The recent discovery of dark energy has prompted an investigation of ways in which the accelerated expansion of the universe can be realized. In this dissertation, we present two separate projects related to dark energy. The first project analyzes a class of braneworld models in which multiple branes float in a five-dimensional anti-de Sitter bulk, while the second investigates a class of dark energy models from an effective field theory perspective. Investigations of models including extra dimensions have led to modifications of gravity involving a number of interesting features. In particular, the Randall-Sundrum model is well-known for achieving an amelioration of the hierarchy problem. However, the basic model relies on Minkowski branes and is subject to solar system constraints in the absence of a radion stabilization mechanism. We present a method by which a four-dimensional low-energy description can be obtained for braneworld scenarios, allowing for a number of generalizations to the original models. This method is applied to orbifolded and uncompactified N-brane models, deriving an effective four-dimensional action. The parameter space of this theory is constrained using observational evidence, and it is found that the generalizations do not weaken solar system constraints on the original model. Furthermore, we find that general

fi nite element methods by building applications

algorithms for the inde¯nite generalized

. We develop a new theoretical framework to analyze this extreme case and develop new algorithms for signal reconstruction from such coarsely quantized measurements. The 1-bit CS framework leads us to scenarios where it may be more appropriate to reduce bit-depth instead of sampling rate. We find

and backchase

Caractérisation des problèmes conjoints de détection et d'estimation

We explore two fundamental questions at the intersection of sampling theory and information theory: how is channel capacity affected by sampling below the channel’s Nyquist rate, and what sub-Nyquist sampling strategy should be employed to maximize capacity. In particular, we first derive the capacity of sampled analog channels for two prevalent sampling mechanisms: filtering followed by sampling and sampling following filter banks. Connections between sampling and MIMO Gaussian channels are illuminated based on this analysis. Optimal prefilters that maximize capacity are identified for both cases, as well as several kinds of channels for which these sampling mechanisms are optimal to maximize capacity at sub-Nyquist rates. We also highlight connections between sampled analog channel capacity and minimum mean squared error estimation from sampled data. In particular, it is shown that for both filtering and filter-bank sampling strategies, the filters maximizing capacity and minimizing mean squared error are equivalent. We also investigate a more general sampling strategy by adding modulation banks to filter-bank sampling. This general sampling method subsumes most nonuniform sampling techniques applied in both theory and practice. We also show a connection between this general sampling method and MIMO Gaussian channels. We then identify the optimal sampling strategy for piece-wise flat sampled

de nationalité suisse et originaire de Zurich (ZH) et Lucerne (LU) acceptée sur proposition du jury:

Commutative, idempotent groupoids and the

Abstract-The ion microprobe has been used to determine the concentrations of a number oftrace elements (including all 14 stable rare earth elements) and the isotopic compositions of Ca and Mg in seven hibonite-rich refractory inclusions. Hibonite in every inclusion analyzed shows the effect of RAZE partitioning with other solid or liquid phases, in which hibonite is enriched in light over heavy REE compared to other existing phases. This partitioning causes Cl chondrite-normalized enrichments to drop by factors of 4 to 22 from lanthanum to lutetium. Fractionations due to gas-solid partitioning of REE are sometimes uperimposed on the pattern produced by partitioning between condensed phases. Two inclusions, HAL and DH-H 1, show evidence for formation in extremely oxidizing environments and their Ca is mass fm~ionat ~ in favor of the heavy isotopes by 7 and 12~/~u, reape&vely. Two ~~ndurn-hi~nite inclu~ons show evidence of extremely high temperature gas-solid fractionation, such that the more volatile tight REE are depleted as a group relative to the more refractory heavy REE. These inciusions both have Ca that is mass fractionated in favor of the light isotopes. Hibonite preserves the largest nuclear isotope anomalies inTi found to date (FAHEY ef al., 198Sa; HINTON et al., 1987b), but there does not seem to be a simple relationship between chemical composition, isotopic mass fractionation and the sign and magnitude of nuclear isotopic anomalies in hibonite-rich inclusions. One possibility for producing a variety of chemical compositions and degrem of mass fractionation while preserving nuclear isotope anomalies is by multiple cycles of condensation and evaporation in a turbulent protoplanetary accretion disk (MORL~LL,