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Search Results: 1 - 10 of 31542 matches for " Silke Thomas "
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Quantum gravity without Lorentz invariance
Thomas P Sotiriou,Matt Visser,Silke Weinfurtner
Physics , 2009, DOI: 10.1088/1126-6708/2009/10/033
Abstract: There has been a significant surge of interest in Horava's model for 3+1 dimensional quantum gravity, this model being based on anisotropic scaling at a z=3 Lifshitz point. Horava's model, and its variants, show dramatically improved ultra-violet behaviour at the cost of exhibiting violation of Lorentz invariance at ultra-high momenta. Following up on our earlier note, [arXiv:0904.4464 [hep-th]], we discuss in more detail our variant of Horava's model. In contrast to Horava's original model, we abandon "detailed balance" and restore parity invariance. We retain, however, Horava's "projectability condition" and explore its implications. Under these conditions, we explicitly exhibit the most general model, and extract the full classical equations of motion in ADM form. We analyze both spin-2 and spin-0 graviton propagators around flat Minkowski space. We furthermore analyze the classical evolution of FLRW cosmologies in this model, demonstrating that the higher-derivative spatial curvature terms can be used to mimic radiation fluid and stiff matter. We conclude with some observations concerning future prospects.
Projectable Horava-Lifshitz gravity in a nutshell
Silke Weinfurtner,Thomas P. Sotiriou,Matt Visser
Physics , 2010, DOI: 10.1088/1742-6596/222/1/012054
Abstract: Approximately one year ago Horava proposed a power-counting renormalizable theory of gravity which abandons local Lorentz invariance. The proposal has been received with growing interest and resulted in various different versions of Horava-Lifshitz gravity theories, involving a colourful potpourri of new terminology. In this proceedings contribution we first motivate and briefly overview the various different approaches, clarifying their differences and similarities. We then focus on a model referred to as projectable Horava-Lifshitz gravity and summarize the key results regarding its viability.
Phenomenologically viable Lorentz-violating quantum gravity
Thomas P. Sotiriou,Matt Visser,Silke Weinfurtner
Physics , 2009, DOI: 10.1103/PhysRevLett.102.251601
Abstract: Horava's "Lifschitz point gravity" has many desirable features, but in its original incarnation one is forced to accept a non-zero cosmological constant of the wrong sign to be compatible with observation. We develop an extension of Horava's model that abandons "detailed balance", and in 3+1 dimensions exhibit all five marginal (renormalizable) and four relevant (super-renormalizable) operators, as determined by power counting. We also consider the classical limit of this theory, evaluate the Hamiltonian and super-momentum constraints, and extract the classical equations of motion in a form similar to the ADM formulation of general relativity. This puts the model in a framework amenable to developing detailed precision tests.
Spectral Properties of Correlated Materials: Local Vertex and Non-Local Two-Particle Correlations from Combined GW and Dynamical Mean Field Theory
Thomas Ayral,Philipp Werner,Silke Biermann
Physics , 2012, DOI: 10.1103/PhysRevLett.109.226401
Abstract: We present a fully self-consistent combined GW and dynamical mean field (GW+DMFT) study of the spectral properties of the extended two-dimensional Hubbard model. The inclusion of the local dynamical vertex stemming from the DMFT self-energy and polarization is shown to cure the problems of self-consistent GW in the description of spectral properties. We calculate the momentum-resolved spectral functions, the two-particle polarization and electron loss spectra, and show that the inclusion of GW in extended DMFT leads to a narrowing of the quasi-particle width and more pronounced Hubbard bands in the metallic regime as one approaches the charge-ordering transition. Finally, the momentum-dependence introduced by GW into the extended DMFT description of collective modes is found to affect their shape, giving rise to dispersive plasmon-like long-wavelength and stripe modes.
From dispersion relations to spectral dimension - and back again
Thomas P. Sotiriou,Matt Visser,Silke Weinfurtner
Physics , 2011, DOI: 10.1103/PhysRevD.84.104018
Abstract: The so-called spectral dimension is a scale-dependent number associated with both geometries and field theories that has recently attracted much attention, driven largely though not exclusively by investigations of causal dynamical triangulations (CDT) and Horava gravity as possible candidates for quantum gravity. We advocate the use of the spectral dimension as a probe for the kinematics of these (and other) systems in the region where spacetime curvature is small, and the manifold is flat to a good approximation. In particular, we show how to assign a spectral dimension (as a function of so-called diffusion time) to any arbitrarily specified dispersion relation. We also analyze the fundamental properties of spectral dimension using extensions of the usual Seeley-DeWitt and Feynman expansions, and by saddle point techniques. The spectral dimension turns out to be a useful, robust and powerful probe, not only of geometry, but also of kinematics.
Lower-dimensional Horava-Lifshitz gravity
Thomas P. Sotiriou,Matt Visser,Silke Weinfurtner
Physics , 2011, DOI: 10.1103/PhysRevD.83.124021
Abstract: We consider Horava-Lifshitz gravity in both 1+1 and 2+1 dimensions. These lower-dimensional versions of Horava-Lifshitz gravity are simple enough to be explicitly tractable, but still complex enough to be interesting. We write the most general (non-projectable) action for each case and discuss the resulting dynamics. In the 1+1 case we utilize the equivalence with 2-dimensional Einstein-aether theory to argue that, even though non-trivial, the theory does not have any local degrees of freedom. In the 2+1 case we show that the only dynamical degree of freedom is a scalar, which qualitatively has the same dynamical behaviour as the scalar mode in (non-projectable) Horava-Lifshitz gravity in 3+1 dimensions. We discuss the suitability of these lower-dimensional theories as simpler playgrounds that could help us gain insight into the 3+1 theory. As special cases we also discuss the projectable limit of these theories. Finally, we present an algorithm that extends the equivalence with (higher order) Einstein-aether theory to full Horava-Lifshitz gravity (instead of just the low energy limit), and we use this extension to comment on the apparent naturalness of the covariant formulation of the latter.
