The First Magnetic Fields

We review current ideas on the origin of galactic and extragalactic magnetic fields. We begin by summarizing observations of magnetic fields at cosmological redshifts and on cosmological scales. These observations translate into constraints on the strength and scale magnetic fields must have during the early stages of galaxy formation in order to seed the galactic dynamo. We examine mechanisms for the generation of magnetic fields that operate prior during inflation and during subsequent phase transitions such as electroweak symmetry breaking and the quark-hadron phase transition. The implications of strong primordial magnetic fields for the reionization epoch as well as the first generation of stars is discussed in detail. The exotic, early-Universe mechanisms are contrasted with astrophysical processes that generate fields after recombination. For example, a Biermann-type battery can operate in a proto-galaxy during the early stages of structure formation. Moreover, magnetic fields in either an early generation of stars or active galactic nuclei can be dispersed into the intergalactic medium.Comment: Accepted for publication in Space Science Reviews. Pdf can be also downloaded from http://canopus.cnu.ac.kr/ryu/cosmic-mag1.pd

Tamanho de amostras para a determinação de parâmetros físicos em planossolo por tomografia computadorizada

A técnica da tomografia computadorizada (TC) permite medir a densidade e a umidade de amostras de solo, constituindo uma importante ferramenta na Ciência do Solo. Este trabalho tem como objetivos descrever os aspectos da adequação do tamanho de amostras de um Planossolo e os procedimentos de avaliação e estudos por análise estatística, empregando-se um minitomógrafo computadorizado de raios gama com fonte de 241Am. O valor do erro atribuído ao equipamento são 0,051 e 0,046 Mg m-3, respectivamente, para os horizontes A e B. O valor teórico da espessura da amostra do Planossolo para uso na técnica de TC com fonte de 241Am é, aproximadamente, 4,0 cm para os horizontes A e B. Já a espessura ideal de amostras é de aproximadamente 6,0 cm, sendo menor para amostras do horizonte B em relação ao A. Obteve-se boa precisão e adaptabilidade no emprego da TC para estudos de Planossolos._________________________________________________________________________________ ABSTRACT: Computerized tomography (CT) is an important tool in Soil Science for noninvasive measurement of density and water content of soil samples. This work aims to describe the aspects of sample size adequacy for Planosol (Albaqualf) and to evaluate procedures for statistical analysis, using a CT scanner with a 241Am source. Density errors attributed to the equipment are 0.051 and 0.046 Mg m-3 for horizons A and B, respectively. The theoretical value for sample thickness for the Planosol, using this equipment, is 4.0 cm for the horizons A and B. The ideal thickness of samples is approximately 6.0 cm, being smaller for samples of the horizon B in relation to A. Alternatives for the improvement of the efficiency analysis and the reliability of the results obtained by CT are also discussed, and indicate good precision and adaptability of the application of this technology in Planosol (Albaqualf) studies