted for by calculations of nucleosynthesis that match observed cosmic element abundances.

What I don't know is whether these calculations are the only justification for the very bold assumption that the extra DM is EDM of a kind otherwise unknown to physics. Is this indeed so?

Galactic rotation curves were the first indicators of dark matter [Zwicky]. It was later discovered that galactic cluster mostions could not be adequately explained without some amount of dark matter.

A possibility that seems to have been rejected on this account is that the unobserved DM in the universe is planetary sytems, Pluto-like objects and stuff like the postulated Oort cloud on the fringes of our system, brown and black dwarfs and possibly shoals of isolated black holes -- i.e. that it is miscellaneous Big Lumps of Junk (BLJ) that we can't detect. Such BLJ would be unscattered as it moved through hot gas, like the recently observed DM in the Bullet Cluster.

Baryonic mass, like planets, dim stars and black holes [re: 'hot dark matter'] has been conclusively ruled out by many sigmas

I also don't know if there is any evidence (say from tiny deviations of Keplerian orbital speeds of outer planets) for the existence of either the Oort cloud or a halo of BLJ in deep space beyond the solar system.

Does anybody know for sure that there are not enough BLJ out there to account for the gravitationally observed DM?

Gravitational lensing studies of our own galaxy have already ruled out this model. Your questions are very much in line with the ones astrophysicists have already explored. I encourage you to research this on arxiv, but, I will gladly provide references if desired. I'm just feeling a bit lazy tonite.

 

[oldman]

...Your questions are very much in line with the ones astrophysicists have already explored. I encourage you to research this on arxiv...

Thanks for your replies Chronos and El --- and your offer to help, Chronos. I suspected that such obvious possibilities would have been ruled out as far as astronomers can do this, and your repies confirm that this is indeed the consensus.

The conclusion that so much of the universe is exotic matter that is unknown to physics still seems to me pretty fanciful; the truth often turns out to be more ordinary and unexciting than we first imagine.

A cosmological example is how after 80 years of speculation, textbook analysis and observational effort the universe's spatial geometry has after all turned out to be just the old familiar Euclidean stuff of our schooldays.

 

[EL]

The conclusion that so much of the universe is exotic matter that is unknown to physics still seems to me pretty fanciful; the truth often turns out to be more ordinary and unexciting than we first imagine.

Yes, exotic matter is an exciting thought, but there are more hints, than those from cosmological observations, of the need of new physics.
The standard model of particle physics indeed suffers from some fine tuning problems (the Higgs mass divergence) that can be solved by inwoking new fields into the theory (e.g. supersymmetric fields). The mismatch between General Relativity and Quantum Field Theory also stresses the need of new physics at higher energies.
And new fields means new particles.
So the question is not really wheter there is new physics or not, but wheter at what energy scales these particles show up! If they are just around the corner (100GeV - a few TeV), which from theory there are some good arguments for, they may indeed make up the dark matter.

 

[SpaceTiger]

I interpret this as meaning that dark matter has again to be invoked to compensate for a deficiency in gravitational attraction, as in the earlier work I mentioned. But I'm not clear why in this case such extra matter has to be non-baryonic and exotic. I guess I don't know enough about how/if other details of the standard model prescribe the proportions and state of the baryonic matter involved.

The trouble with using baryonic dark matter (e.g. hunks of junk) to explain structure formation is that you have to form it before galaxies form. I don't think there's any straightforward way of doing this. Anyway, nucleosynthesis and the CMB are enough to convince most astronomers of non-baryonic dark matter, especially since they both suggest the same relative fraction of it. There still exists the possibility of dark matter being made of primordial black holes with a mass distribution that has so far evaded our detection, but I think most astronomers consider that to be a longshot.

 

[oldman]

...nucleosynthesis and the CMB are enough to convince most astronomers of non-baryonic dark matter, especially since they both suggest the same relative fraction of it...

Just a quickie: the bit I have emphasized in bold: do they both suggest independently the same relative fraction?

