Dark matter talks July 3, 2007
Posted by dorigo in astronomy, news, physics, science.12 comments
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…
