47,707 research outputs found
Spatial and observational homogeneities of the galaxy distribution in standard cosmologies
This work discusses the possible empirical verification of the geometrical
concept of homogeneity of the standard relativistic cosmology considering its
various definitions of distance. We study the physical consequences of the
distinction between the usual concept of spatial homogeneity (SH), as defined
by the Cosmological Principle, and the concept of observational homogeneity
(OH), arguing that OH is in principle falsifiable by means of astronomical
observations, whereas verifying SH is only possible indirectly. Simulated
counts of cosmological sources are produced by means of a generalized
number-distance expression that can be specialized to produce either the counts
of the Einstein-de Sitter (EdS) cosmology, which has SH by construction, or
other types of counts, which do, or do not, have OH by construction.
Expressions for observational volumes and differential densities are derived
with the various cosmological distance definitions in the EdS model. Simulated
counts that have OH by construction do not always exhibit SH features. The
reverse situation is also true. Besides, simulated counts with no OH features
at low redshift start showing OH characteristics at high redshift. The comoving
distance seems to be the only distance definition where both SH and OH appear
simultaneously. The results show that observations indicating possible lack of
OH do not necessarily falsify the standard Friedmannian cosmology, meaning that
this cosmology will not necessarily always produce observable homogeneous
densities. The general conclusion is that the use of different cosmological
distances in the characterization of the galaxy distribution lead to
significant ambiguities in reaching conclusions about the behavior of the
large-scale galaxy distribution in the Universe.Comment: 12 pages, 12 figures, LaTeX. Matches the final version sent to the
journal. Accepted for publication in "Astronomy and Astrophysics
Scale Invariance in a Perturbed Einstein-de Sitter Cosmology
This paper seeks to check the validity of the "apparent fractal conjecture"
(Ribeiro 2001ab: gr-qc/9909093, astro-ph/0104181), which states that the
observed power-law behaviour for the average density of large-scale
distribution of galaxies arises when some observational quantities, selected by
their relevance in average density profile determination, are calculated along
the past light cone. Implementing these conditions in the proposed set of
observational relations profoundly changes the behaviour of many observables in
the standard cosmological models. In particular, the average density becomes
observationally inhomogeneous, even in the spatially homogeneous spacetime of
standard cosmology, change which was already analysed by Ribeiro (1992b, 1993,
1994, 1995: astro-ph/9910145) for a non-perturbed model. Here we derive
observational relations in a perturbed Einstein-de Sitter cosmology by means of
the perturbation scheme proposed by Abdalla and Mohayaee (1999:
astro-ph/9810146), where the scale factor is expanded in power series to yield
perturbative terms. The differential equations derived in this perturbative
context, and other observables necessary in our analysis, are solved
numerically. The results show that our perturbed Einstein-de Sitter cosmology
can be approximately described by a decaying power-law like average density
profile, meaning that the dust distribution of this cosmology has a scaling
behaviour compatible with the power-law profile of the density-distance
correlation observed in the galaxy catalogues. These results show that, in the
context of this work, the apparent fractal conjecture is correct.Comment: 18 pages, 1 figure, LaTeX. Final version (small changes in the figure
plus some references update). Fortran code included with the LaTeX source. To
be published in "Fractals
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
Nova Eruptions with Infrared Interferometric Observations
Infrared interferometric observations have a great deal of potential to
unravel the nature of the nova eruptions. We suggest that techniques, already
in place, to derive the ejection details at optical wavelengths be used with
infrared interferometric observations to derive parameters such as the ejected
mass in a nova eruption. This is achievable based on modelling the initial
phase of the eruption when the infrared light is dominated by the free-free
thermal process.Comment: To appear in the proceedings of "Physics of Evolved Stars 2015 - A
conference dedicated to the memory of Olivier Chesneau
A Uniform Approximation for the Coherent State Propagator using a Conjugate Application of the Bargmann Representation
We propose a conjugate application of the Bargmann representation of quantum
mechanics. Applying the Maslov method to the semiclassical connection formula
between the two representations, we derive a uniform semiclassical
approximation for the coherent state propagator which is finite at phase space
caustics.Comment: 4 pages, 1 figur
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