Is there a fundamental acceleration scale in galaxies?
زهرا داوری
دانشکده فیزیک دانشگاه صنعتی شریف
Abstract: As we know, the mass discrepancy or missing mass problem has been found for many years in the galactic scale. Therefore, it seems that there is a need for a significant amount of non-luminous matter, i.e. the dark matter in galaxy system but up to now, no direct evidence of its existence has been found. One of the successful alternatives to the dark matter is Milgrom’s modified Newtonian dynamics (MOND) that could explain well the mass discrepancy problem in the galaxy without invoking dark matter. The MOND theory predicts a universal constant acceleration scale in the galaxy, below which the Newtonian dynamics is no longer valid. We analyzed 193 high-quality disk galaxies by using Bayesian inference and we concluded that the probability of the existence of a fundamental acceleration scale is essentially zero. However, there still exist evidence that rejects MOND as a fundamental theory and we conclude that a0 is of emergent nature.
عبدالعلی بنی هاشمی (دانشکده فیزیک دانشگاه شهید بهشتی)
Abstract: In this talk, first we will take a brief look at some LCDM tensions; then we explain our idea for lessening them by giving a new model for the evolution of the universe. Our idea is based on supposing a phase transition for Dark Energy, where we study this phase transition in the framework of Landau-Ginzburg theory. In the end, we will show that the results of our model are promising and reduce the tensions.
Department of Theoretical Physics of the University of Geneva
Abstract: I’m going to introduce a new instability in partial differential equations, which also can be present in some dark energy and cosmological models. We have studied k-essence scalar field and have developed an N-body code, k-evolution, in which we consider non-linearities. We have proved that the k-essence non-linearities suffer from the new instability and blowup in finite time; I’ll show the solution and blowup time for different initial conditions. Moreover, to understand the new instability, I will solve the traffic flow as a similar dynamical system and show how shock waves are produced in the traffic flow.
Department of Physics and Astronomy at the University of Pennsylvania
Abstract: Nonlinear structure formation is an important area in the modern cosmology, where deep questions can be addressed by large-scale structure data, especially the questions which are related to the dark universe – the physics of dark matter and dark energy. In this arena, the abundance and clustering of dark matter halos probe important features of the standard model of cosmology. The excursion set approach is a framework for estimating how the statistical properties of nonlinear structures in the cosmos depend on the history of the universe and the nature of gravity. In this theory, one associates a random walk with each position in the initial density field, and the barrier to be crossed is determined by the collapse model. The main problem here is the first passage time problem and the first up-crossing distribution is the quantity which is related to the abundance of nonlinear objects. My recent work is about approximations of the first up-crossing distribution (for non-Markovian walks) and modeling the abundance of halos, and in this talk, I would like to show the associated implications for halo bias.
