BMS Symmetries of Gravitational Scattering

After motivating the relevance of the Bondi-Metzner-Sachs (BMS) group over the last decades, we review how concepts such as Penrose diagrams and the covariant phase space formalism can be used to understand the asymptotic structure of asymptotically flat spacetimes (AFS). We then explicitly construct the asymptotic symmetry group of AFS in $3+1$ dimensions, the BMS group. Next, we apply this knowledge to the usual far-field scattering problem in general relativity, which leads to the unravelling of the intrinsic features of gravity in the infrared. In particular, we work out the connections between asymptotic symmetries, soft theorems in quantum field theories and gravitational memory effects. We restrict to the study of this infrared triangle through the lens of supertranslations here, but the analogous features that can be found in the case of superrotations or for other gauge theories are also motivated at the end of our discussion. We conclude with an overview of the implications of the infrared triangle of gravity for the formulation of an approach to quantum gravity through holography, as well as a brief discussion of its potential in tackling the black hole information paradox.Comment: 63 pages, 9 figures. Essay submitted for the partial fulfillment of the requirements for the degree of Master of Advanced Study in Applied Mathematics at the University of Cambridge, aimed at students willing to learn about the role of asymptotic symmetries in the context of flat space holography, with only basic knowledge of quantum field theory and general relativity assume

Characterizing analogue caldera collapse with computerized X-ray tomography

Analogue models of caldera collapse were imaged by computerized X-ray tomography (μCT). Interval μCT radiography sequences document ‘2.5D’ surface and internal model deformation in an unprecedented way, and carry the potential for a better understanding of the kinematics of various volcano-tectonic processes, of which caldera collapse is a mere illustration. A semi-automatic subsidence velocity analysis was carried out on radiographs. The developed method is a step towards the quantitative documentation of volcano-tectonic modelling that would render data interpretations immediately comparable to monitoring data available from recent deformation at natural volcanoes

Monitoring and modelling volcanoes with assessment of their hazards by means of remote sensing and analogue modelling

Many active volcanoes in developing countries are poorly-known and not monitored. This thesis investigates low cost solutions to map the topography, to identify hazards and to document the eruptions at volcanoes with satellite data. Using a combination of remote sensing techniques and analogue modelling, this thesis also contributes to the understanding of volcanic processes such as the controls upon the 3D shape of sub-volcanic intrusive systems, upon the location of eruption outbreaks, upon the variations in eruption intensity through time and upon the transition between contrasted eruptive styles at a single volcano. After reviewing previous applications of low cost remote sensing in volcanology, the accuracy of two topographic datasets derived from contrasted remote sensing data (ASTER and SRTM) is assessed for volcanic terrains. Oldoinyo Lengai, a natrocarbonatite stratovolcano in Tanzania, is used as an illustrative example of poorly-known volcanoes whose hazards need to be assessed and whose eruptive activity has to be monitored. Satellite images enable mapping, constraining volumes and characterizing surface features of three flank collapses and their associated deposits. An existing numerical model is applied to constrain the emplacement dynamics and the velocity of one of those debris avalanche flows. An algorithm is then presented to retrieve daily information about eruptive activity and its variation over an 8-year period using nighttime MODIS satellite data. Analysis of this time series enable to highlight the control of Earth tides on the timing of high intensity eruptions. The same algorithm, combined with field data and petrologic analyses, is used to document a voluminous lava flow eruption that occurred at Oldoinyo Lengai at the end of March 2006, providing insights into the structure of the shallow plumbing system of the volcano. Satellite data are finally combined with laboratory experiments simulating magma propagation in the Earth crust with sand and syrup or gelatin and water, to provide a better understanding of the control exerted by volcanic edifice load upon magma ascent. These experiments also enable to explain the links between magma ascent, volcano load, sub-volcanic intrusions, volcano surface deformation and location of volcanic vents at the base of large volcanoes