697 research outputs found

### Observations in the Einstein--de Sitter Cosmology: Dust Statistics and Limits of Apparent Homogeneity

The two-point correlation function for the dust distribution in the
unperturbed Einstein-de Sitter cosmological model is studied along the past
light cone. It was found that this function seems unable to represent the
theoretical distribution of dust along the backward null cone of this
unperturbed model, which has already been determined in a previous paper as
being apparently inhomogeneous at ranges usually considered as local. Such
result was revisited in order to determine more precisely the quantitative
limits where, in theory, we can detect apparent homogeneity, and it was found
that this may only happen up to $z \sim 10^{-2}$. A different statistical
analysis proposed by Pietronero is used, and it appears to be able to represent
more accurately the theoretical distribution of dust in this cosmology. In the
light of these results, it is argued that the usual practice of disregarding
relativistic effects in studies of distribution of galaxies, by considering
them as being placed on local regions, seems to be valid only on much closer
scales than it is commonly believed. In the Einstein-de Sitter cosmology with
$H_0=75 km s^{-1} Mpc^{-1}$, that may only happen in redshifts as low as $z
\approx 0.04$, which means that the local approximation seems to be valid up to
zeroth order of approximation only. As at present there are many redshift
surveys which have already probed at deeper ranges, it seems that in order to
compare the Friedmann models with observations we have to be very careful when
ignoring the past light cone problem in observational cosmology, either in
theoretical calculations or in data analysis, due to relativistic effects which
produce observable inhomogeneity even in spatially homogeneous cosmological
models.Comment: standard LaTeX; 23 pages, 2 tables, 3 LaTeX figures; paper published
in 1995 is archived her

### Cosmological Distances and Fractal Statistics of Galaxy Distribution

This paper studies the effect of the distance choice in radial (non-average)
statistical tools used for fractal characterization of galaxy distribution.
After reviewing the basics of measuring distances of cosmological sources,
various distance definitions are used to calculate the differential density
$\gamma$ and the integral differential density $\gamma^\ast$} of the dust
distribution in the Einstein-de Sitter cosmology. The main results are as
follows: (1) the choice of distance plays a crucial role in determining the
scale where relativistic corrections must be taken into account, as both
$\gamma$ and $\gamma^\ast$ are strongly affected by such a choice; (2)
inappropriate distance choices may lead to failure to find evidence of a galaxy
fractal structure when one calculates those quantities, even if such a
structure does occur in the galaxy distribution; (3) the comoving distance and
the distance given by Mattig's formula are unsuitable to probe for a possible
fractal pattern as they render $\gamma$ and $\gamma^\ast$ constant for all
redshifts; (4) a possible galaxy fractal system at scales larger than 100Mpc (z
\~ 0.03) may only be found if those statistics are calculated with the
luminosity or redshift distances, as they are the ones where $\gamma$ and
$\gamma^\ast$ decrease at higher redshifts; (5) C\'el\'erier and Thieberger's
(2001) critique of Ribeiro's (1995: astro-ph/9910145) earlier study are
rendered impaired as their objections were based on misconceptions regarding
relativistic distance definitions.Comment: 14 pages, 4 figures, A&A LaTeX macro. Minor linguistic changes to
match the version sent to the publisher. Accepted for publication in
"Astronomy and Astrophysics

### Dogmatism and Theoretical Pluralism in Modern Cosmology

This work discusses the presence of a dogmatic tendency within modern
cosmology, and some ideas capable of neutralizing its negative influence. It is
verified that warnings about the dangers of dogmatic thinking in cosmology can
be found as early as the 1930's, and we discuss the modern appearance of
"scientific dogmatism". The solution proposed to counteract such an influence,
which is capable of neutralizing this dogmatic tendency, has its origins in the
philosophical thinking of the Austrian physicist Ludwig Boltzmann (1844-1906).
In particular we use his two main epistemological theses, scientific theories
as representations of nature and theoretical pluralism, to show that once they
are embodied in the research practice of modern cosmology, there is no longer
any reason for dogmatic behaviours.Comment: 14 pages; LaTeX sourc

### Boltzmann's Concept of Reality

In this article we describe and analyze the concept of reality developed by
the Austrian theoretical physicist Ludwig Boltzmann. It is our thesis that
Boltzmann was fully aware that reality could, and actually was, described by
different points of view. In spite of this, Boltzmann did not renounce the idea
that reality is real. We also discuss his main motivations to be strongly
involved with philosophy of science, as well as further developments made by
Boltzmann himself of his main philosophical ideas, namely scientific theories
as images of Nature and its consequences. We end the paper with a discussion
about the modernity of Boltzmann's philosophy of science.Comment: 13 pages, pdf only. To appear in the book on Ludwig Boltzmann
scientific philosophy, published by Nova Science. Edited by A. Eftekhar

### A Fortran Code for Null Geodesic Solutions in the Lemaitre-Tolman-Bondi Spacetime

This paper describes the Fortran 77 code SIMU, version 1.1, designed for
numerical simulations of observational relations along the past null geodesic
in the Lemaitre-Tolman-Bondi (LTB) spacetime. SIMU aims at finding scale
invariant solutions of the average density, but due to its full modularity it
can be easily adapted to any application which requires LTB's null geodesic
solutions. In version 1.1 the numerical output can be read by the GNUPLOT
plotting package to produce a fully graphical output, although other plotting
routines can be easily adapted. Details of the code's subroutines are
discussed, and an example of its output is shown.Comment: 13 pages, 10 figures, LaTeX. Fortran code included with the LaTeX
source code (also available at http://www.if.ufrj.br/~mbr/codes). Accepted
for publication in "Computer Physics Communications

### The Apparent Fractal Conjecture

This short communication advances the hypothesis that the observed fractal
structure of large-scale distribution of galaxies is due to a geometrical
effect, which arises when observational quantities relevant for the
characterization of a cosmological fractal structure are calculated along the
past light cone. If this hypothesis proves, even partially, correct, most, if
not all, objections raised against fractals in cosmology may be solved. For
instance, under this view the standard cosmology has zero average density, as
predicted by an infinite fractal structure, with, at the same time, the
cosmological principle remaining valid. The theoretical results which suggest
this conjecture are reviewed, as well as possible ways of checking its
validity.Comment: 6 pages, LaTeX. Text unchanged. Two references corrected. Contributed
paper presented at the "South Africa Relativistic Cosmology Conference in
Honour of George F. R. Ellis 60th Birthday"; University of Cape Town,
February 1-5, 199

### On Modelling a Relativistic Hierarchical (Fractal) Cosmology by Tolman's Spacetime. II. Analysis of the Einstein-de Sitter Model

This paper studies the spatially homogeneous Einstein-de Sitter cosmological
model in the context of a relativistic hierarchical (fractal) cosmology as
developed in paper I (0807.0866). The Einstein-de Sitter model is treated as a
special case of Lemaitre-Tolman's spacetime, obtained by the appropriate choice
of the latter's three arbitrary functions. The observational relations along
the past light cone of the model under consideration are calculated, and an
investigation of whether or not it has fractal behaviour is performed. It was
found that the Einstein-de Sitter model does not seem to remain homogeneous
along the geodesic and that it also has no fractal features along the backward
null cone.Comment: 13 pages. 1 figure. Paper published in 199

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