Entropy generation and inflation in wave collision induced pre-big-bang cosmology

We study inflation and entropy generation in a recently proposed pre-big-bang model universe produced in a collision of gravitational and dilaton waves. It is shown that enough inflation occurs provided the incoming waves are sufficiently weak. We also find that entropy in this model is dynamically generated as the result of the nonlinear interaction of the incoming waves, before the universe enters the phase of dilaton driven inflation. In particular, we introduce a measure for the entropy produced in the collision in terms of the focusing lengths of the incoming waves

Comments on noncommutative gravity

We study the possibility of obtaining noncommutative gravity dynamics from string theory in the Seiberg-Witten limit. We find that the resulting low-energy theory contains more interaction terms than those proposed in noncommutative deformations of gravity. The role of twisted diffeomorphisms in string theory is studied and it is found that they are not standard physical symmetries. It is argued that this might be the reason why twisted diffeomorphisms are not preserved by string theory in the low energy limit. Twisted gauge transformations are also discussed.Comment: 37 pages. Typos corrected. Final version to appear in Nuclear Physics

Gravitational shocks as a key ingredient of Gamma-Ray Bursts

We identify a novel physical mechanism that may be responsible for energy release in $\gamma$-ray bursts. Radial perturbations in the neutron core, induced by its collision with collapsing outer layers during the early stages of supernova explosions, can trigger a gravitational shock, which can readily eject a small but significant fraction of the collapsing material at ultra-relativistic speeds. The development of such shocks is a strong-field effect arising in near-critical collapse in General Relativity and has been observed in numerical simulations in various contexts, including in particular radially perturbed neutron star collapse, albeit for a tiny range of initial conditions. Therefore, this effect can be easily missed in numerical simulations if the relevant parameter space is not exhaustively investigated. In the proposed picture, the observed rarity of $\gamma$-ray bursts would be explained if the relevant conditions for this mechanism appear in only about one in every $10^4-10^5$ core collapse supernovae. We also mention the possibility that near-critical collapse could play a role in powering the central engines of Active Galactic Nuclei.Comment: 9 pages, 3 figure

On the stringy nature of winding modes in noncommutative thermal field theories

We show that thermal noncommutative field theories admit a version of `channel duality' reminiscent of open/closed string duality, where non-planar thermal loops can be replaced by an infinite tower of tree-level exchanges of effective fields. These effective fields resemble closed strings in three aspects: their mass spectrum is that of closed-string winding modes, their interaction vertices contain extra moduli, and they can be regarded as propagating in a higher-dimensional `bulk' space-time. In noncommutative models that can be embedded in a D-brane, we show the precise relation between the effective `winding fields' and closed strings propagating off the D-brane. The winding fields represent the coherent coupling of the infinite tower of closed-string oscillator states. We derive a sum rule that expresses this effective coupling in terms of the elementary couplings of closed strings to the D-brane. We furthermore clarify the relation between the effective propagating dimension of the winding fields and the true codimension of the D-brane

Critical gravitational collapse: towards a holographic understanding of the Regge region

81 pages, 31 figures.We study the possible holographic connection between the Regge limit in QCD and critical gravitational collapse of a perfect fluid in higher dimensions. We begin by analyzing the problem of critical gravitational collapse of a perfect fluid in any number of dimensions and numerically compute the associated Choptuik exponent in d=5, 6 and 7 for a range of values of the speed of sound of the fluid. Using continuous self-similarity as guiding principle, a holographic correspondence between this process and the phenomenon of parton saturation in high-energy scattering in QCD is proposed. This holographic connection relates strong gravitational physics in the bulk with (nonsupersymmetric) QCD at weak coupling in four dimensions.The work of C.G. has been partially supported by the Spanish DGI contract FPA2003-02877 and the CAM grant HEPHACOS P-ESP-00346. A.T. thanks the Marie Curie and the Freydoon Mansouri foundations for support, and the CERN Theory Group for hospitality. M.A.V.-M. acknowledges partial support from the Spanish Government Grants PA2005-04823, FIS2006-05319 and Spanish Consolider-Ingenio 2010 Programme CPAN (CSD2007-00042), and thanks the CERN Theory Group for hospitality

Scaling Phenomena in Gravity from QCD

We present holographic arguments to predict properties of strongly coupled gravitational systems in terms of weakly coupled gauge theories. In particular we relate the latest computed value for the Choptuik critical exponent in black hole formation in five dimensions, \gamma_{5D}=0.412 \pm 1%, to the saturation exponent of four-dimensional Yang-Mills theory in the Regge limit, \gamma_{BFKL}\simeq 0.410.Comment: 13 pages. To Pere Pascual, in memoriam. v2: minor changes. Typos corrected and references added. v3: conclusions expanded, references added. To appear in Physics Letters