 

[SpaceTiger]

Just a quickie: the bit I have emphasized in bold: do they both suggest independently the same relative fraction?

Yes, you can compute $\Omega_b$ from nucleosynthesis using light element abundances, while fits to the WMAP power spectrum are used to constrain it in the CMB.

 

[oldman]

there are more hints, than those from cosmological observations, of the need of new physics...the question is not really wheter there is new physics or not, but wheter at what energy scales these particles show up! If they are just around the corner (100GeV - a few TeV), which from theory there are some good arguments for, they may indeed make up the dark matter.

Thanks EL. These are positive thoughts in times when the present mystery of what 95% of the universe is made of needs urgently to be solved. It is high time that physics was refreshed by the observation of new phenomena, as it continually was between say 1890 and 1970. Roll on discovery!

And thanks, Space Tiger, for your relevant and incisive clarifications.

 

[EL]

It is high time that physics was refreshed by the observation of new phenomena, as it continually was between say 1890 and 1970. Roll on discovery!

I do not agree with you here. I think the last 10-20 years have been outstandingly the most exciting, productive and successfull years in cosmology throughout the entire history! We have been able to map our universe with a precision which you could only dream about in earlier years.
And what's even more exciting, there's yet much more to come!

 

[oldman]

I do not agree with you here. I think the last 10-20 years have been outstandingly the most exciting, productive and successfull years in cosmology throughout the entire history! We have been able to map our universe with a precision which you could only dream about in earlier years.
And what's even more exciting, there's yet much more to come!

You're quite right. It is indeed the golden age of cosmology. But I was thinking of the "new physics" you were referring to. Read "Not Even Wrong" by Peter Woit sometime! Kind regards.

 

[SpaceTiger]

You're quite right. It is indeed the golden age of cosmology. But I was thinking of the "new physics" you were referring to. Read "Not Even Wrong" by Peter Woit sometime! Kind regards.

So this is a borrowed opinion? I would say that cosmology has offered many exciting observations that hint at new physics. Dark matter, dark energy, and inflation have all been studied both astronomers and physicists alike. The last 10 years, in particular, has seen a lot of collaboration between the two communities.

 

[selfAdjoint]

So this is a borrowed opinion? I would say that cosmology has offered many exciting observations that hint at new physics. Dark matter, dark energy, and inflation have all been studied both astronomers and physicists alike. The last 10 years, in particular, has seen a lot of collaboration between the two communities.

Since all of the "new physics" addresses higher energies than we can produce on earth, physicists are learning to look at astrophysics for clues, and the history of astrophysics, which is I guesss one side of cosmology, is very important to them.

 

[oldman]

So this is a borrowed opinion? I would say that cosmology has offered many exciting observations that hint at new physics. Dark matter, dark energy, and inflation have all been studied both astronomers and physicists alike. The last 10 years, in particular, has seen a lot of collaboration between the two communities.

It is a borrowed opinion I share. But you may be misunderstanding me.

Physics, as distinct from cosmology, is in desparate need of some new observational or experimental input to resolve its present impasse, namely the question of whether string theory or something else is the best way forward with fundamental questions. Inevitably, experimental progress is now hampered by the impossible difficulty and expense of building large enough accelerators.

So the best hope for substantial progress physics at present is , in my opinion, to look towards cosmology for new inputs. As you say, there is lots of collaboration between the two communities. As there should be. Cosmology, via astronomy, has the input of new observations, like the WMAP and Chandra results, that is so lacking in physics. Results tether balloons of theoretical hot air with the ropes of reality. When ideas like the anthropic principle, string theory, inflation, exotic matter, dark energy and the landscape are floating around it is essential to separate the dross from the gold by observation.

Which is what I'm fortunate enough to see happening.

 

[marcus]

... hampered by the impossible difficulty and expense of building large enough accelerators.

So the best hope for substantial progress in physics at present is, in my opinion, to look towards cosmology for new inputs.

[Empirical] results tether balloons of theoretical hot air with the ropes of reality.

