82 research outputs found
Curvature invariants, geodesics, and the strength of singularities in Bianchi-I loop quantum cosmology
We investigate the effects of the underlying quantum geometry in loop quantum cosmology on spacetime curvature invariants and the extendibility of geodesics in the Bianchi-I model for matter with a vanishing anisotropic stress. Using the effective Hamiltonian approach, we find that even though quantum geometric effects bound the energy density and expansion and shear scalars, divergences of curvature invariants are potentially possible under special conditions. However, as in the isotropic models in LQC, these do not necessarily imply a physical singularity. Analysis of geodesics and strength of such singular events, point towards a general resolution of all known types of strong singularities. We illustrate these results for the case of a perfect fluid with an arbitrary finite equation of state w\u3e-1, and show that curvature invariants turn out to be bounded, leading to the absence of strong singularities. Unlike classical theory, geodesic evolution does not break down. We also discuss possible generalizations of sudden singularities which may arise at a nonvanishing volume, causing a divergence in curvature invariants. Such finite volume singularities are shown to be weak and harmless. © 2012 American Physical Society
Towards Molecular Simulations that are Transparent, Reproducible, Usable By Others, and Extensible (TRUE)
Systems composed of soft matter (e.g., liquids, polymers, foams, gels,
colloids, and most biological materials) are ubiquitous in science and
engineering, but molecular simulations of such systems pose particular
computational challenges, requiring time and/or ensemble-averaged data to be
collected over long simulation trajectories for property evaluation. Performing
a molecular simulation of a soft matter system involves multiple steps, which
have traditionally been performed by researchers in a "bespoke" fashion,
resulting in many published soft matter simulations not being reproducible
based on the information provided in the publications. To address the issue of
reproducibility and to provide tools for computational screening, we have been
developing the open-source Molecular Simulation and Design Framework (MoSDeF)
software suite. In this paper, we propose a set of principles to create
Transparent, Reproducible, Usable by others, and Extensible (TRUE) molecular
simulations. MoSDeF facilitates the publication and dissemination of TRUE
simulations by automating many of the critical steps in molecular simulation,
thus enhancing their reproducibility. We provide several examples of TRUE
molecular simulations: All of the steps involved in creating, running and
extracting properties from the simulations are distributed on open-source
platforms (within MoSDeF and on GitHub), thus meeting the definition of TRUE
simulations
Continuum thermodynamics of chemically reacting fluid mixtures
We consider viscous and heat conducting mixtures of molecularly
miscible chemical species forming a fluid in which the constituents can
undergo chemical reactions. Assuming a common temperature for all components,
a first main aim is the derivation of a closed system of partial mass and
partial momentum balances plus a common balance of internal energy. This is
achieved by careful exploitation of the entropy principle which, in
particular, requires appropriate definitions of absolute temperature and
chemical potentials based on an adequate definition of thermal energy that
excludes diffusive contributions. The latter is crucial in order to obtain a
closure framework for the interaction forces between the different species.
The interaction forces split into a thermo-mechanical and a chemical part,
where the former turns out to be symmetric if binary interactions are
assumed. In the non-reactive case, this leads to a system of Navier-Stokes
type sub-systems, coupled by interspecies friction forces. For chemically
reacting systems and as a new result, the chemical interaction force is
identified as a contribution which is non-symmetric, unless chemical
equilibrium holds. The theory also provides a rigorous derivation of the
so-called generalized thermodynamic driving forces, avoiding the use of
approximate solutions to the Boltzmann equations which is common in the
engineering literature. Moreover, starting with a continuum thermodynamic
field theory right away, local versions of fundamental relations known from
thermodynamics of homogeneous systems, like the Gibbs-Duhem equation, are
derived. Furthermore, using an appropriately extended version of the entropy
principle and introducing cross-effects already before closure as entropy
invariant couplings between principal dissipative mechanisms, the Onsager
symmetry relations are a strict consequence. With a classification of the
factors forming the binary products in the entropy production according to
their parity instead of the classical distinction between so-called fluxes
and driving forces, the apparent anti-symmetry of certain couplings is
thereby also revealed. If the diffusion velocities are small compared to the
speed of sound, the well-known Maxwell-Stefan equations together with the
so-called generalized thermodynamic driving forces follow in the special case
without chemical reactions, thereby neglecting wave phenomena in the
diffusive motion. This results in a reduced model having only the
constituents mass balances individually. In the reactive case, this
approximation via a scale separation argument is no longer possible. Instead,
we first employ the partial mass and mixture internal energy balances, common
to both model classes, to identify all constitutive quantities. Combined with
the concept of entropy invariant model reduction, leaving the entropy
production unchanged under the reduction from partial momentum balances to a
single common mixture momentum balance, the chemical interactions yield an
additional contribution to the transport coefficients, leading to an
extension of the Maxwell-Stefan equations to chemically active mixtures.
Within the considered model class for reactive fluid mixtures the new results
are achieved for arbitrary free energy functions
Model-Based Run-time Verification of Software Components by Integrating OCL into Treaty
Model Driven Development is used to improve software quality and efficiency by automatically transforming abstract and formal models into software implementations. This is particularly sensible if the model’s integrity can be proven formally and is preserved during the model’s transformation.
A standard to specify software model integrity is the Object Constraint Language (OCL). Another topic of research is the dynamic development of software components, enabling software system composition at component run-time. As a consequence, the system’s verification must be realized during system run-time (and not during transformation or compile time). Many established verification techniques cannot be used for run-time verification.
