# Why is Dark Matter a puzzle?

1. Aug 24, 2006

### 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?

2. Aug 24, 2006

### EL

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

3. Aug 24, 2006

### oldman

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

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?

Last edited: Aug 24, 2006
4. Aug 24, 2006

### EL

Maybe this will help you:

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

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

Last edited: Aug 24, 2006
5. Aug 25, 2006

### Chronos

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

Last edited: Aug 25, 2006
6. Aug 25, 2006

### oldman

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, text book 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.

7. Aug 25, 2006

### EL

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 missmatch 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.

8. Aug 25, 2006

### SpaceTiger

Staff Emeritus
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.

9. Aug 25, 2006

### oldman

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

Last edited: Aug 25, 2006
10. Aug 25, 2006

### SpaceTiger

Staff Emeritus
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.

11. Aug 25, 2006

### oldman

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.

Last edited: Aug 25, 2006
12. Aug 25, 2006

### EL

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!

Last edited: Aug 25, 2006
13. Aug 26, 2006

### oldman

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.

14. Aug 26, 2006

### SpaceTiger

Staff Emeritus
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.

15. Aug 26, 2006

### selfAdjoint

Staff Emeritus
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.

16. Aug 26, 2006

### oldman

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.

Last edited: Aug 26, 2006
17. Aug 26, 2006

### marcus

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.

18. Aug 27, 2006

### Chronos

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?'.

19. Aug 27, 2006

### oldman

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.

20. Aug 28, 2006

### SpaceTiger

Staff Emeritus
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.

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