2,863 research outputs found
U(N) tools for Loop Quantum Gravity: The Return of the Spinor
We explore the classical setting for the U(N) framework for SU(2)
intertwiners for loop quantum gravity (LQG) and describe the corresponding
phase space in terms of spinors with appropriate constraints. We show how its
quantization leads back to the standard Hilbert space of intertwiner states
defined as holomorphic functionals. We then explain how to glue these
intertwiners states in order to construct spin network states as wave-functions
on the spinor phase space. In particular, we translate the usual loop gravity
holonomy observables to our classical framework. Finally, we propose how to
derive our phase space structure from an action principle which induces
non-trivial dynamics for the spin network states. We conclude by applying
explicitly our framework to states living on the simple 2-vertex graph and
discuss the properties of the resulting Hamiltonian.Comment: 23 page
New tools for Loop Quantum Gravity with applications to a simple model
Loop Quantum Gravity is now a well established approach to quantum gravity.
One of the main challenges still faced by the theory is constructing a
consistent dynamics which would lead back to the standard dynamics of the
gravitational field at large scales. Here we will review the recent U(N)
framework for Loop Quantum Gravity and the new spinor representation (that
provides a classical setting for the U(N) framework). Then, we will apply these
techniques to a simple model in order to propose a dynamics for a symmetry
reduced sector of the theory. Furthermore, we will explore certain analogies of
this model with Loop Quantum Cosmology.Comment: 4 pages, to appear in Proceedings of Spanish Relativity Meeting 2011
(ERE 2011) held in Madrid, Spai
Using APOGEE Wide Binaries to Test Chemical Tagging with Dwarf Stars
Stars of a common origin are thought to have similar, if not nearly
identical, chemistry. Chemical tagging seeks to exploit this fact to identify
Milky Way subpopulations through their unique chemical fingerprints. In this
work, we compare the chemical abundances of dwarf stars in wide binaries to
test the abundance consistency of stars of a common origin. Our sample of 31
wide binaries is identified from a catalog produced by cross-matching APOGEE
stars with UCAC5 astrometry, and we confirm the fidelity of this sample with
precision parallaxes from Gaia DR2. For as many as 14 separate elements, we
compare the abundances between components of our wide binaries, finding they
have very similar chemistry (typically within 0.1 dex). This level of
consistency is more similar than can be expected from stars with different
origins (which show typical abundance differences of 0.3-0.4 dex within our
sample). For the best measured elements, Fe, Si, K, Ca, Mn, and Ni, these
differences are reduced to 0.05-0.08 dex when selecting pairs of dwarf stars
with similar temperatures. Our results suggest that APOGEE dwarf stars may
currently be used for chemical tagging at the level of 0.1 dex or at the
level of 0.05 dex when restricting for the best-measured elements in
stars of similar temperatures. Larger wide binary catalogs may provide
calibration sets, in complement to open cluster samples, for on-going
spectroscopic surveys.Comment: 21 pages, 14 figures, accepted for publication in Ap
Dynamics for a 2-vertex Quantum Gravity Model
We use the recently introduced U(N) framework for loop quantum gravity to
study the dynamics of spin network states on the simplest class of graphs: two
vertices linked with an arbitrary number N of edges. Such graphs represent two
regions, in and out, separated by a boundary surface. We study the algebraic
structure of the Hilbert space of spin networks from the U(N) perspective. In
particular, we describe the algebra of operators acting on that space and
discuss their relation to the standard holonomy operator of loop quantum
gravity. Furthermore, we show that it is possible to make the restriction to
the isotropic/homogeneous sector of the model by imposing the invariance under
a global U(N) symmetry. We then propose a U(N) invariant Hamiltonian operator
and study the induced dynamics. Finally, we explore the analogies between this
model and loop quantum cosmology and sketch some possible generalizations of
it.Comment: 28 pages, v2: typos correcte
Holomorphic Simplicity Constraints for 4d Riemannian Spinfoam Models
Starting from the reformulation of the classical phase space of Loop Quantum
Gravity in terms of spinor variables and spinor networks, we build coherent
spin network states and show how to use them to write the spinfoam path
integral for topological BF theory in terms of Gaussian integrals in the
spinors. Finally, we use this framework to revisit the simplicity constraints
reducing topological BF theory to 4d Riemannian gravity. These holomorphic
simplicity constraints lead us to a new spinfoam model for quantum gravity
whose amplitudes are defined as the evaluation of the coherent spin networks.Comment: 4 pages. Proceedings of Loops'11, Madrid. To appear in Journal of
Physics: Conference Series (JPCS
Sugar nucleotide quantification by liquid chromatography tandem mass spectrometry reveals a distinct profile in Plasmodium falciparum sexual stage parasites
The obligate intracellular lifestyle of Plasmodium falciparum
and the difficulties to obtain sufficient amounts of biological
material has hampered the study of specific metabolic pathways
in the malaria parasite. Thus, for example, the pools of sugar
nucleotides required to fuel glycosylation reactions have never
been studied in-depth in well-synchronized asexual parasites or
in other stages of its life cycle. These metabolites are of
critical importance, especially considering the renewed interest
in the presence of N -, O - and other glycans in key parasite
proteins. In this work, we adapted a liquid chromatography
tandem mass spectrometry method based on the use of porous
graphitic carbon (PGC) columns and MS-friendly solvents to
quantify sugar nucleotides in the malaria parasite. We report on
the thorough quantification of the pools of these metabolites
throughout the intraerythrocytic cycle of P. falciparum The
sensitivity of the method enabled for the first time the
targeted analysis of these glycosylation precursors in
gametocytes, the parasite sexual stages that are transmissible
to the mosquito vector
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