Local Voids as the Origin of Large-angle Cosmic Microwave Background Anomalies: The Effect of a Cosmological Constant

We explore the large angular scale temperature anisotropies in the cosmic microwave background (CMB) due to homogeneous local dust-filled voids in a flat Friedmann-Robertson-Walker universe with a cosmological constant. In comparison with the equivalent dust-filled void model in the Einstein-de Sitter background, we find that the anisotropy for compensated asymptotically expanding local voids can be larger because second-order effects enhance the linear integrated Sachs-Wolfe (ISW) effect. However, for local voids that expand sufficiently faster than the asymptotic velocity of the wall, the second-order effect can suppress the fluctuation due to the linear ISW effect. A pair of quasi-linear compensated asymptotic local voids with radius (2-3)*10^2 ~h^{-1} Mpc and a matter density contrast ~-0.3 can be observed as cold spots with a temperature anisotropy Delta T/T~O(10^{-5}) that might help explain the observed large-angle CMB anomalies. We predict that the associated anisotropy in the local Hubble constant in the direction of the voids could be as large as a few percent.Comment: 23 pages, 5 figures, version accepted for publication in ApJ with minor revisio

A fluid-structure solver for confined microcapsule flows

We present a fluid-structure coupling method designed to study capsules flowing in a confined environment. The fluid solver is based on the Finite Volume Method and is coupled to a Finite Elements solid solver using the Immersed Boundary Method. We study the relaxation of a spherical capsule, initially deformed into an ellipsoid, and released in a square cross-section channel within a quiescent fluid environment. We perform a convergence study in order to validate the numerical method and consider the effect of the inertial forces on the capsule relaxation

On the Rees-Sciama effect: maps and statistics

Small maps of the Rees-Sciama (RS) effect are simulated by using an appropriate N-body code and a certain ray-tracing procedure. A method designed for the statistical analysis of cosmic microwave background (CMB) maps is applied to study the resulting simulations. These techniques, recently proposed --by our team-- to consider lens deformations of the CMB, are adapted to deal with the RS effect. This effect and the deviations from Gaussianity associated to it seem to be too small to be detected in the near future. This conclusion follows from our estimation of both the RS angular power spectrum and the RS reduced n-direction correlation functions for n<7.Comment: 11 pages, 13 figures, to appear in MNRA

Local Voids as the Origin of Large-angle Cosmic Microwave Background Anomalies I

We explore the large angular scale temperature anisotropies in the cosmic microwave background due to expanding homogeneous local voids at redshift z~1. A compensated spherically symmetric homogeneous dust-filled void with radius \~3*10^2 h^{-1}Mpc, and density contrast ~-0.3 can be observed as a cold spot with a temperature anisotropy -1*10^{-5} surrounded by a slightly hotter ring. We find that a pair of these circular cold spots separated by ~50 degree can account both for the planarity of the octopole and for the alignment between the quadrupole and the octopole in the cosmic microwave background (CMB) anisotropy. The cold spot in the Galactic southern hemisphere which is anomalous at the ~3sigma level can be explained by such a large void at z~1. The observed north-south asymmetry in the large-angle CMB power can be attributed to the asymmetric distribution of these local voids between the two hemispheres. The statistical significance of the low quadrupole is further reduced in this interpretation of the large angular scale CMB anomalies.Comment: 8 pages, 5 eps files, Version accepted for ApJ. New maps for non-overlapping voids (Fig. 4) is adde

CMB anisotropy: deviations from Gaussianity due to non-linear gravity

Non-linear evolution of cosmological energy density fluctuations triggers deviations from Gaussianity in the temperature distribution of the cosmic microwave background. A method to estimate these deviations is proposed. N-body simulations -- in a $\Lambda$CDM cosmology -- are used to simulate the strongly non-linear evolution of cosmological structures. It is proved that these simulations can be combined with the potential approximation to calculate the statistical moments of the CMB anisotropies produced by non-linear gravity. Some of these moments are computed and the resulting values are different from those corresponding to Gaussianity.Comment: 6 latex pages with mn.sty, 3 eps figures. Accepted in MNRA

Estimating small angular scale CMB anisotropy with high resolution N-body simulations: weak lensing

We estimate the impact of weak lensing by strongly nonlinear cosmological structures on the cosmic microwave background. Accurate calculation of large $\ell$ multipoles requires N-body simulations and ray-tracing schemes with both high spatial and temporal resolution. To this end we have developed a new code that combines a gravitational Adaptive Particle-Particle, Particle-Mesh (AP3M) solver with a weak lensing evaluation routine. The lensing deviations are evaluated while structure evolves during the simulation so that all evolution steps--rather than just a few outputs--are used in the lensing computations. The new code also includes a ray-tracing procedure that avoids periodicity effects in a universe that is modeled as a 3-D torus in the standard way. Results from our new simulations are compared with previous ones based on Particle-Mesh simulations. We also systematically investigate the impact of box volume, resolution, and ray-tracing directions on the variance of the computed power spectra. We find that a box size of $512 h^{-1}$ Mpc is sufficient to provide a robust estimate of the weak lensing angular power spectrum in the $\ell$-interval (2,000--7,000). For a reaslistic cosmological model the power $[\ell(\ell+1)C_{\ell}/2\pi]^{1/2}$ takes on values of a few $\mu K$ in this interval, which suggests that a future detection is feasible and may explain the excess power at high $\ell$ in the BIMA and CBI observations.Comment: 49 pages, 13 figures, accepted for publication in Ap

The challenges and possibilities of reflective learning in higher education. Research focused from the perspective of university students on four different degree courses

