251 research outputs found
Solution of a Braneworld Big Crunch/Big Bang Cosmology
We solve for the cosmological perturbations in a five-dimensional background
consisting of two separating or colliding boundary branes, as an expansion in
the collision speed V divided by the speed of light c. Our solution permits a
detailed check of the validity of four-dimensional effective theory in the
vicinity of the event corresponding to the big crunch/big bang singularity. We
show that the four-dimensional description fails at the first nontrivial order
in (V/c)^2. At this order, there is nontrivial mixing of the two relevant
four-dimensional perturbation modes (the growing and decaying modes) as the
boundary branes move from the narrowly-separated limit described by
Kaluza-Klein theory to the well-separated limit where gravity is confined to
the positive-tension brane. We comment on the cosmological significance of the
result and compute other quantities of interest in five-dimensional
cosmological scenarios.Comment: 54 pages, 12 figures, URL updated & 3 references adde
Sparse Graph Codes for Quantum Error-Correction
We present sparse graph codes appropriate for use in quantum
error-correction. Quantum error-correcting codes based on sparse graphs are of
interest for three reasons. First, the best codes currently known for classical
channels are based on sparse graphs. Second, sparse graph codes keep the number
of quantum interactions associated with the quantum error correction process
small: a constant number per quantum bit, independent of the blocklength.
Third, sparse graph codes often offer great flexibility with respect to
blocklength and rate. We believe some of the codes we present are unsurpassed
by previously published quantum error-correcting codes.Comment: Version 7.3e: 42 pages. Extended version, Feb 2004. A shortened
version was resubmitted to IEEE Transactions on Information Theory Jan 20,
200
Conformal symmetry of brane world effective actions
A simple derivation of the low-energy effective action for brane worlds is
given, highlighting the role of conformal invariance. We show how to improve
the effective action for a positive- and negative-tension brane pair using the
AdS/CFT correspondence.Comment: 5 pages, published versio
Holographic predictions for cosmological 3-point functions
We present the holographic predictions for cosmological 3-point correlators,
involving both scalar and tensor modes, for a universe which started in a
non-geometric holographic phase. Holographic formulae relate the cosmological
3-point functions to stress tensor correlation functions of a holographically
dual three-dimensional non-gravitational QFT. We compute these correlators at
1-loop order for a theory containing massless scalars, fermions and gauge
fields, and present an extensive analysis of the constraints due to Ward
identities showing that they uniquely determine the correlators up to a few
constants. We define shapes for all cosmological bispectra and compare the
holographic shapes to the slow-roll ones, finding that some are distinguishable
while others, perhaps surprisingly, are not.Comment: 51pp; 4 fig
Colliding Branes in Heterotic M-theory
We study the collision of two flat, parallel end-of-the-world branes in
heterotic M-theory. By insisting that there is no divergence in the Riemann
curvature as the collision approaches, we are able to single out a unique
solution possessing the local geometry of (2d compactified Milne)/Z_2 x R_3,
times a finite-volume Calabi-Yau manifold in the vicinity of the collision. At
a finite time before and after the collision, a second type of singularity
appears momentarily on the negative-tension brane, representing its bouncing
off a zero of the bulk warp factor. We find this singularity to be remarkably
mild and easily regularised. The various different cosmological solutions to
heterotic M-theory previously found by other authors are shown to merely
represent different portions of a unique flat cosmological solution to
heterotic M-theory.Comment: 29 pp, 4 figs; Appendix B revised to include junction condition for
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The holographic fluid dual to vacuum Einstein gravity
We present an algorithm for systematically reconstructing a solution of the
(d+2)-dimensional vacuum Einstein equations from a (d+1)-dimensional fluid,
extending the non-relativistic hydrodynamic expansion of Bredberg et al in
arXiv:1101.2451 to arbitrary order. The fluid satisfies equations of motion
which are the incompressible Navier-Stokes equations, corrected by specific
higher derivative terms. The uniqueness and regularity of this solution is
established to all orders and explicit results are given for the bulk metric
and the stress tensor of the dual fluid through fifth order in the hydrodynamic
expansion. We establish the validity of a relativistic hydrodynamic description
for the dual fluid, which has the unusual property of having a vanishing
equilibrium energy density. The gravitational results are used to identify
transport coefficients of the dual fluid, which also obeys an interesting and
exact constraint on its stress tensor. We propose novel Lagrangian models which
realise key properties of the holographic fluid.Comment: 31 pages; v2: references added and minor improvements, published
versio
Educational intervention improves fruit and vegetable intake in young adults with metabolic syndrome components
The FRUVEDomics study investigates the effect of a diet intervention focused on increasing fruit and vegetable intake on the gut microbiome and cardiovascular health of young adults with/at risk for metabolic syndrome(MetS). It was hypothesized that the recommended diet would result in metabolic and gut microbiome changes. The 9-week dietary intervention adhered to the US Department of Agriculture Dietary Guidelines for Americansand focused on increasing fruit and vegetable intake to equal half of the diet. Seventeen eligible young adults with/or at high risk of MetS consented and completed preintervention and postintervention measurements, including anthropometric, body composition, cardiovascular, complete blood lipid panel, and collection of stool sample for microbial analysis. Participants attended weekly consultations to assess food logs, food receipts, and adherence to the diet. Following intention-to-treat guidelines, all 17 individuals were included in the dietary, clinical, and anthropometric analysis. Fruit and vegetable intake increased from 1.6 to 3.4 cups of fruits and vegetables (P \u3c .001) daily. Total fiber (P = .02) and insoluble fiber (P \u3c .0001) also increased. Clinical laboratory changes included an increase in sodium (P = .0006) and low-density lipoprotein cholesterol (P = .04). In the fecal microbiome, Erysipelotrichaceae (phylum Firmicutes) decreased (log2 fold change: −1.78, P = .01) and Caulobacteraceae (phylum Proteobacteria) increased (log2 fold change = 1.07, P = .01). Implementing a free-living 9-week diet, with intensive education and accountability, gave young adults at high risk for/or diagnosed with MetS the knowledge, skills, and feedback to improve diet. To yield greater impact, a longer diet intervention may be needed in this population
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