Suggestions for monday february 2nd

For coming monday we have suggestions of diverse nature:

Patrick volunteers to lead the discussion on the paper 

http://arxiv.org/abs/0901.3474

where Brandenberger et al. propose an explanation of the Pamela positron excess in terms of non-superconducting cosmic strings.

We also have a paper which was not discussed last week, proposed by Lidia and Gianfranco, about the anisotropy properties of galactic and extra-galactic dark matter annihilation

http://arxiv.org/abs/0901.2921

by Mattia Fornasa et al.

I have also spotted a paper on a explanation of the Pamela + ATIC signals in terms of a nearby GRB 

http://arxiv.org/abs/0812.4851

by Kunihito Ioka

and I am sure there are experts on acceleration mechanisms who are eager to comment on it... :)

Please feel free to post (or send me, and I will post them for you) other suggestions at your leisure.

Another suggestion for monday 26th

Joe will lead the discussion on this paper, wich proposes a solution of the missing satellite problem.

"A quantitative explanation of the observed population of Milky Way satellite galaxies"

http://arxiv.org/abs/0901.2116

by S. Koposov, J. Yoo, H.W. Rix, D.H. Weinberg, A.V. Maccio', and J. Miralda-Escude'

Suggestions for Jan 26

Anisotropy probe of galactic and extra-galactic Dark Matter annihilations

Authors: Mattia Fornasa, Lidia Pieri, Gianfranco Bertone, Enzo Branchini

(Submitted on 19 Jan 2009)

Abstract: We study the flux and the angular power spectrum of gamma-rays produced by Dark Matter (DM) annihilations in the Milky Way (MW) and in extra-galactic halos. The annihilation signal receives contributions from: a) the smooth MW halo, b) resolved and unresolved substructures in the MW, c) external DM halos at all redshifts, including d) their substructures. Adopting a self-consistent description of local and extra-galactic substructures, we show that the annihilation flux from substructures in the MW dominates over all the other components for angles larger than O(1) degrees from the Galactic Center, unless an extreme prescription is adopted for the substructures concentration. We also compute the angular power spectrum of gamma-ray anisotropies and find that, for an optimistic choice of the particle physics parameters, an interesting signature of DM annihilations could soon be discovered by the Fermi LAT satellite at low multipoles, l<100,>10^4 solar masses. For the substructures models we have adopted, we find that the contribution of extra-galactic annihilations is instead negligible at all scales.

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Dipolar Dark Matter and Dark Energy

Authors: Luc Blanchet, Alexandre Le Tiec

(Submitted on 20 Jan 2009)

Abstract: In previous work [L. Blanchet and A. Le Tiec, Phys. Rev. D 78, 024031 (2008)], a model of dark matter and dark energy based on the concept of gravitational polarization was investigated. This model was shown to recover the concordance cosmological scenario (Lambda-CDM) at cosmological scales, and the phenomenology of the modified Newtonian dynamics (MOND) at galactic scales. In this Letter we prove that the model can be formulated with a simple and physically meaningful matter action in general relativity. We also provide alternative derivations of the main results of the model.

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Dark Matter Sees The Light

Authors: Patrick Meade, Michele Papucci, Tomer Volansky

(Submitted on 20 Jan 2009)

Abstract: We construct a Dark Matter (DM) annihilation module that can encompass the predictions from a wide array of models built to explain the recently reported PAMELA and ATIC/PPB-BETS excesses. We present a detailed analysis of the injection spectrums for DM annihilation and quantitatively demonstrate effects that have previously not been included from the particle physics perspective. With this module we demonstrate the parameter space that can account for the aforementioned excesses and be compatible with existing high energy gamma ray and neutrino experiments. However, we find that it is relatively generic to have some tension between the results of the HESS experiment and the ATIC/PPB-BETS experiments within the context of annihilating DM. We discuss ways to alleviate this tension and how upcoming experiments will be able to differentiate amongst the various possible explanations of the purported excesses.

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Cafe on monday january 19th

Hello everybody,

coming monday I would like to discuss a paper from D. Rapetti and the rest of S. Allen's group:

http://arXiv.org/abs/0812.2259

on a test of modified gravity theories from the observed X-ray luminosity function of galaxy clusters. I am a little sceptical about some things in this paper, and there are few others I still do not understand so I would be glad if someone else could join efforts with me.

Also, I can propose you something that reminds me of my first astrophysical love: reionization and first stars.

In the paper

http://arXiv.org/abs/0812.3405

M. Alvarez et al. made a first ever attempt to connect reionization to the local Universe within the framework of a simulation. 

I have spotted few more papers about Pamela excess (they grow like mushrooms after a rainy shower) but we might keep for the future (unless someone else wishes differently). 

Anybody else is as usual welcome to propose whatever they found intriguing. 

Also, I would care to remind graduate students that the Cafe is a good gym to exercise their independence and curiosity (namely: post papers!) and that the only stupid thing about a question is not asking it (namely: if there is something you don't understand speak aloud!).

See you at the usual place, usual time.

Cafe on monday january the 12th

Hello everybody,

we start the first Cafe of the new Year with a joint presentation from Kumiko and myself: we will discuss the recent paper

http://arXiv.org/abs/0812.0597

from Jeltema, Kehayas and Profumo on gamma rays from clusters of galaxies, produced from cosmic rays and dark matter annihilation.

Gianfranco has proposed a paper by C. Alard

http://arXiv.org/abs/0901.0344

on a peculiar lensed system which seems to be compatible only with dark matter matter envelope around luminous objects, the discrepancy between light and gravitational mass being very hard to explain in modified gravity models.

If you have noticed any interesting paper in the last month please don't be shy and post it for our Cafe!

See you on monday same time, same place

Salut, and a cold and shining weekend.

No Cafe on monday, january 5th

Given the null response to the call, the first Cafe GReCO of 2009 will be on january 12th.