Master Mason
Other interesting things in ArXiv (12 Jun 2009)
Wayne Hu. Acceleration from modified gravity: lessons from worked examples. The main question Wayne Hu discusses in this minireview is “How can we distinguish dark energy from modified gravity theories if the former and the latter provide the same predictions for cosmological dynamics?” He is particularly focused on DGP and 
models. His answer is the presence of anisotropic stress for modified gravity models which is coupled directly to lensing potential – if we are able to measure this effect, we will be able to discriminate between IR modified gravity and dark energy. Another idea is to study behavior of theories in non-linear regimes, for example, by means of N-body simulation, and compare it to LSS.
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Lithium problem
Apart from being a very nice review of Big Bang Nucleosynthesis (BBN), recent paper by Karsten Jedamzik and Maxim Pospelov discusses an important open problem in the physics of BBN.
In short, the lithium problem in physics of Big Bang Nucleosynthesis is seeming underproduction of 
. What do we mean by that?
Workshop on tests of gravity in Case Western – day 2 and Arkani-Hamed’s talk
The second day of the Workshop on Tests of Gravity (and here is my blog post about the first day) was mostly devoted to analog models (Bill Unruh, Michael Uhlmann, George Pickett) and models of modified gravity (Nima Arkani-Hamed, Justin Khoury, Stacy McGaugh, Ted Jacobson, Levon Pogosyan and Mark Wyman).
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Workshop on tests of gravity at Case Western – day 1
Dear friends,
I am sorry for being rather quiet for a while. As many of you may already know, my laptop has decided to enter coma during the trip, as a result, I was left without appropriate internet access (sporadic use of Pascal Vaudrevange’s computer is not counted, thanks, Pascal!). Although the laptop tried to revive after we came back home, demonstrating some kind of “brain” activity, in a couple of days I understood that the growth of entropy is as inevitable as a finally victory of string theory over loop quantum gravity.
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Real-time gauge/gravity duality
Balt van Rees from the U. of Amsterdam continues the discussion of non-equilibrium AdS/CFT we have started not so long time ago. Since his recent paper with Skenderis was one of the major achievements in the field, I highly recommend going through his post. Dmitry.
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A bound on the speed of sound from holography?
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This post is authored by Aleksey Cherman (on the left) and Abhinav Nellore (on the right). Aleksey is a graduate student in the nuclear theory group at the University of Maryland, College Park, working with Tom Cohen, and Abhi is a graduate student in Steve Gubser’s group at Princeton. Dmitry. |  |
We all know that sound travels at about 343 m/s in air, and much faster than that in many solids. But just how much faster could sound travel if given the chance? Could there be a medium in which the speed of sound can approach the speed of light? Or might there be some more stringent fundamental bound on the speed of sound?
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The in-in formalism and cosmological perturbations
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This post is written by two great guys Peter Adshead (his photo is on the left) and Eugene Lim (on the right). Peter is a PhD student of Richard Easther at Yale U., while Eugene is Richard’s former postdoc now working at Columbia U. Dmitry. |  |
The discovery of the anisotropies in the cosmic microwave background (CMB) by the COBE satellite in 1992 heralded in a new era of cosmology. Instead of simply studying the gross evolution of the universe (usually termed “background evolution”), cosmologists now study the structure and evolution of the tiny perturbations about this background. The anisotropies observed in the CMB are believed to be the seeds from which all of the structure (stars, galaxies etc.) we observe today eventually grew. These perturbations, detected as an average of over/under-density of 
, are thought to be generated during an early period of accelerated expansion – inflation.
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On Moore-Read states
Raoul Santachiara is a stuff member at Laboratoire de Physique Theorique et Modeles Statistiques, Universite de Paris-Sud. His interests include statistical and mathematical physics, critical phenomena, disordered systems and entanglement properties of many-body systems. Dmitry.
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Fermionic Schwinger-Keldysh propagators from AdS/CFT
Gregory Giecold is a PhD student at CEA, Saclay. Dmitry.
In this post I will describe recent work on fermionic Schwinger-Keldysh propagators from AdS/CFT. For further details and references see ArXiv: 0904.4869.
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381. M2 branes and Chern-Simons theories
John Davey is a PhD student of Amihay Hanany at Physics Department of Imperial College, London. Dmitry.
Recently Chern-Simons theories have attracted much attention as they are thought to describe the world volume theory of the elusive M2 brane. In this short article I will attempt to outline some of the recent developments in this field and describe how brane tilings can be used to find and investigate a large class of these theories.
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363. Vector inflation
Alexey Golovnev is a research associate in the Arnold Sommerfeld Center for Theoretical Physics , U. of Munich. Dmitry.
I would like to blog about the recently proposed model of vector inflation, Golovnev, Mukhanov, Vanchurin, arXiv:0802.2068. (See also Golovnev, Mukhanov, Vanchurin, arXiv:0810.4304 and Golovnev, Vanchurin, arXiv:0903.2977 for further developments.) Inflation is one of the basic concepts in cosmology, it solves the well-known cosmological problems (for which I refer the reader to any modern textbook on cosmology, see below) by a period of very rapid (nearly exponential) increase of the size of the Universe. By a straightforward inspection of the Friedman equations, this period of accelerated expansion can be achieved with a matter content of negative pressure. As calculating the pressure basically amounts to taking the difference of kinetic and potential energies, the most natural idea to realize this type of equation of state is to “freeze” an inflaton field at a high value of its potential energy.
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357. Vortex line representation. Coulomb interaction of vortex lines
After brief introduction into vortex line representation we are probably ready to discuss the interaction of vortex lines between each other. But before I proceed to the actual derivation, let me focus for a bit on not so terribly popular (but powerful) formulation of ideal hydrodynamics – Hamiltonian formulation.
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356. Cosmological parameters in the context of time varying w
Rahul Biswas is a graduate student in Benjamin Wandelt’s research group working on estimation of cosmological parameters. His main interests also include models of dark energy and supernova cosmology. Dmitry.
Let me start by saying that I highly appreciate Dmitry’s idea of having this online journal club. I hope to learn something from the discussions here while talking about a recent paper (arxiv:0903.2532) with Benjamin Wandelt about parameter constraints in the context of dark energy models with a time varying equation of state 
.
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342. Thermal equilibrium in special relativity
David Cubero is professor at the Department of Applied Physics of the University of Sevilla. Dmitry.
Special relativity, despite being more than a hundred years old, still shows an intriguing capacity to surprise us in very fundamental issues, such as thermal equililbrium. In this post, we will review a recent controversy about the proper velocity distribution of dilute gases at thermal equilibrium.
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329. Human Activity in the Web
Filippo Radicchi is a research scientist in Complex Systems Lagrange Lab, ISI Foundation, Turin. He is interested in non-equilibrium diagrammatic methods, RG group analysis of complex networks and community detection. Dmitry.
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