Physical cosmology

Résumé

Physical cosmology is a branch of cosmology concerned with the study of cosmological models. A cosmological model, or simply cosmology, provides a description of the largest-scale structures and dynamics of the universe and allows study of fundamental questions about its origin, structure, evolution, and ultimate fate. Cosmology as a science originated with the Copernican principle, which implies that celestial bodies obey identical physical laws to those on Earth, and Newtonian mechanics, which first allowed those physical laws to be understood. Physical cosmology, as it is now understood, began with the development in 1915 of Albert Einstein's general theory of relativity, followed by major observational discoveries in the 1920s: first, Edwin Hubble discovered that the universe contains a huge number of external galaxies beyond the Milky Way; then, work by Vesto Slipher and others showed that the universe is expanding. These advances made it possible to speculate about the orig

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Adaptive finite element simulations of dendritic growth including fluid flow induced by shrinkage

Jacek Narski

A phase field model describing the solidification of a binary alloy is investigated. The location of the solid and liquid phases in the computational domain is described by introducing an order parameter, the phase-field, which varies smoothly from one in the solid to zero in the liquid through a slightly diffused interface. The solidification process of binary alloys is controlled by the local concentration of the alloy and the temperature. The concentration is altered by the existing flows in the melt. With temperature being a given constant, the model corresponds to coupling the phase-field equation, the concentration equation and the compressible Navier-Stokes equations. The main difficulty when solving numerically phase field models is due to the very rapid change of the phase field and the concentration field across the diffused interface, whose thickness has to be taken very small in comparison to the dimension of the computational domain in order to correctly capture the physics of the phase transformation. A high spatial resolution is therefore needed to describe the smooth transition. In this work, we present a physical model governing the solidification process. In order to reduce the number of grid points required for the reliable simulations, we introduce an adaptive algorithm that aims to build successive meshes with large aspect ratio such that the relative estimated error of the concentration and/or velocity in the H1-norm is close to a preset tolerance TOL. For this purpose, we introduce error indicators which measure the error of the concentration and the velocity in the directions of maximum and minimum stretching of the element. Finally, we apply our method to 2D and 3D simulations of the dendritic growth proving its efficiency.

EPFL2007

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The standard Lambda Cold Dark Matter (Lambda CDM) cosmological model provides a good description of a wide range of astrophysical and cosmological data. However, there are a few big open questions that make the standard model look like an approximation to a more realistic scenario yet to be found. In this paper, we list a few important goals that need to be addressed in the next decade, taking into account the current discordances between the different cosmological probes, such as the disagreement in the value of the Hubble constant H-0, the sigma(8)-S-8 tension, and other less statistically significant anomalies. While these discordances can still be in part the result of systematic errors, their persistence after several years of accurate analysis strongly hints at cracks in the standard cosmological scenario and the necessity for new physics or generalisations beyond the standard model. In this paper, we focus on the 5.0 sigma tension between the Planck CMB estimate of the Hubble constant H-0 and the SH0ES collaboration measurements. After showing the H-0 evaluations made from different teams using different methods and geometric calibrations, we list a few interesting new physics models that could alleviate this tension and discuss how the next decade's experiments will be crucial. Moreover, we focus on the tension of the Planck CMB data with weak lensing measurements and redshift surveys, about the value of the matter energy density Omega(m), and the amplitude or rate of the growth of structure (sigma(8), f sigma(8)). We list a few interesting models proposed for alleviating this tension, and we discuss the importance of trying to fit a full array of data with a single model and not just one parameter at a time. Additionally, we present a wide range of other less discussed anomalies at a statistical significance level lower than the H-0-S-8 tensions which may also constitute hints towards new physics, and we discuss possible generic theoretical approaches that can collectively explain the non-standard nature of these signals. Finally, we give an overview of upgraded experiments and next-generation space missions and facilities on Earth that will be of crucial importance to address all these open questions. (C) 2022 The Author(s). Published by Elsevier B.V.

ELSEVIER2022

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Hydrogen Intensity and Real-Time Analysis Experiment: 256-element array status and overview

Jean-Paul Richard Kneib, Martin Kunz, Emma Elizabeth Tolley, Zeyu Zhang

The Hydrogen Intensity and Real-time Analysis Experiment (HIRAX) is a radio interferometer array currently in development, with an initial 256-element array to be deployed at the South African Radio Astronomy Observatory Square Kilometer Array site in South Africa. Each of the 6 m, f/0.23 dishes will be instrumented with dual-polarization feeds operating over a frequency range of 400 to 800 MHz. Through intensity mapping of the 21 cm emission line of neutral hydrogen, HIRAX will provide a cosmological survey of the distribution of large-scale structure over the redshift range of 0.775 < z < 2.55 over similar to 15,000 square degrees of the southern sky. The statistical power of such a survey is sufficient to produce similar to 7% constraints on the dark energy equation of state parameter when combined with measurements from the Planck satellite. Additionally, HIRAX will provide a highly competitive platform for radio transient and HI absorber science while enabling a multitude of cross-correlation studies. We describe the science goals of the experiment, overview of the design and status of the subcomponents of the telescope system, and describe the expected performance of the initial 256-element array as well as the planned future expansion to the final, 1024-element array. (C) 2022 Society of Photo Optical Instrumentation Engineers (SPIE)

SPIE-SOC PHOTO-OPTICAL INSTRUMENTATION ENGINEERS2022

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ELSEVIER2022