Prof. David Alonso

Abstract:

We study the non-linear gravitational collapse of dark matter into halos through numerical N-body simulations of Lemaître–Tolman–Bondi void models. We extend the halo mass function formalism to these models in a consistent way. This extension not only compares well with the simulated data at all times and radii, but it also gives interesting clues about the impact of the background shear on the growth of perturbations. Our results give hints about the possibility of constraining the background shear via cluster number counts, which could then give rise to strong constraints on general inhomogeneous models, of any scale.

Abstract:

We propose a new method for the extraction cosmological parameters using the baryon acoustic oscillation (BAO) scale as a standard ruler in deep galaxy surveys with photometric determination of redshifts. The method consists in a simple empirical parametric fit to the angular two-point correlation function ω(θ). It is parametrized as a power law to describe the continuum and as a Gaussian to describe the BAO bump. The location of the Gaussian is used as the basis for the measurement of the sound horizon scale. This method, although simple, actually provides a robust estimation, since the inclusion of the power law and the use of the Gaussian remove the shifts which affect the local maximum. We discuss the effects of projection bias, non-linearities, redshift space distortions and photo-z precision and apply our method to a mock catalogue of the Dark Energy Survey, built upon a large N-body simulation provided by the MICE collaboration. We discuss the main systematic errors associated with our method and show that they are dominated by the photo-z uncertainty.

Abstract:

We perform numerical simulations of large scale structure evolution in an inhomogeneous Lemaître-Tolman-Bondi (LTB) model of the Universe. We follow the gravitational collapse of a large underdense region (a void) in an otherwise flat matter-dominated Einstein–de Sitter model. We observe how the (background) density contrast at the center of the void grows to be of order one, and show that the density and velocity profiles follow the exact nonlinear LTB solution to the full Einstein equations for all but the most extreme voids. This result seems to contradict previous claims that fully relativistic codes are needed to properly handle the nonlinear evolution of large scale structures, and that local Newtonian dynamics with an explicit expansion term is not adequate. We also find that the (local) matter density contrast grows with the scale factor in a way analogous to that of an open universe with a value of the matter density Ω M ( r ) corresponding to the appropriate location within the void.