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Dark matter talks July 3, 2007

Posted by dorigo in astronomy, news, physics, science.

This morning the plenary session of PASCOS 2007 included three talks on dark matter. I was not tidy enough to take detailed notes, but I will try to give some flavour of the discussion below. I apologize in advance for any misrepresentation, which is entirely due to my insufficient knowledge of the field rather than to the talks themselves. Also, I should warn the less cosmology-learned readers that what follows can be a bit technical in some places.

Joseph Silk gave a talk about “Dark matter status and perspectives“. He started by stating that dark matter is something we unambiguously measure, and that we do understand the physics of it, and he went on to give a short summary of the evidences. Of course, the rotational curves of galaxies: the velocity of rotation increases with R and then stays constant way off the center to 30 kparsecs and more, something which is at odds with the distribution of luminous matter. And then gravitational lensing: we see arcs, as parts of Einstein rings, due to the lens effects of clumps of matter on the path of light. One can actually make a map of the distorsions of galaxies, thereby mapping the distribution of dark matter. Joe then gave a review of the various candidates for dark matter, excluding them one by one until he concentrated on the most likely one, the neutralino. The neutralino is a particle predicted by Supersymmetric models: it is weakly interacting, massive, and having nothing to decay into it is stable: a perfect WIMP (weakly interacting massive particle). It may have a mass in the 100-1000 GeV range, which would fit with the fraction of dark matter of standard cosmology. Now, it can be discovered at the LHC, but that would be suggestive but indecisive for dark matter: one has to find direct evidence from fluxes through the earth or in the universe. Underground detectors have a chance to probe a mass range which is beyond the reach of any future collider. He finished his talk by remarking that a direct detection is essential for credibility, but indirect detection is needed in multiple windows to demonstrate a cosmological significance.

After Silk there was a talk by Nigel Smith, “The current status of direct dark matter searches“. It was an experimental talk about the various experiments seeking dark matter. I unfortunately did not take any notes, since I understood the matter better…

And then there was a nice talk by Carlos Frenk, “Testing the Lambda-CDM paradigm on large and small scales“. I took a picture during his presentation:

Carlos started by noting that the Lambda-CDM model is quite old, goes back to a paper by Peebles in 1982, when he worked out the expected fluctuations spectrum. Since then, the data has measured the amplitude of CMB ripples, but the fine match between prediction and measurement is sometimes shown without pointing out that the former had been predicted by inflationary cold dark matter model 20 years before.

He then showed a nice video of the Millennium simulation, a cosmological n-body simulation with 10 billion particles in a 500/h Megaparsec box. I had seen the simulation video before, but I must admit it is always awesome to see.

Then he discussed some of the “issues” with the model, starting with the tentative disagreement between the Sloand and 2-degree field galaxy surveys, They appear to be incompatible in their power spectra at 2-sigma level, even when the power spectra are computed with the very same method. The answer is that the two surveys are based on galaxies selected differently: there is a selection bias that favors red galaxies in SDSS and blue galaxies in 2dfGRS. So the two surveys actually agree if you take the color bias in account.

The two surveys have allowed to detect conclusively the baryon acoustic oscillations. It demonstrates that structure grew by gravitational instability in the Lambda-CDM universe. The discovery of oscillations has started an industry. We can measure the wavelength of this oscillation at low redshift and constrain the value of w. However, a computer simulation with 20 times the millennium volume is needed to understand whether we can measure these baryon acoustic oscillations to a value that we need to get real information on dark energy.

Carlos then discussed the mass profile in cluster cores, which can be determined from strong lensing combined with weak lensing, in the few instances of clusters showing radial arcs together with long arcs from the einstein ring. One finds a disagreement with lambda-CDM, but agreement can be restored by hypothesizing a non-spherically symmetric distribution of dark matter, which biases the result (a spherical symmetry is assumed in the calculations).

