21 research outputs found
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
Critical gravitational collapse: towards a holographic understanding of the Regge region
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.Comment: 81 pages, 31 figure
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
Holographic Flow of Anomalous Transport Coefficients
We study the holographic flow of anomalous conductivities induced by gauge
and gravitational Chern-Simons terms. We find that the contribution from the
gauge Chern-Simons term gives rise to a flow that can be interpreted in terms
of an effective, cutoff dependent chemical potential. In contrast the
contribution of the gauge-gravitational Chern-Simons term is just the
temperature squared and does not flow.Comment: 26 pages, no figure
Holographic Gravitational Anomaly and Chiral Vortical Effect
We analyze a holographic model with a pure gauge and a mixed
gauge-gravitational Chern-Simons term in the action. These are the holographic
implementations of the usual chiral and the mixed gauge-gravitational anomalies
in four dimensional field theories with chiral fermions. We discuss the
holographic renormalization and show that the gauge-gravitational Chern-Simons
term does not induce new divergences. In order to cancel contributions from the
extrinsic curvature at a boundary at finite distance a new type of counterterm
has to be added however. This counterterm can also serve to make the Dirichlet
problem well defined in case the gauge field strength vanishes on the boundary.
A charged asymptotically AdS black hole is a solution to the theory and as an
application we compute the chiral magnetic and chiral vortical conductivities
via Kubo formulas. We find that the characteristic term proportional to T^2 is
present also at strong coupling and that its numerical value is not
renormalized compared to the weak coupling result.Comment: 27 pages, no figure