13. Eye on ArXiv: 7 Apr 2008 - Gravity cutoff in theories with large descrete symmetries
DmitryIt is a development of one year old extremely elegant Dvali’s idea that the possible maximal number of species is bounded by the gravitational physics.
Let me demonstrate what they have found. One takes a large black hole and throws N of 
-particles into it ( is a scalar field with 
symmetry). Therefore, black hole acquires a charge of the order of 
.
The symmetry is exact and evaporating black hole has to return the exact amount of the eaten charge (i.e., ). However, the usual black hole cannot return the charge, since it emits thermal radiation and this radiation consists of as many particles as antiparticles. We conclude, that properties of the black hole should be modified as long as it reaches certain size. Suppose the black hole just reached this size. Its mass should be enough to produce quanta back:
.
On the other hand,
and we come to the conclusion that
Extremely simple and beautiful.
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12. Eye on ArXiv: 4 Apr 2008 - KS throat cloaking, non-gaussianity and Mathur’s fuzzballs
Dmitry1. Alex Buchel, Lev Kofman “Black Universe epoch in string cosmology”
Alex and Lev write about multithroat scenario in string cosmology. As we remember, typically one needs more than a single throat in KKLMMT-type scenarios because the single throat setup does not solve the hierarchy problem (the energy scale of the throat is inflationary, while we naturally need a TeV scale of the Standard Model after the end of inflation). In two throat scenario history of the Universe looks as follows:
a) inflation. D3- brane travels in a high energy throat towards the tip of the throat where it is annihilated with one of anti-D3’s.
b) reheating. Products of this annihilation (KK modes, SM particles) tunnel into the low energy throat where we are living in.
c) thermalization. Lev had a paper about it: KK modes and SM particles interact with each other in order for KK modes to completely decay into SM particles and subsequently thermalize. It is actually hard for them to do it quickly, because KK modes carry associated conserved quantum numbers. In overall, the thing is dangerous because KK modes unable to decay will overclose the Universe.
In today’s paper, Alex and Lev show that SM throat is in fact cloaked by the Schwartzild horizon produced by KK modes tunneling from the high energy throat.
I will have to take a much closer look on the paper, but for the beginning it is clear that, as follows from the presence of horizon, the process of tunneling from short throat to long throat is one way.
2. Teruaki Suyama, Fuminobu Takahashi “Non-Gaussianity from Symmetry”
As the authors claim, non-gaussianity can be generated by light scalar fields “with symmetries”. The mechanism they are talking about is of course nothng else but a standard curvaton mechanism. Namely, during inflation light scalar field fluctuates strongly with its expectation values being different in different Hubble patches. These slight deviations in 
among different Hubble patches later transform into adiabatic fluctuations of the scalar mode.
By “symmetries” authors mean symmetries of the potential of scalar fields and the fact that different symmetries may lead to different conditions on the derivatives of the total number of efolds 
.
3. K. Skenderis, M. Taylor “The fuzzball proposal for black holes”
Very large review paper on the subject. Mathur’s fuzzball is the following tricky idea: let us consider a class of horizon free non-singular solutions (fuzzballs) of the Einstein equations such that they asymptotically look like a black hole with horizon at 
(where 
is an arbitrary scale) but deviate from the Schwartzild solution at 
. One is in pronciple able to count all such solutions, and it turns out that there are exactly 
of them, where 
is the entropy of the black hole with horizon at !
So, one can think of fuzzballs as black hole microstates and of the corresponding Schwartzild solution - as an averaged description of the system, where averaging is made over all possible fuzzballs.
Existence of fuzzballs can in principle be a resolution of the information loss’ paradox.
Have a nice weekend!
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