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TOPIC: Greetings from Aspen! Let’s understand the dark sector [refresh]
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Blogger Niayesh Afshordi wrote on Jan. 29, 2009 @ 14:29 GMT
I promised the editor to blog about the cosmology workshop that I am attending this week, here at the Aspen center for physics. Now, it turns out that I’m much less reliable than I thought I was, and now I have to try to summarize three days of meeting in one post. But first a word about the Aspen center for physics:



Aspen center for physics is an amazing establishment which hosts week-long workshops in winters, and longer focus programs in summers. As far as I can tell, it aims at bringing top-notch physicists (who spend most of their time in front of computer screens) back in touch with the beautiful nature that surrounds Aspen (and makes so hard to get to). I have been coming to the cosmology winter programs for the past six years, and have always been impressed by the quality of the talks and the participants. Of course, if you look up Aspen on google maps, you won’t be surprised that the workshop has a weird schedule to accommodate people’s skiing plans.

As to the topics so far, there is still a lot of excitement about possible discovery of dark matter signature, via the observation of positron excess in PAMELA data (at energies of ~ 100 GeV). Doug Finkbeiner and Graham Kribs talked about their favourite dark matter models to explain the excess. However, Greg Tarle from Michigan, who was involved with the HEAT experiment (which detected a similar signal a few years ago), warned about the calibration problems, and how mistaking a tiny fraction of protons as positrons could lead to a similar excess. He also didn’t believe the ATIC detection of excess electron+positrons at higher energies. Two points: I) Pulsars seem to have a very uncertain physics and can also possibly explain all the excesses. II) Fermi/GLAST satellite will be able to confirm/rule out the ATIC excess within a few months.

The next big theme of the workshop (and cosmology in general, for the past few years) was how modifications of Einstein gravity can be important and observed on cosmological scales. Many discussed consistency conditions that you expect for dark energy models, and if they’re broken in observations, it means that General Relativity is modified. The catch is that you can probably always model any observation as an interacting model of dark matter and dark energy. Wayne Hu presented the first simulations of an f(R) model, which is a specific class of scalar-tensor theories. Ghazal Geshnizjani talked about a specific modified gravity model, with two or more extra dimensions, which we have developed with Justin Khoury, and can potentially explain a host of anomalies on cosmological scales. I also talked about my own solution to the cosmological constant problem, gravitational aether. An amazing result, which I presented for the first time here, was that my student Chanda Prescod-Weinstein and I had shown that formation of stellar black holes in this model can trigger cosmic acceleration. I’m really excited about this, and hope that the community catches on too!

Some other highlights:

Katie Freese talked about Dark Stars, the first generation of stars that are fuelled by dark matter annihilation, rather than nuclear fusion. It’ll be very exciting if we get to see any signature of these.

Kev Abazajian discussed the relation between the Lyman-alpha forest and the temperature of dark matter. He seems to think that an earlier measurement of matter power spectrum by McDonald et al. from SDSS quasars, had some calibration issues, and an apparent discrepancy with LCDM, highlighted by Seljak, Slosar and McDonald, may go away.

Alexey Vikhlinin and Brian Gerke presented new cosmological constraints based on the number counts of X-ray clusters (detected in ROSAT and followed up with Chandra) and galaxy groups (seen in DEEP2), respectively. They’re both very interesting, as it’s the first time that such accurate constraints can come from halo mass function.

Apparently, there are discrepancies between different ways of reducing SNeIa data to get cosmological constraints. I wonder how long we can run with SNeIa without really understanding what they are!

Rich Holman talked about unparticles (a weird invention of Howard Georgi, just in case particle physics isn’t complicated enough!) Rich discussed how they could couple to standard model particles, and its implications. I was wondering whether they can have a fluid description or not. I still don’t know.

Lam Hui discussed how modified gravity theories can lead to a breakdown of equivalence principle on the scale of galaxies: big galaxies can be moving faster/slower than small ones! He also pointed out that in modified gravity theories, galaxies don’t follow mass on large scales, i.e. bias is scale-dependent, a fact that only few seem to be aware of!

Dan Coe from CalTech, reconstructed the mass distribution of a galaxy cluster with more than a hundred strongly lensed images! This was the first time I saw a systematic approach to mass reconstruction from strong lensing.

Finally, Andrey Kravstov talked about his suit of N-body simulations, which he used to calibrate properties of dark matter haloes. I got very excited about this, because he confirmed a prediction which Renyue and I made back in 2001; the concentration of dark matter haloes reaches a minimum at high masses. Back then, all the simulations showed that it was a monotonically deceasing power-law as a function of mass. Isn’t it great when people confirm your predictions? :-)

By the way, I’m sorry if I missed somebody’s talk. Email me, and I’ll make it up to you!

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amrit wrote on Jan. 29, 2009 @ 14:49 GMT
Hi Niayesh

Some people out there discuss also that in black holes energy of matter might be transformed in energy of the space? According my research energy flow matter-space-matter is continuous. Universe is a system in a permanent dynamic equilibrium.

attachments: Relation_between_Time_Space_And_Motion_Sorli__2009.doc, Dynamic_Equilibrium_of_The_Atemporal_Universe.mht

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atomiton1 wrote on Jan. 29, 2009 @ 21:58 GMT
I believe that the dencity of the star in the center of a galaxy has a major effect on what we see happenning to it. In fact im working on a theory to help describe how it works. To chat email is atomiton1@yahoo.com

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