A method to enable model-based run-time verification will be developed during this work. How OCL constraints can be transformed into executable software artifacts and how they can be used in the component-based system Treaty will be the major task of this diploma thesis.Modellgetriebene Entwicklung dient der Verbesserung von Qualität und Effizienz in der Software-Entwicklung durch Automatisierung der notwendigen Transformationen von abstrakten bzw. formalen Modellen bis zur Implementierung. Dies ist insbesondere dann sinnvoll, wenn die Integrität der ursprünglichen Modelle formal bewiesen werden kann und durch die Transformation gewährleistet wird. Ein Standard zur Spezifikation der Integrität von Softwaremodellen ist die Object Constraint Language (OCL). Eine weitere Forschungsrichtung im Software-Engineering ist die Entwicklung von dynamischen Komponenten-Modellen, die die Komposition von Softwaresystemen im laufenden Betrieb ermöglichen. Dies bedeutet, dass die Systemverifikation im laufenden Betrieb realisiert werden muss. Die meisten der etablierten Verifikationstechniken sind dazu nicht geeignet.
In der Diplomarbeit soll ausgehend von diesem Stand der Technik eine Methode zur modellbasierten Verifikation zur Laufzeit entwickelt werden. Insbesondere soll untersucht werden, wie OCL-Constraints zur Laufzeit in ausführbare Software-Artefakte übersetzt und in dem komponentenbasierten System Treaty verwendet werden können
Hamiltonian Theory: Dynamics
This chapter focuses on the status of the implementation of the dynamics in
the canonical version of Loop Quantum Gravity (LQG). Concretely this means to
provide a mathematical meaning of the quantum Einstein equations, sometimes
called Wheeler-DeWitt equations, to give a physical interpretation and Hilbert
space structure to its solutions and to construct a representation of the
algebra of observables including a physical Hamiltonian. This is a structural
overview intentionally skipping technical details.Comment: 52 pages. This is a preprint of a chapter to appear in the "Handbook
of Quantum Gravity", edited by Cosimo Bambi, Leonardo Modesto and Ilya
Shapiro, 2023, Springer, reproduced with permission of Springe
Students’ Perspectives Of Simon Says Game To Practice Listening Comprehension At Grade Eight Students Of MTsS Madinatussalam in 2020/2021 Academic Year
This research was aimed to find out students’ perspective of Simon Says
game for practicing their listening class. The subject of this research was grade
VIII-3 students at MTsS Madinatussalam in 2020 academic year. This research of
this study was conducted by using qualitative research. The data used in this
research such as; interview sheet and photography evidence. The researcher did
interview for some students at grade VIII-3 in MTsS Madinatussalam. The
researcher gives six questions too in interview session to students that still related
to students’ perspectives of Simon Says game for practicing their listening. The
result of this research showed that almost all students have positive perspectives
in Simon says game for practicing their listening. They agree that Simon Says
game makes them easier to practice their listening in a fun way. They also agree
that Simon says game can make their class atmospheres’ be active. They said they
are being attractive while playing this game for listening practice class
Classical and quantum perturbations to the primordial universe
In this Ph.D. thesis we analyse both classical and quantum effects relevant for the study of cosmological perturbations. We choose this particular topic because, through the analysis of cosmological perturbations, it is possible to explore a wide range of different physical phenomena. Moreover, they are a central and important piece in the puzzle of the history of the universe.
The most obvious relevance of cosmological perturbations is the study of structure formation and the large scale structure of the universe. In this regard, such perturbations are related to primordial gravitational waves and primordial magnetic fields. Given their dependence on pre-recombination phenomena, they could give us some information on the universe before hydrogen recombination.
Classical perturbations have been widely studied in literature, with the main focus on isotropic cosmological models. While this is usually a good approximation, the presence of a primordial magnetic field causes a coupling between different algebraic modes of the usual decomposition, connecting density perturbations, primordial magnetic fields and primordial gravitational waves. Moreover, the presence of the magnetic field requires the use of an anisotropic cosmological model. While small, these relations are important in the evolution of anisotropic structures. Furthermore, such primordial seeds of the magnetic fields are widely believed to be the origin of the magnetic fields measured today in galaxies. In the first part of this thesis, we analyse these relations, together with the possible effects that a non ideal, i.e. viscous, cosmological fluid could have on the growth of perturbations. We focus our attention to a Bianchi I model, improving the results of some preceding papers.
The second part of the thesis focuses on the semiclassical approximation of quantum gravity. Quantum effects are believed to influence the birth and dynamics of perturbation seeds and, in general, the dynamics of the primordial universe. This way, the mathematical scheme used to represent these effects is a central point in the description of quantum gravity regarding such seeds.
Furthermore, even more care is required to split the WKB action between embedding variables and physical degrees of freedom, and in many models the quantum gravity corrections to the Schrödinger equation violate the unitarity of the system evolution. This decomposition shares some similarities with the Born-Oppenheimer approximation of molecular physics.
We perform a critical analysis of two different ways to apply this decomposition. In particular, we analyse limits and perspectives of the different proposals to solve the non unitarity problem, even comparing expansions in different fundamental physical constants (Planck constant and mass). We find the source of non-unitary effects in a common assumption in the definition of WKB time, and we propose an alternative formulation. Also, we show how the usual assumptions of classicality of the physical quantities must be handled with care, focusing our attention to the implementation of the classical background in the perturbation scheme.
Studies in this research field are very important because they could bind CMB measurements and primordial gravitational waves to quantum gravity, bringing us finally an experimental playground
- …