5,955 research outputs found
Product CFTs, gravitational cloning, massive gravitons and the space of gravitational duals
The question of graviton cloning in the context of the bulk/boundary
correspondence is considered. It is shown that multi-graviton theories can be
obtained from products of large-N CFTs. No more than one interacting massless
graviton is possible. There can be however, many interacting massive gravitons.
This is achieved by coupling CFTs via multi-trace marginal or relevant
perturbations. The geometrical structure of the gravitational duals of such
theories is that of product manifolds with their boundaries identified. The
calculational formalism is described and the interpretation of such theories is
discussed.Comment: Latex, 25 pages. (v2) Minor corrections and references adde
Infra-red modification of gravity from asymmetric branes
We consider a single Minkowski brane sandwiched in between two copies of
anti-de Sitter space. We allow the bulk Planck mass and cosmological constant
to differ on either side of the brane. Linearised perturbations about this
background reveal that gravity can be modified in the infra-red. At
intermediate scales, the braneworld propagator mimics four-dimensional GR in
that it has the correct momentum dependance. However it has the wrong tensor
structure. Beyond a source dependant scale, we show that quadratic brane
bending contributions become important, and conspire to correct the tensor
structure of the propagator. We argue that even higher order terms can
consistently be ignored up to very high energies, and suggest that there is no
problem with strong coupling. We also consider scalar and vector perturbations
in the bulk, checking for scalar ghosts.Comment: Version appearing in CQ
Stealth Acceleration and Modified Gravity
We show how to construct consistent braneworld models which exhibit late time
acceleration. Unlike self-acceleration, which has a de Sitter vacuum state, our
models have the standard Minkowski vacuum and accelerate only in the presence
of matter, which we dub ``stealth-acceleration''. We use an effective action
for the brane which includes an induced gravity term, and allow for an
asymmetric set-up. We study the linear stability of flat brane vacua and find
the regions of parameter space where the set-up is stable. The 4-dimensional
graviton is only quasi-localised in this set-up and as a result gravity is
modified at late times. One of the two regions is strongly coupled and the
scalar mode is eaten up by an extra symmetry that arises in this limit. Having
filtered the well-defined theories we then focus on their cosmology. When the
graviton is quasi-localised we find two main examples of acceleration. In each
case, we provide an illustrative model and compare it to LambdaCDM.Comment: 32 pages, 5 figure
Structural and morphological evolution of powders nanostructured ceramics: transitional aluminas
This work aims with the study of the transformation of boehmite into transitional aluminas. Boehmite was obtained by a sol-gel method from an aluminium hazardous waste. The thermal behaviour of boehmite was followed by thermogravimetry and differential thermal analysis to determine the transformation temperatures. By calcinations of boehmite at temperatures ranging between 250-1000ÂșC, transitional aluminas (?, ? , ?-Al2O3) were synthesized and characterized by XRD, TEM and FTIR. All the transitional aluminas exhibit nanometric crystallite size, ranging from 2.5-15nm. ?-Al2O3 was obtained as a nanostructured material at 500ÂșC with a cell parameter a=7.923Ă
. ?-phase stars to appear at 850ÂșC with a crystallite size of 6nm and cell parameters a=5.672Ă
and c=24.600Ă
. For ?-Al2O3 the cell parameters, in Ă
, were a=11.817, b=2.912, c=5.621 and ?=103.8Âș. The progressive conversion of the transitional phase ?-Al2O3 into the stable polymorph ?-alumina, takes place gradually and a four-phases region is achieved at 1000ÂșC, where coexist with other transitional phase such as ?- and ?-Al2O3
Aging in lattice-gas models with constrained dynamics
We investigate the aging behavior of lattice-gas models with constrained
dynamics in which particle exchange with a reservoir is allowed. Such models
provide a particularly simple interpretation of aging phenomena as a slow
approach to criticality. They appear as the simplest three dimensional models
