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SUMMARY:Ludovic Ducobu (Transilvania University of Brasov\, Romania)
DESCRIPTION:Title: Compact objects and scalar fields in theories of gravity with torsion\, ep. 1 : modelisation \nAbstract:  \nGeneral Relativity [GR] offers an extremely successful framework to describe the gravitational interaction. That being said\, the necessity to question the framework of GR is clear at both the experimental (origin and composition of dark matter and dark energy) and theoretical (search for quantum gravity) levels.\nSince not all these puzzles can be purely reduced to quantum correction problems\, this motivates the study of alternative theories of gravitation already at the classical level.\n\nTo this purpose\, one can follow two distinct but complementary routes. On the one hand\, one can try to modify Einstein’s equations by introducing new fields while maintaining the assumptions related to the geometrical structure of spacetime. On the other hand\, one can precisely question the framework of pseudo-Riemannian geometry as being adapted to the full description of spacetime (typically\, at cosmological scales).\nIn the first case\, scalar-tensor theories of gravity (and especially Horndeski gravity)\, where the new degrees of freedom are encoded by means of a scalar field\, have been extensively investigated in the litterature.\nIn the later case\, a minimal generalisation of GR’s framework consist in relaxing the hypothesis linking the connection to the metric\, leading to the framework of metric-affine gravity. This reveals new starting points to generalise GR through\, for instance\, an examination of the so-called geometrical trinity of gravity.\n\nThe aim of this talk is to present the main motivations and features of modified theories of gravity involving scalar fields in both the usual (GR) and metric-affine approaches.\n\nI will first propose a quick overview of Horndeski gravity and of the metric-affine approach of GR; focussing on the so-called Teleparallel theories of gravity and on the Teleparallel equivalent to GR.\nThen\, “unifying” the two approaches\, I will advocate for the interest of modified theories of gravity involving scalar fields in the metric-affine framework.\nI will then present recent results from my own research and discuss the first study of assymptotically flat black holes endowed with scalar hair in a teleparallel generalisation of Horndeski gravity.\nOur study provides an explicit demonstration of how scalar-torsion gravity (and possibly other non-Riemaniann models) provides a richer canvas for the study of phenomenology beyond GR and open the way for novel studies regarding scalarized black hole solutions in non-Riemannian theories of gravity.\n\nThis presentation is based on the work done in [arXiv:2201.11445 [gr-qc]] (https://arxiv.org/abs/2201.11445) and [arXiv:2212.07653 [gr-qc]] (https://arxiv.org/abs/2212.07653) in collaboration with Sebastian Bahamonde\, Daniela Doneva\, Christian Pfeifer and Stoytcho Yazadjiev.\n\n\n\nThe seminar will take place in Room S08 at the Faculty of Sciences.
URL:https://www.naxys.be/event/ludovic-ducobu/
CATEGORIES:NAXYS Seminar
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DTSTART;TZID=Europe/Brussels:20230630T130000
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DTSTAMP:20260502T025721
CREATED:20230602T080740Z
LAST-MODIFIED:20230602T080740Z
UID:2011-1688130000-1688133600@www.naxys.be
SUMMARY:Enoch Yeung (UC Santa Barbara (UCSB)\, USA)
DESCRIPTION:Title: Data-Driven State Criticality and Observability with Koopman\nOperator Methods \nAbstract: I will present two major results to show the use of data-\ndriven Koopman methods to identify critical states and observable\nsubspaces to solve problems in synthetic biology. In the first\, I\npresent the use of dynamic mode decomposition (DMD) to model the\ntranscriptome-wide response of a root-isolate bacterium to a novel\nchemical compound. By solving an observability maximization problem\nfor the DMD model\, we find a panel of biomarkers that can act as\neffective biosensors for the compound\, even in field studies with the\nbacterium. This study establishes a new precedent for reasoning about\nstate criticality and system observability\, even without prior\nknowledge of a network model. Second\, I present new theoretical\nresults that show how Koopman methods can be used to evaluate\ncriticality of states to optimize performance of a nonlinear system.\nHistorically\, this problem is solved using either direct sensitivity\nanalysis on a known model or by generating local function\ndistributions that span the nonlinear observable subspace of a system.\nIn the absence of a known model\, I present a new Koopman-based method\nfor estimating the observable subspace of a nonlinear system purely\nfrom data. Our results provide a route for data-driven discovery of\ncritical states that affect an output-based performance measure. \nThe seminar will take place in Room S08 at the Faculty of Sciences.
URL:https://www.naxys.be/event/enoch-yeung-uc-santa-barbara-ucsb-usa/
CATEGORIES:NAXYS Seminar
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