108 research outputs found
An Issue to the Cosmological Constant Problem
According to general relativity, the present analysis shows on geometrical
grounds that the cosmological constant problem is an artifact due to the
unfounded link of this fundamental constant to vacuum energy density of quantum
fluctuations.Comment: 7 page
The Tully-Fisher relation : Correspondence between the Inverse and Direct approaches
In a previous paper, we have demonstrated the importance to define a
statistical model describing the observed linear correlation between the
absolute magnitude and the log line width distance indicator of
galaxies (the Tully-Fisher relation). As long as the same statistical model is
used during the calibration step of the relation and the step of the
determination of the distances of galaxies, standard statistical methods such
as the maximum likelihood technic permits us to derive bias free estimators of
the distances of galaxies. However in practice, it is convenient to use a
different statistical model for calibrating the Tully-Fisher relation (because
of its robustness, the Inverse Tully-Fisher relation is prefered during this
step) and for determining the distances of galaxies (the Direct Tully-Fisher
relation is more accurate and robust in this case). Herein, we establish a
correspondence between the Inverse and the Direct Tully-Fisher approaches.
Assuming a gaussian luminosity function, we prove that the ITF and DTF models
are in fact mathematically equivalent (i.e. they describe the same physical
data distribution in the TF diagram). It thus turns out that as long as the
calibration parameters are obtained for a given model, we can deduce the
corresponding parameters of the other model. We present these formulae of
correspondence and discuss their validitity for non-gaussian luminosity
functions.Comment: 10 pages, uuencoded en compressed Postscript file, figures avaible
under requests. To be published in A\&
Anisotropic Hubble expansion of large scale structures
We investigate the dynamics of an homogenous distribution of galaxies moving
under the cosmological expansion through Euler-Poisson equations system. The
solutions are interpreted with the aim of understanding the cosmic velocity
fields in the Local Super Cluster, and in particular the presence of a bulk
flow. Among several solutions, we shows a planar kinematics with constant
(eternal) and rotational distortion, the velocity field is not potential
Exact Solutions in Multidimensional Cosmology with Shear and Bulk Viscosity
Multidimensional cosmological model describing the evolution of a fluid with shear and bulk viscosity in Ricci-flat spaces is investigated. The barotropic equation of state for the density and the pressure in each space is assumed. The second equation of state is chosen in the form when the bulk and the shear viscosity coefficients are inversely proportional to the volume of the Universe. The integrability of Einstein equations reads as a colinearity constraint between vectors which are related to constant parameters in the first and second equations of state. We give exact solutions in a Kasner-like form. The processes of dynamical compactification and the entropy production are discussed. The non-singular -dimensional isotropic viscous solution is singled out
About the Malmquist bias in the determination of H0 and of distances of galaxies
We provide the mathematical framework which elucidates the way of using a
Tully-Fisher (TF) like relation in the determination of the Hubble constant
, as well as for distances of galaxies. The methods related to the
so-called Direct and Inverse TF Relations (herein DTF and ITF) are interpreted
as maximum likelihood statistics. We show that, as long as the same model is
used for the calibration of the TF relation and for the determination of ,
we obtain a coherent Hubble's constant. The choice of the model is motivated by
reasons of robustness of statistics, it depends on selection effects in
observation which are present in the sample. The difference on the distance
estimates when using either the ITF or the DTF model is only due to random
fluctuations. It is interesting to point out that the DTF estimate does not
depend on the luminosity distribution of sources. Both statistics show a
correction for a bias, inadequately believed to be of Malmquist type. The
repercussion of measurement errors, and additional selection effects are also
analyzedComment: 37 pages,cpt-93/p.2808,latex A&A,4fig available on cpt.univ-mrs.fr
directory ftp/pub/preprints/93/cosmology/93-P.280
The determination of by using the TF relation : About particular selection effects
This paper completes the statistical modeling of the Hubble flow when a
Tully-Fisher type relation is used for estimating the absolute magnitude
from a line width distance indicator . Our investigation
is performed with the aim of providing us with a full understanding of
statistical biases due to selection effects in observation, regardless of
peculiar velocities of galaxies. We show that unbiased -statistics
can be obtained by means of the maximum likelihood method as long as the
statistical model can be defined. We focus on the statistical models related to
the Direct, resp. Inverse, Tully-Fisher relation, when selection effects on
distance, resp. on , are present. It turns out that the use of the Inverse
relation should be preferred, according to robustness criteria. The formal
results are ensured by simulations with samples which are randomly generated
according to usual characteristics.Comment: 8 pages, Postscript compressed file, to be published in A\&
Northernmost record of the pantropical portunid crab <i>Cronius ruber</i> in the eastern Atlantic (Canary Islands): natural range extension or human-mediated introduction?
