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Growth data: null test for Lambda-Cold Dark Matter Imprimir
Domenico Sapone
Departamento de Física
FCFM, Universidad de Chile

Viernes 24 de octubre, 16:15hrs
Departamento de Física (DFI), F12
Av Blanco Encalada 2008.

Current and upcoming surveys will measure the cosmological parameters with an extremely high accuracy. The primary goal of these observations is to eliminate some of the currently viable cosmological models created to explain the late time accelerated expansion (either real or only inferred). However, most of the statistical tests used in cosmology have a strong requirement: the use of a model to fit the data. Recently there has been an increased interest on finding tests that are model independent, i.e. to have a function that depends entirely on observed quantities and not on the model. In this talk I will present an alternative consistency check at the perturbative level for a homogeneous and isotropic Universe filled with a dark energy component. This test makes use of the growth of matter perturbations data and it is able to not only test the homogeneous and isotropic Universe but also, within the framework of a Friedmann-Lemaître-Robertson-Walker Universe, if the dark energy component is able to cluster, if there is a tension in the data or if we are dealing with a modification of gravity. I will also show the reconstruction of the null test using recent and future data. 

Observación: Charla en Inglés.

Seminarios Anteriores

El LED azul, el Nobel y otros colores Imprimir
Profesor Víctor Fuenzalida
Departamento de Fisica
Universidad de Chile
Viernes 17 de octubre a las 16:00 
Audutorio de Geología (G-108)
Plaza Ercilla 803
Facultad de Ciencias Físicas y Matemáticas
Universidad de Chile
La semana pasada se otorgó el premio Nobel de Física a los investigadores japoneses que inventaron el LED azul. ¿Por qué el Nobel se otorgó a estos investigadores? ¿Por qué no al inventor del primer LED? ¿Qué tiene de importante el azul? Se presentará un breve resumen de la física de semiconductores, el funcionamiento del LED -de cualquier color- y el impacto del LED azul en las tecnologías actuales de despliegue digital e iluminación. 
Plasmas z-pinch: Dinámica de ablación y aplicaciones a astrofísica en el laboratorio Imprimir
Felipe Veloso
Instituto de Física
Pontificia Universidad Católica de Chile

Viernes 10 de octubre, 16:15hrs
Departamento de Física (DFI), Sala F12
Av Blanco Encalada 2008


El estudio de plasmas tipo z-pinch de arreglos de alambres ha sido un área activa de investigación en las últimas décadas con aplicaciones muy diversas que incluyen producción de rayos X, estudios de opacidad para modelos espectroscópicos, de flujos supersónicos para estudios astrófisicos en el laboratorio entre muchos otros. Los arreglos de alambres se forman cuando corrientes de cientos o miles de kiloamperes circulan a través de alambres de unos pocos micrones de diámetro. Estos plasmas interactúan con el campo magnético global de la configuración, cuya dinámica está gobernada por su topología que este campo presenta. En esta charla, se presentarán resultados obtenidos en el generador Llampudken (~350kA en ~350ns) donde se estudia la interacción de plasmas densos con diferentes topologías de campo magnético, junto con las dinámicas características de los procesos de ablación. Finalmente, se discutirá sobre la escalabilidad y similitudes en la emisión de jets de plasmas de estas configuraciones para su potencial interés en el estudio de procesos astrofísicos de formación de estrellas observado por los distintos telescopios.
Over 25 years of access to the velocity gradient tensor: what we have learned from experiments... Imprimir
James M. Wallace
Professor Emeritus, Dept. of Mechanical Engineering
Director, Burgers Program for Fluid Dynamics
Institute for Physical Science & Technology
University of Maryland
Over 25 years of access to the velocity gradient tensor: what we have learned from experiments and DNS about turbulence? 

Viernes 3 de octubre, 16:15hrs
Departamento de Física (DFI), F12
Av Blanco Encalada 2008.

A little over Twenty-five  years ago there was no experimental access to the velocity gradient tensor for turbulent flows. Without such access, knowledge of fundamental and defining properties of turbulence, such as vorticity, dissipation and strain rates and helicity, could not be studied in the laboratory.  Although a few direct simulations at very low Reynolds numbers had been performed, most of these did not focus on properties of the small scales of turbulence defined by the velocity gradient tensor.   In 1987 the results of the development and first successful use of a multi-sensor hot-wire probe for measurements of all the components of the velocity gradient tensor in a turbulent boundary layer were published by Balint, Vukoslavcevic and Wallace (Adv. in Turbulence: Proc. of 1st Euro. Turb. Conf.,Springer-Verlag, 456). That same year the first DNS of a turbulent channel flow was successfully carried out and reported by Kim, Moin and Moser (J. Fluid Mech. 177), including statistics of the vorticity field.  Also in that year a DNS of homogeneous shear flow by Rogers and Moin (J. Fluid Mech. 176) was published in which the authors examined the structure of the vorticity field. Additionally, Ashurst, Kerstein, Kerr and Gibson (Phys. Fluids 30) examined alignment of the vorticity and strainrate fields using this homogeneous shear flow data as well as the DNS of isotropic turbulence of Kerr (J. Fluid Mech. 153) who had initiated such studies.  Furthermore, Metcalfe, Orszag, Brachet, Menon and Riley (J. Fluid Mech. 184) also published in 1987 results from their direct simulation of a temporally developing planar mixing layer in which they examined coherent vortical states resulting from secondary instabilities.  Since then several experimentalists have used multi-sensor hot-wire probes of increasing complexity in turbulent boundary layers, wakes, jets, mixing layers and grid flows. Numerous computationalists have employed DNS in a wide variety of turbulent flows at ever increasing Reynolds numbers.  PIV and other optical methods have been rapidly developed and advanced during these two and a half decades which have provided other means of access to these fundamental properties of turbulence. This presentations reviews highlights of these remarkable developments and points out some of the most important things we have learned about turbulence as a result.
Red de paredes tipo zig-zag en un cristal líquido nemático Imprimir
Ignacio Andrade
Departamento de Física
FCFM, U. de Chile

Viernes 26 de septiembre, 16:15hrs
Departamento de Física (DFI), F12
Av Blanco Encalada 2008.

Liquid crystals displays (LCD's) with tailoring electrodes exhibit complex spatiotemporal dynamics when a large voltage is applied. We report experimental observations of the appearance of a programmable zig-zag lattice using an in-plane switching cell filled with a nematic liquid crystal. Applying a small voltage to a wide range of frequencies, the system exhibits an Ising wall lattice. Increasing the voltage, this lattice presents a spatial instability generating an undulating wall lattice, and to higher voltages it becomes zig-zag type. Experimentally, we characterize the bifurcations and phase diagram of the wall lattice. Theoretically, we develop, from first principles, a descriptive model. This model has a good qualitative agreement with experimental observations. 
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