Despite an increase in university teaching practices based on reflective learning methodology (RL), only very few studies are found in the context of higher education aimed at determining students? perception of this approach to teaching and learning. The aim of this research was to ascertain the opinions of students on different university degree courses regarding the challenges, difficulties and contributions arising from the application of reflective learning methodology in their learning process so as to propose strategies for improving education. The study was carried out on four Bachelor degree courses at the University of Girona: Social Education, Environmental Sciences, Nursing and Psychology. The research was conducted in two stages. In the first (2011-12 academic year), a questionnaire was administered to 162 students (43 from Social Education; 33 from Environmental Sciences; 31 from Nursing; and 55 from Psychology). One section of the questionnaire was specifically devoted to studying the perceptions of students participating in various RL experiences regarding the main difficulties they faced and the contributions of the RL to their learning process. Among the highlighted difficulties was the fact that RL requires a work process they are not used to and their lack of reflective writing skills. Among the contributions, the students felt that RL results in more complex and selfregulated knowledge, develops greater dynamic capabilities and increases the degree of reflection on learning processes and areas for improvement. In the second stage of the study (2012-13 academic year), four focus groups were held with students who had participated in the previous years? experience with the aim of gaining further insight into their perceptions regarding the challenges and contributions of RL. A total of 20 students participated, all of whom gave informed written consent. The sessions were recorded and transcribed in full and a thematic content analysis was performed. In all four groups the students stated that the experience had allowed them to improve their learning and become aware of their current situation and areas for improvement. As for the challenges, they cited difficulties in understanding the aims and purpose of RL, particularly at the beginning of the experience, together with problems experienced in writing about it, doubts about the level of openness required and uncertainty about how they would be assessed. The research conducted suggested that RL has significant potential to connect academic activity with professional action. It also provided working guidelines for improving experiences carried out on the basis of RL. These include the need to clarify the methodology and present arguments for its use, so that students understand the type of work it will mean for them and the objectives they pursued. Students should also be provided with sufficiently clear guidelines regarding how they will be assessed (in relation to both activities and level of reflection). There is a need to build a group climate based on mutual trust, continuous feedback and the establishment of a support process that maintains this trust throughout the learning process

On the estimation and detection of the Rees Sciama effect

This article has been accepted for publication in: Monthly Notices of the Royal Astronomical Society. ©: 2016 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.Maps of the Rees–Sciama (RS) effect are simulated using the parallel N-body code, hydra, and a run-time ray-tracing procedure. A method designed for the analysis of small, square cosmic microwave background (CMB) maps is applied to our RS maps. Each of these techniques has been tested and successfully applied in previous papers. Within a range of angular scales, our estimate of the RS angular power spectrum due to variations in the peculiar gravitational potential on scales smaller than 42/h megaparsecs is shown to be robust. An exhaustive study of the redshifts and spatial scales relevant for the production of RS anisotropy is developed for the first time. Results from this study demonstrate that (i) to estimate the full integrated RS effect, the initial redshift for the calculations (integration) must be greater than 25, (ii) the effect produced by strongly non-linear structures is very small and peaks at angular scales close to 4.3 arcmin, and (iii) the RS anisotropy cannot be detected either directly—in temperature CMB maps—or by looking for cross-correlations between these maps and tracers of the dark matter distribution. To estimate the RS effect produced by scales larger than 42/h megaparsecs, where the density contrast is not strongly non-linear, high accuracy N-body simulations appear unnecessary. Simulations based on approximations such as the Zel'dovich approximation and adhesion prescriptions, for example, may be adequate. These results can be used to guide the design of future RS simulations.This work has been supported by the Spanish Ministry of Economia y Competitividad, MICINN-FEDER project FIS2015-64552-P. Calculations were carried out at the Centre de Calcul de la Universitat de Valencia. RJT thanks NSERC, the Canada Research Chairs Program, Canada Foundation for Innovation and the Nova Scotia Research and Innovation Trust for support. HMPC thank NSERC for support.Fullana Alfonso, MJ.; Arnau, J.; Thacker, R.; Couchman, H.; Sáez, D. (2017). On the estimation and detection of the Rees Sciama effect. Monthly Notices of the Royal Astronomical Society. 464(4):3784-1795. https://doi.org/10.1093/mnras/stw2615S37841795464

Voids in the Large-Scale Structure

Voids are the most prominent feature of the LSS of the universe. Still, they have been generally ignored in quantitative analysis of it, essentially due to the lack of an objective tool to identify and quantify the voids. To overcome this, we present the Void-Finder algorithm, a novel tool for objectively quantifying galaxy voids. The algorithm classifies galaxies as either wall- or field-galaxies. Then it identifies voids in the wall-galaxy distribution. Voids are defined as continuous volumes that do not contain any wall-galaxies. The voids must be thicker than an adjustable limit, which is refined in successive iterations. We test the algorithm using Voronoi tessellations. By appropriate scaling of the parameters we apply it to the SSRS2 survey and to the IRAS 1.2 Jy. Both surveys show similar properties: ~50% of the volume is filled by the voids, which have a scale of at least 40 Mpc, and a -0.9 under-density. Faint galaxies populate the voids more than bright ones. These results suggest that both optically and IRAS selected galaxies delineate the same LSS. Comparison with the recovered mass distribution further suggests that the observed voids in the galaxy distribution correspond well to under-dense regions in the mass distribution. This confirms the gravitational origin of the voids.Comment: Submitted to ApJ; 33 pages, aaspp4 LaTeX file, using epsfig and natbib, 1 table, 12 PS figures. Complete gzipped version is available at http://shemesh.fiz.huji.ac.il/hagai/; uuencoded file is available at http://shemesh.fiz.huji.ac.il/papers/ep3.uu or ftp://shemesh.fiz.huji.ac.i