Structure formation | EPFL Graph Search

In physical cosmology, structure formation is the formation of galaxies, galaxy clusters and larger structures from small early density fluctuations. The universe, as is now known from observations of the cosmic microwave background radiation, began in a hot, dense, nearly uniform state approximately 13.8 billion years ago. However, looking at the night sky today, structures on all scales can be seen, from stars and planets to galaxies. On even larger scales, galaxy clusters and sheet-like structures of galaxies are separated by enormous voids containing few galaxies. Structure formation attempts to model how these structures were formed by gravitational instability of small early ripples in spacetime density or another emergence. The modern Lambda-CDM model is successful at predicting the observed large-scale distribution of galaxies, clusters and voids; but on the scale of individual galaxies there are many complications due to highly nonlinear processes involving baryonic physics

Centre d'accueil et de réinsertion pour les marginaux et nécessiteux de Lausanne

En se plaçant à la rue de Genève, au cœur du quartier chaud de Lausanne, ce travail propose une réflexion sur le thème de l'accueil des personnes en marge de notre société. Il s'agit ici de s'intéresser à une question grandissante de notre époque mais pour laquelle il n'existe pas de réponses dont on puisse dire qu'elles sont des règles absolues. Certes, quelques organismes privés ont déjà tenté de mettre en place des structures d'accueil, de traitement et d'orientation professionnelle, mais cela reste trop ponctuel à la vue des besoins grandissants. Le programme proposé est organisé sous la forme d'une prise en charge du marginal en trois étapes: une unité d'accueil ponctuel de jour proposant des services sanitaires et médicaux, un sleep-in pour passer la nuit à l'abri, et une unité de réinsertion composée de logements et de structures de formation et d'orientation professionnelle. Appliquées successivement, ces trois phases constituent un véritable processus qui serait un moyen de réinsérer l'individu dans un rythme de vie commun à la majorité de la population. Davantage que les questions organisationnelles du projet, celle de la limite entre les différentes sphères de la privacité reste au centre des enjeux. Comment intégrer sans exposer? Comment protéger sans emprisonner, sans créer de nouveaux ghettos?

2003

A Framework for Numerical Simulations of Structure Formation

Claude Becker

In the absence of a full analytical treatment of nonlinear structure formation in the universe, numerical simulations provide the critical link between the properties of the underlying model and the features of the observed structures. Currently N-body simulations are the main tool to study structure growth. We explore an alternative framework for numerical simulations of structure formation. The underlying idea is to replace the long-range gravitational force in the Vlasov-Poisson system by a purely local interaction. To this end we trade the classical phase space distribution for its quantum mechanical counterpart, the Wigner distribution function. Its dynamical equation is equivalent to the Schrödinger equation and reduces to the Vlasov equation in the formal semi-classical limit. The proposed framework allows in principle to simulate systems with arbitrary phase space distributions and could for instance be beneficial for simulations of warm dark matter, where the velocity dispersion is important. We discuss several methods to obtain a set of wavefunctions whose Wigner distribution is close to a given initial phase space distribution function. An auxiliary gauge field is introduced to mediate the gravitational interaction, thereby obtaining a local Schrödinger-Maxwell system. We also use the ideas of lattice gauge theories to obtain a fully gauge-invariant discretization of the equations of motion. Their iterative solution was implemented in a three-dimensional simulation code. We discuss its computational complexity and memory requirements. Several testbed simulations were performed with this method. We compared the gravitational collapse of a Gaussian wavefunction with an independent numerical solution of the spherically symmetric Schrödinger-Newton system. The results were found to be in good agreement. Finally, a simple example of the growth of cosmic perturbations is investigated within our framework. We conclude by outlining various possible directions to optimize and develop our method.

EPFL2011

A new framework for numerical simulations of structure formation

Claude Becker, Alexey Boyarsky, Oleg Ruchayskiy, Matthieu Schaller

The diversity of structures in the Universe (from the smallest galaxies to the largest superclusters) has formed under the pull of gravity from the tiny primordial perturbations that we see imprinted in the cosmic microwave background. A quantitative description of this process would require description of motion of zillions of dark matter particles. This impossible task is usually circumvented by coarse graining the problem: one either considers a Newtonian dynamics of 'particles' with macroscopically large masses or approximates the dark matter distribution with a continuous density field. There is no closed system of equations for the evolution of the matter density field alone and instead it should still be discretized at each time step. In this work, we describe a method of solving the full six-dimensional Vlasov-Poisson equation via a system of auxiliary Schrodinger-like equations. The complexity of the problem gets shifted into the choice of the number and shape of the initial wavefunctions that should only be specified at the beginning of the computation (we stress that these wavefunctions have nothing to do with quantum nature of the actual dark matter particles). We discuss different prescriptions to generate the initial wavefunctions from the initial conditions and demonstrate the validity of the technique on two simple test cases. This new simulation algorithm can in principle be used on an arbitrary distribution function, enabling the simulation of warm and hot dark matter structure formation scenarios.

Oxford Univ Press2014

A Framework for Numerical Simulations of Structure Formation

Claude Becker

EPFL2011