Spectral dimension as a probe of the ultraviolet continuum regime of causal dynamical triangulations
Thomas P. Sotiriou,Matt Visser,Silke Weinfurtner
Physics , 2011, DOI: 10.1103/PhysRevLett.107.131303
Abstract: We explore the ultraviolet continuum regime of causal dynamical triangulations, as probed by the flow of the spectral dimension. We set up a framework in which one can find continuum theories that can in principle fully reproduce the behaviour of the latter in this regime. In particular, we show that, in 2+1 dimensions, Hovrava-Lifshitz gravity can mimic the flow of the spectral dimension in causal dynamical triangulations to high accuracy and over a wide range of scales. This seems to provide evidence for an important connection between the two theories.
Comment on: Detecting Vanishing Dimensions Via Primordial Gravitational Wave Astronomy
Thomas P. Sotiriou,Matt Visser,Silke Weinfurtner
Physics , 2011, DOI: 10.1103/PhysRevLett.107.169001
Abstract: It has been recently claimed [arXiv:1102.3434] that quantum gravity models where the number of dimensions reduces at the ultraviolet exhibit a potentially observable cutoff in the primordial gravitational wave spectrum, and that this is a "generic" and "robust" test for such models, since "(2+1)-dimensional spacetimes have no gravitational degrees of freedom". We argue that such a claim is misleading.
Screening and Non-local Correlations in the Extended Hubbard Model from Self-Consistent Combined GW and Dynamical Mean Field Theory
Thomas Ayral,Silke Biermann,Philipp Werner
Physics , 2012, DOI: 10.1103/PhysRevB.87.125149
Abstract: We describe a recent implementation of the combined GW and dynamical mean field (DMFT) method "GW+DMFT" for the two-dimensional Hubbard model with on-site and nearest-neighbor repulsion. We clarify the relation of the GW+DMFT scheme to alternative approaches in the literature, and discuss the corresponding approximations to the free energy functional of the model. Furthermore, we describe a numerically exact technique for the self-consistent solution of the GW+DMFT equations, namely the hybridization expansion continuous-time algorithm for solving the dynamical impurity model that arises within the GW+DMFT scheme. We compute the low-temperature phase diagram of the extended Hubbard model, addressing the metal-insulator transition at small intersite interactions and the transition to a charge-ordered state for stronger intersite repulsions. Within the GW+DMFT framework, as in extended DMFT, the intersite repulsion translates into a frequency-dependence of the local effective interaction. We analyze this dependence by extracting a charateristic plasma frequency and show how it affects the local spectral function.
Ranking hospitals for outcomes in total hip replacement - administrative data with or without additional patient surveys? - Part 1: Administrative data
Sch?fer, Thomas,Neusser, Silke,Lorenz, Christoph,D?rning, Hans
GMS Medizinische Informatik, Biometrie und Epidemiologie , 2007,
Abstract: Background: Many hospital rankings rely on the frequency of adverse outcomes and are based on administrative data. In the study presented here, we tried to find out, to what extent available administrative data of German Sickness Funds allow for an adequate hospital ranking and compared this with rankings based on additional information derived from a patient survey. Total hip replacement was chosen as an example procedure. In part I of the publication, we present the results of the approach based on administrative data. Methods: We used administrative data from the AOK-Lower Saxony of the years 2000, 2001 and 2002. The study population comprised all beneficiaries, who received total hip replacement in the years 2000 or 2001. Performance indicators used where “critical incident (Mortality or revision)” and “number of revisions” within the first year. Hospitals were ranked if they performed at least 20 procedures on AOK-beneficiaries in each of the two years. Multivariate modelling (logistic and poisson regression) was used to estimate the performance indicators by case-mix variables (age, sex, co-diagnoses) and hospital characteristics (hospital size, surgical volume). The actual ranking was based on these multivariate models, excluding hospital variables and adding dummy-variables for each hospital. Hospitals were ranked by their case-mix adjusted odds ratio or SMR respectively with respect to a pre-selected reference hospital. The resulting rankings were compared with each other, with regard to temporal stability, and the impact of case-mix variables.Results: About 4500 beneficiaries received total hip replacement in each year (n2000: 4482; n2001: 4579). The ranking included 65 hospitals. Comparing the years 2000 and 2001, the temporal stability of the rankings based on a single performance indicator was low (Spearman rang correlation coefficients 0.158 and 0.191). The agreement of rankings based on different performance indicators in the same year was high (Spearman: 0.80 and 0.85). Including case-mix variables improved the model fit remarkably. Odds ratios for hospitals varied from 0.0 to 10.0 (critical incident) and SMRs from 0.0 to 6.1 (number of revisions). Conclusions: Using data of two adjacent years together improves the reliability of hospital rankings. Adding the administrative data derived patient variables improves the explanation of the performance indicators. Whether this is sufficient to account for case-mix can not be determined at this point. If the case-mix was addressed properly, the rankings showed large differences in the quality
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