*We study stars in M theory. First, we obtain the analog of Oppenheimer -- Volkoff equations in a suitably general set up. We obtain analytically the asymptotic solutions to these equations when the equations of state are linear. We study perturbations around such solutions in several examples and, following a standard method, use their behaviour to determine whether an instability is present or not. In this way, we obtain a generalisation of the corresponding results of Chavanis. We also find that stars in M theory have instabilities. Therefore, if sufficiently massive, such stars will collapse. We discuss the significance of these (in)stabilities within the context of Mathur's fuzz ball proposal.*The last paper I consider is this: Inflationary paradigm after Planck 2013. The first author is the very creator of the idea of Inflation, Alan guth, and at first sight it would seem a review of resoluts. In fact is somewhat of a reply to another paper, but still it is a good way to get an idea of how the Planck results have affected our view on Inflation. One last words for my previous entry. After a discussion of it in an spanish forum I was pointed to an old paper by Tholman - http://authors.library.caltech.edu/2596/1/TOLpr30d.pdf- and, seemengly, it could be considered in the kind of ideas discussed there. My example could be translated to emission of photons between bodies orbitating a central one. The gravitational blue/red shift would play the role of the expanding/contracting universe red/blue shift and that would accommodate into a stationary situation. If the difference of blue/red shift energy is greater than the temperature difference (boltzman factors mediated) then an inverse Clausius behaviour is possible. The key point is that the Clausius Law is not equivalent to the formulation of increase of entropy in a general relativistic setup and this last kind of law still holds, at least in static (probably stationary) gravitational fields, but I am not sure about how the same would still hold in a general spacetime.

## Tuesday, December 31, 2013

### A prety good year ending in Arxiv

After a somewhat disappointing year in theorethical physics where the greater topic seems to have been the black holes firewall discussion (seemingly settled in a long - around 90 pages- paper) the end year in a promising way.
One one hand we have the second paper of the famous Amplituhedron construction (I told about it in my spanish blog when it appeared) that was announced by Nima Arkani-Hamed, but of which we only had seen a somewhat introductory paper. This is the second one: Into the Amplituhedron. I have not read it, neither I did it with the first. I still am reading, from time to time, a previous long paper about grassmanians and all that. I hope I'll give a reading to the first, and to the one of today, soon, but I guess that Lubos will write a fair better post about it that anything that I could do.
I must acknowledge to Lubos the pointing to another paper. Gravitation from Entanglement in Holographic CFTs. I had not paid attention to it because almost like a question of principles I don't pay attention to papers with the fancy names "holographic" or "entanglement" in the topic. But this time it looks pretty important because it looks like is they have made a great advance in a constructive derivation of general relativity from some thermodynamics consideration, improving the previous work by Ted Jacobson. But better read the entry in Lubos Blog Einstein's equations from first law of thermodynamics in AdS.
A very different type of article is Stars in M theory (made up of intersecting branes). The title is surprising and the subject certifies that we are in front of a very exotic paper.

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