exhibiting a glassy behavior similar to that of mean field (low temperature
mode-coupling) models.Comment: 5 pages and 3 figures, REVTeX. Submitted to Europhysics Letter
Braneworld holography in Gauss-Bonnet gravity
We investigate holography on an (n-1)-dimensional brane embedded in a
background of AdS black holes, in n-dimensional Gauss-Bonnet gravity. We
demonstrate that for a critical brane near the AdS boundary, the Friedmann
equation corresponds to that of the standard cosmology driven by a CFT dual to
the AdS bulk. We show that there is no holographic description for non-critical
branes, or when the brane is further away from the AdS boundary. We then derive
a Cardy-Verlinde formula for the dual CFT on the critical brane near the
boundary. This gives us insight into the remarkable correspondence between
Cardy-Verlinde formulae and Friedmann equations in Einstein gravity.Comment: 24 pages, no figures; references added, minor changes, version to
appear in CQ
Chiral gravity as a covariant formulation of massive gravity
We present a covariant nonlinear completion of the Fierz-Pauli (FP) mass term
for the graviton. The starting observation is that the FP mass is immediately
obtained by expanding the cosmological constant term, i.e. the determinant of
the vielbein, around Minkowski space to second order in the vielbein
perturbations. Since this is an unstable expansion in the standard case, we
consider an extended theory of gravity which describes two vielbeins that give
rise to chiral spin--connections (consequently, fermions of a definite
chirality only couple to one of the gravitational sectors). As for Einstein
gravity with a cosmological constant, a single fine-tuning is needed to recover
a Minkowski background; the two sectors then differ only by a constant
conformal factor. The spectrum of this theory consists of a massless and a
massive graviton, with FP mass term. The theory possesses interesting limits in
which only the massive graviton is coupled to matter at the linearized level.Comment: 1+16 pages LaTeX, typos corrected, and some references adde
The evolution of Balmer jump selected galaxies in the ALHAMBRA survey
We present a new color-selection technique, based on the Bruzual & Charlot
models convolved with the bands of the ALHAMBRA survey, and the redshifted
position of the Balmer jump to select star-forming galaxies in the redshift
range 0.5 < z < 1.5. These galaxies are dubbed Balmer jump Galaxies BJGs. We
apply the iSEDfit Bayesian approach to fit each detailed SED and determine
star-formation rate (SFR), stellar mass, age and absolute magnitudes. The mass
of the haloes where these samples reside are found via a clustering analysis.
Five volume-limited BJG sub-samples with different mean redshifts are found to
reside in haloes of median masses slightly
increasing toward z=0.5. This increment is similar to numerical simulations
results which suggests that we are tracing the evolution of an evolving
population of haloes as they grow to reach a mass of at z=0.5. The likely progenitors of our samples at z3 are Lyman
Break Galaxies, which at z2 would evolve into star-forming BzK galaxies,
and their descendants in the local Universe are elliptical galaxies.Hence, this
allows us to follow the putative evolution of the SFR, stellar mass and age of
these galaxies. From z1.0 to z0.5, the stellar mass of the volume
limited BJG samples nearly does not change with redshift, suggesting that major
mergers play a minor role on the evolution of these galaxies. The SFR evolution
accounts for the small variations of stellar mass, suggesting that star
formation and possible minor mergers are the main channels of mass assembly.Comment: 14 pages, 10 figures. Submitted to A&A. It includes first referee's
comments. Abstract abridged due to arXiv requirement
Ghost-free braneworld bigravity
We consider a generalisation of the DGP model, by adding a second brane with
localised curvature, and allowing for a bulk cosmological constant and brane
tensions. We study radion and graviton fluctuations in detail, enabling us to
check for ghosts and tachyons. By tuning our parameters accordingly, we find
bigravity models that are free from ghosts and tachyons. These models will lead
to large distance modifications of gravity that could be observable in the near
future.Comment: Dedicated to the memory of Ian Kogan. Version to appear in Classical
and Quantum Gravit
- âŠ