The pantropical crab Cronius ruber (Lamarck, 1818) (Brachyura: Portunidae) is recorded for the first time from the Canary Islands. Previously known from off Cape Verde Islands and Senegal, this is the northernmost record of the species in the eastern Atlantic Ocean. Crabs have been caught by means of a collecting small trap for sampling in shallow waters, and then identified by both morphological characters and DNA barcoding (16S). Cytochrome c oxidase I partial sequence has been obtained for this species for the first time. This relatively large and very aggressive crab species seems to be rapidly occupying both hard substrates (sublittoral caves) and soft substrates (sand with seagrass meadow) adjacent to shallow rocky bottoms, at depths between 2 and 10 m, in the warm southern waters of Gran Canaria Island. The reasons for this species’ occurrence are discussed herein. Among them, natural range extension may be a consequence of tropicalization in the eastern Atlantic. Also, a human-mediated introduction could be based on the heavy traffic of ships (ballast waters or oil platforms) arriving at the Canary Islands from African countries and from Brazil in the last decade
Cambios en la composición de la captura y de la captura accesoria y en diversidad especÃfica en una pesquerÃa con nasas camaroneras semi-flotantes en tres ecosistemas insulares del Atlántico oriental sujetos a distinta presión antrópica
Composition and bycatch of semi-floating shrimp-trap fisheries (SSTF) were compared among areas with different levels of anthropogenic alteration of marine ecosystems. The three areas selected were Madeira, the Canary Islands and Cape Verde. Mean species richness and diversity of the SSTF did not show significant differences among areas. The dominant species in catches of the SSTF for all regions studied was the main target species, Plesionika edwardsii, which accounted for 96.0% of the catch in Cape Verde, 75.8% in Madeira and 59.1% in the Canary Islands. Targeted pandalid shrimps accounted for more than 96.8% of total catches for all areas combined. Numbers of non-target species caught were 18 (Madeira), 14 (Canary Islands) and 16 (Cape Verde), of which 13 (Madeira), 8 (Canary Islands) and 11 (Cape Verde) were always discarded. Bycatch accounted for 0.5% (Madeira), 0.7% (Canary Islands) and 3.1% (Cape Verde) in numbers. Shark species accounted for 0.11% of all individuals caught. A total of 5 species in Madeira, 6 in the Canary Islands and 4 in Cape Verde, accounting for 0.2% to 0.8% of total catches, were not landed due to the small size of individuals or low numbers of individuals caught (self-consumption). The present results suggest that the selectivity of traps for the main target species, P. edwardsii, in SSTF changes due to changes in species dominance, which are probably linked to the degree of human fishing exploitation of the marine ecosystems in each area.Se comparó la composición de las capturas y la captura accesoria de la pesquerÃa de camarones con nasas semi-flotantes (SSTF) entre áreas con diferentes niveles de alteración de sus ecosistemas marinos por la acción humana. Se seleccionaron tres áreas: Madeira, Canarias y Cabo Verde. La riqueza de especies y la diversidad de la pesquerÃa no mostraron diferencias significativas entre áreas. La especie dominante en las capturas de la SSTF fue la principal especie objetivo, Plesionika edwardsii, en las tres regiones estudiadas, variando desde 96.0% (Cabo Verde) hasta 59.1% (Canarias), con 75.8% en Madeira. Los camarones pandálidos objetivo de la pesca representaron más del 96.8% de la captura total en el conjunto de las tres áreas. El número de especies no-objetivo capturadas fue 18 (Madeira), 14 (Canarias) y 16 (Cabo Verde), de las cuales siempre fueron descartadas 13 especies en Madeira, 8 en Canarias y 11 en Cabo Verde. El porcentaje de individuos (en número) de las capturas accesorias en relación con las capturas totales representó el 0.5% en Madeira, 0.7% en Canarias y 3.1% en Cabo Verde. Las especies de tiburones representaron solo el 0.11% de los individuos capturados. Cinco especies en Madeira, seis en Canarias y cuatro en Cabo Verde representaron entre 0.2 y 0.8% del total de las capturas no desembarcadas debido a su pequeño tamaño o al bajo número de individuos capturados (auto consumo). Los resultados obtenidos sugieren que las nasas de la SSTF cambian sus caracterÃsticas de selectividad con respecto a la especie objetivo principal, P. edwardsii, debido a cambios relativos entre las especies dominantes, lo que probablemente está relacionado con la explotación pesquera de los ecosistemas marinos en cada área
On the presence of Trachinus pellegrini (Trachinidae) in the Canary and Cape Verde Islands (north-eastern Atlantic)
Présence de Trachinus pellegrini (Trachinidae) aux îles Canaries etCanaries et aux îles du Cap-Vert (Atlantique nord-est).
Trachinus pellegrini Cadenat, 1937 est signalée pour la première fois aux îles Canaries, ce qui représente sa limite de répartition la plus septentrionale. Les différences morphologiques entre adultes
et juvéniles sont également présentées. La présence de cette espèce aux îles du Cap-Vert est aussi confirmée.Postprin
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