Which is what I'm fortunate enough to see happening.

I agree. Well put.

BTW if you started a "blog" online diary --- thoughts of a nonprofessional cosmology watching curmudgeon, or what you will----you might pick up a following of like-minded.

If you keep the style up to that level (not to take for granted)

the combination of obsession with frustrated skepticism is resonant.

you could critique popular science books if you wanted----there is a "market" for that I think.

 

[Chronos]

Since all of the "new physics" addresses higher energies than we can produce on earth, physicists are learning to look at astrophysics for clues, and the history of astrophysics, which is I guesss one side of cosmology, is very important to them.

Good point, SA. We are crippled by the puny energies Earth bound laboratories can produce. New observational strategies are, IMO, the key. Natural laboratories, like binary neutron stars, GRB's, colliding galaxies, etc., are more promising. The recent NASA release is a prime example. The clues toward a deeper understanding of high energy physics abound, we just haven't figured out what they are telling us. We have, however, came a long way in the last twenty years. The data we have currently accumulated is so vast it will take another twenty years before we will, IMO, be slapping our foreheads saying 'how did we miss seeing that?'.

 

[oldman]

...a nonprofessional cosmology-watching curmudgeon...

Thanks for those remarks, Marcus; this is pretty much what I am. But I'm a curmudgeon who wishes the cosmology enterprise, and those involved in it, much good fortune.

 

[SpaceTiger]

Physics, as distinct from cosmology...

Actually, what I'm saying is that "cosmology" is done by both physicists and astronomers alike and is not distinct from either. Rather, I would say that it is a subfield of both. For example, both Alan Guth (physicist) and David Spergel (astronomer) are called "cosmologists", among other things.

Other than that semantic quibble, however, I think we're on the same page.

 

[oldman]

Other than (a) semantic quibble ... I think we're on the same page.

Well, at least we're reading the same chapter together!

But I'm still rather puzzled about way the mysterious non-baryonic dark matter of the late 1990's seems nowadays to have metamorphosed into just plain old dark matter whose distinguishing feature is simply that it is invisible. (Note how the Chandra result is reported at http://www.nasa.gov/centers/marshall...06/06-096.html ).

This may also be only a matter of semantics, but at the same time it also blurs the fact that the nucleosynthesis calculations of the standard model have yet to be reconciled with the existence of the quantities of unseen matter inferred from the old Zwicky-Rubin results (and now required by the WMAP and lambda CDM model?)

It seems to me that this conflict and the failure to find suitable candidates for non-baryonic matter led to the strange ideas of MOND. If MOND is now dead because of the Chandra observations let it not be forgotten that there remains an unresolved conflict between observation and theory to be settled.

Semantic quibbles are sometimes important!

 

[SpaceTiger]

But I'm still rather puzzled about way the mysterious non-baryonic dark matter of the late 1990's seems nowadays to have metamorphosed into just plain old dark matter whose distinguishing feature is simply that it is invisible.

What has happened is that people have begun to take it for granted that "dark matter" refers to non-baryonic matter. In the late '90s, we were less sure of the need for exotic matter, so we had to be more careful to distinguish.

I can't access the article you linked, but another reason they might not make the distinction is that those observations alone can, in principle, be explained with baryonic dark matter. The need for non-baryonic dark matter comes from other observations (such as nucleosynthesis and the CMB).

This may also be only a matter of semantics, but at the same time it also blurs the fact that the nucleosynthesis calculations of the standard model have yet to be reconciled with the existence of the quantities of unseen matter inferred from the old Zwicky-Rubin results (and now required by the WMAP and lambda CDM model?)

With the exception of the Lithium problem (which is probably rooted in our understanding of stellar evolution), nucleosynthesis calculations are not inconsistent with the standard model.

 

[EL]

With the exception of the Lithium problem (which is probably rooted in our understanding of stellar evolution), nucleosynthesis calculations are not inconsistent with the standard model.

I consider the lithium problem almost solved:
http://www.arxiv.org/abs/astro-ph/0608201