That was all for the morning. The conference hall of the Brackett physics building was crowded this morning: I see about 180 names in the list of participants… And the hall will be mine this afternoon, when I will give my presentation. Unfortunately, I know from past experience that particle physics session is the least frequented in these conferences, when people prefer to learn about string theory or cosmology (the two other parallel sessions). We will see how much attendence there will be this afternoon…


1. Coin - July 4, 2007

It may have a mass in the 100-1000 GeV range, which would fit with the fraction of dark matter of standard cosmology. Now, it can be discovered at the LHC,

Wouldn’t the Tevatron have been able to find it if it were in that range, though?

2. Adam - July 4, 2007


I suppose it would be possible to see it, since the Tevatron operates around that energy; but they’d need a resonance, or decay to look for.

Accelerators are great because you can set your beam energy near the mass of the particle you’re looking for, but once you see whatever you need to see, you need to look for it in the universe… A particle you got from smashing things together is an isolated incident; and though you can hypothesize that it is the particle you’re looking for, you still need to prove that the WIMP would come from such and such an interaction… so I think he’s trying to kill two birds with one stone: find the particle AND prove that it’s responsible for the dark matter forces.

Though at this point, finding a WIMP would be purdy sweet!

3. Coin - July 4, 2007


I see. So is the idea that the Tevatron might be able to see it, but the LHC would have to find it first so that the Tevatron would know what to look for?

4. dorigo - July 4, 2007

Hi Adam, Coin,

CDF and D0 may find a neutralino if its mass is at the lower end of the quoted range. However, no need for a prior input is needed. The Tevatron and the LHC are both fixed center-of-mass machines, but the parton distribution functions in the proton provide for a variable subprocess energy. The trouble at the Tevatron is the small cross section, which will make detectability hard for masses above 200 GeV. At LHC this problem will arise at a much higher energy.


5. Alejandro Rivero - July 4, 2007

I understand that astronomers are conservative as scientist, and to change the underlying theory once in one hundred years is ago. Should MOND be a better option than darkness?

6. Adam - July 4, 2007

Right, I forgot about the cross-section.

Higher momentum beams are harder to focus, so for a machine like LHC this would be a cakewalk to focus because it’s not as close to the maximum root s.

What’s the cross-section of the Tevatron? I assume you’d need it at least in the picobarn range for collisions near that operational energy. I just started an honours project at LEPP and am learning all the ins and outs of accelerators, fun stuff.

7. dorigo - July 4, 2007

Hi Alejandro,
I think MOND is an excellent alternative, but as far as I understand it has its own issues. I think the issue is not yet resolved, otherwise we would be happy enough with discovering a 100 GeV neutralino in HE collisions – something that does not seem to be deemed sufficient.

8. dorigo - July 4, 2007

Hi Adam, yes, accelerator physics is fun. Classical electrodynamics at work. Neutralino cross sections go from the picobarn to the few femtobarns range, in the mass range of detectability at the Tevatron. I will post about the searches for these things as soon as I have a chance.


9. ryan - July 6, 2007


One can’t have MOND to purely explain the darkness problem The analysis of the Bullet Cluster shows that dark matter uneqivocally exists. The issues with Dark Energy and MOND, are of course, still up in the air and we may one day see an application of MOND in that direction.

10. dorigo - July 7, 2007

Hi Ryan,

sure, I do not think anybody can object that DM exists in some form and quantity – neutrinos have mass, for instance. But I believe that the different evidences are not unequivocal in their interpretation. In my opinion, the bullet cluster is a very striking example of something we do not understand (and fits well the DM interpretation), rather than a proof that we understand the distribution of matter in the universe and just need to find whether it’s a neutralino or something even more exotic.


11. jeff - July 9, 2007

Hi guys. For fun try reading the article
“Fundamentalist Physics: why Dark Energy is bad for astronomy”
by Simon D M White in June volume of REPORT ON PROGRESS IN PHYSICS. To be precise Rep. Prog. Phys. 70 (2007) 883-897

12. dorigo - July 9, 2007

Hi Jeff,

thanks for pointing that out. The pdf can be retrieved at


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