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231	Rôle de l'isospin dans la transition de phase liquide-gaz de la matière nucléaire Ducoin, C. 03 October 2006 (has links) (PDF) La matière nucléaire présente une transition de phase du type liquide-gaz. Ce caractère bien connu est dû au profil de l'interaction nucléaire (attractive à moyenne portée et répulsive à courte portée). Ainsi, la thermodynamique de la matière nucléaire symétrique est analogue à celle d'un fluide de Van der Waals. L'étude se révèle plus complexe dans le cas de la matière asymétrique, composée de neutrons et protons en proportion arbitraire. L'isospin, qui distingue les deux constituants, donne une mesure de cette proportion. Dans l'étude de la matière asymétrique, il s'agit d'un degré de liberté supplémentaire, ajoutant une dimension à l'espace des observables à considérer. <br />La transition liquide-gaz nucléaire est associée au phénomène de multi-fragmentation observé dans les collisions d'ions lourds, ainsi qu'à la physique des étoiles compactes : les systèmes concernés sont riches en neutrons, donc affectés par le degré de liberté d'isospin. <br />Le travail présenté ici est une étude théorique des effets d'isospin apparaissant dans la transition liquide-gaz de la matière nucléaire asymétrique. Une approche de champ moyen est employée, avec une interaction nucléaire effective de type Skyrme. Nous démontrons la présence d'une transition du premier ordre pour la matière asymétrique, et étudions le phénomène de distillation d'isospin qui l'accompagne. Le cas d'une séparation de phase à l'équilibre thermodynamique est comparé à celui d'une décomposition spinodale. Les effets de taille finie sont abordés, ainsi que l'influence du gaz d'électrons présent dans le contexte astrophysique. [PHYS:NUCL] Physics/Nuclear Theory Matière nucléaire Physique statistique Transitions de phase Equilibre chimique Interactions d'ions lours Hartree-Fock Méthode d'approximation Etoiles à neutrons Supernovae
232	Computational Bayesian techniques applied to cosmology Hee, Sonke January 2018 (has links) This thesis presents work around 3 themes: dark energy, gravitational waves and Bayesian inference. Both dark energy and gravitational wave physics are not yet well constrained. They present interesting challenges for Bayesian inference, which attempts to quantify our knowledge of the universe given our astrophysical data. A dark energy equation of state reconstruction analysis finds that the data favours the vacuum dark energy equation of state $w {=} -1$ model. Deviations from vacuum dark energy are shown to favour the super-negative ‘phantom’ dark energy regime of $w {< } -1$, but at low statistical significance. The constraining power of various datasets is quantified, finding that data constraints peak around redshift $z = 0.2$ due to baryonic acoustic oscillation and supernovae data constraints, whilst cosmic microwave background radiation and Lyman-$\alpha$ forest constraints are less significant. Specific models with a conformal time symmetry in the Friedmann equation and with an additional dark energy component are tested and shown to be competitive to the vacuum dark energy model by Bayesian model selection analysis: that they are not ruled out is believed to be largely due to poor data quality for deciding between existing models. Recent detections of gravitational waves by the LIGO collaboration enable the first gravitational wave tests of general relativity. An existing test in the literature is used and sped up significantly by a novel method developed in this thesis. The test computes posterior odds ratios, and the new method is shown to compute these accurately and efficiently. Compared to computing evidences, the method presented provides an approximate 100 times reduction in the number of likelihood calculations required to compute evidences at a given accuracy. Further testing may identify a significant advance in Bayesian model selection using nested sampling, as the method is completely general and straightforward to implement. We note that efficiency gains are not guaranteed and may be problem specific: further research is needed.
233	Implementation and Analysis of Co-Located Virtual Reality for Scientific Data Visualization Jordan M McGraw (8803076) 07 May 2020 (has links) <div>Advancements in virtual reality (VR) technologies have led to overwhelming critique and acclaim in recent years. Academic researchers have already begun to take advantage of these immersive technologies across all manner of settings. Using immersive technologies, educators are able to more easily interpret complex information with students and colleagues. Despite the advantages these technologies bring, some drawbacks still remain. One particular drawback is the difficulty of engaging in immersive environments with others in a shared physical space (i.e., with a shared virtual environment). A common strategy for improving collaborative data exploration has been to use technological substitutions to make distant users feel they are collaborating in the same space. This research, however, is focused on how virtual reality can be used to build upon real-world interactions which take place in the same physical space (i.e., collaborative, co-located, multi-user virtual reality).</div><div><br></div><div>In this study we address two primary dimensions of collaborative data visualization and analysis as follows: [1] we detail the implementation of a novel co-located VR hardware and software system, [2] we conduct a formal user experience study of the novel system using the NASA Task Load Index (Hart, 1986) and introduce the Modified User Experience Inventory, a new user study inventory based upon the Unified User Experience Inventory, (Tcha-Tokey, Christmann, Loup-Escande, Richir, 2016) to empirically observe the dependent measures of Workload, Presence, Engagement, Consequence, and Immersion. A total of 77 participants volunteered to join a demonstration of this technology at Purdue University. In groups ranging from two to four, participants shared a co-located virtual environment built to visualize point cloud measurements of exploded supernovae. This study is not experimental but observational. We found there to be moderately high levels of user experience and moderate levels of workload demand in our results. We describe the implementation of the software platform and present user reactions to the technology that was created. These are described in detail within this manuscript.</div> Software Engineering Computer Software Computer Graphics Simulation and Modelling Virtual Reality and Related Simulation Mobile Technologies Networking and Communications Computer-Human Interaction Collaborative Virtual Reality Virtual Reality Collaborative Virtual Reality Networking Networked VR Mulitplayer Multiplayer VR Data Visualization Data Exploration Head Mounted Display Cyberspace Augmented Reality Emerging Technologies Consumer Devices Virtual Environment Virtual Aesthetics Collaborative VR Simulation Training Simulation and Training Colocated Colocated Virtual Environment Immersive Media Immersive Technology Collaborative Tools Educational Virtual Environment Point Cloud Representations Isosurface Representations Mesh Smoothng Supernova Supernovae Supernova Remnant Modeling Astronomy and Astrophysics Cassiopeia N132D E0102 Crab Nebula CAVE CAVE Theater Mobile VR Virtual Classrooms STEM Workload Workload Assessment Tool Presence Engagement Immersion Experience Consequence Simulator sickness Simulator Sickness Questionnaire NASA TLX NASA Task Load Index Unified User Experience Quesionnaire Modified User Experience Questionnaire Oculus Oculus Go Oculus Quest Novel Solutions Optitrack Motive Motion Capture Mocap User Experience Unity3D Game Engine Lidgren Room Scale VR Hall Scale VR Co-location Colocation Virtual Training Collaborative Virtual Environments Shared Virtual Environments Collaboration Whiteboard Mobile Virtual Reality VR HMD LAN UX CSCL UXQ TLX AR VE CVE 3D UI EVE

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