32 research outputs found
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 -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 -ray bursts would be
explained if the relevant conditions for this mechanism appear in only about
one in every 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
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
Lattice Perturbation Theory in Noncommutative Geometry and Parity Anomaly in 3D Noncommutative QED
We formulate lattice perturbation theory for gauge theories in noncommutative
geometry. We apply it to three-dimensional noncommutative QED and calculate the
effective action induced by Dirac fermions. In particular "parity invariance"
of a massless theory receives an anomaly expressed by the noncommutative
Chern-Simons action. The coefficient of the anomaly is labelled by an integer
depending on the lattice action, which is a noncommutative counterpart of the
phenomenon known in the commutative theory. The parity anomaly can also be
obtained using Ginsparg-Wilson fermions, where the masslessness is guaranteed
at finite lattice spacing. This suggests a natural definition of the
lattice-regularized Chern-Simons theory on a noncommutative torus, which could
enable nonperturbative studies of quantum Hall systems.Comment: 31 pages. LaTeX, feynmf. Minor changes, references added and typos
corrected. Final version published in JHE
Colliding AdS gravitational shock waves in various dimensions and holography
The formation of marginally trapped surfaces in the off-center collision of
two shock waves on AdS_D (with D=4,5,6,7 and 8) is studied numerically. We
focus on the case when the two waves collide with nonvanishing impact parameter
while the sources are located at the same value of the holographic coordinate.
In all cases a critical value of the impact parameter is found above which no
trapped surface is formed. The numerical results show the existence of a simple
scaling relation between the critical impact parameter and the energy of the
colliding waves. Using the isometries of AdS_D we relate the solutions obtained
to the ones describing the collision of two waves with a purely holographic
impact parameter. This provides a gravitational dual for the head-on collision
of two lumps of energy of unequal size.Comment: 25 pages, 11 figures. v2: minor changes, typos corrected. To appear
in JHE
Positioning systems in Minkowski space-time: from emission to inertial coordinates
The coordinate transformation between emission coordinates and inertial
coordinates in Minkowski space-time is obtained for arbitrary configurations of
the emitters. It appears that a positioning system always generates two
different coordinate domains, namely, the front and the back emission
coordinate domains. For both domains, the corresponding covariant expression of
the transformation is explicitly given in terms of the emitter world-lines.
This task requires the notion of orientation of an emitter configuration. The
orientation is shown to be computable from the emission coordinates for the
users of a `central' region of the front emission coordinate domain. Other
space-time regions associated with the emission coordinates are also outlined.Comment: 20 pages; 1 figur
General Properties of Noncommutative Field Theories
In this paper we study general properties of noncommutative field theories
obtained from the Seiberg-Witten limit of string theories in the presence of an
external B-field. We analyze the extension of the Wightman axioms to this
context and explore their consequences, in particular we present a proof of the
CPT theorem for theories with space-space noncommutativity. We analyze as well
questions associated to the spin-statistics connections, and show that
noncommutative N=4, U(1) gauge theory can be softly broken to N=0 satisfying
the axioms and providing an example where the Wilsonian low energy effective
action can be constructed without UV/IR problems, after a judicious choice of
soft breaking parameters is made. We also assess the phenomenological prospects
of such a theory, which are in fact rather negative.Comment: 39 pages. LaTeX. 4 figures. Typos corrected. Comments and references
added. To appear in Nuclear Physics