The cluster abundance in cosmic string models for structure formation

We use the present observed number density of large X-ray clusters to constrain the amplitude of matter density perturbations induced by cosmic strings on the scale of $8 h^{-1}$Mpc ($\sigma_8$), in both open cosmologies and flat models with a non-zero cosmological constant. We find a slightly lower value of $\sigma_8$ than that obtained in the context of primordial Gaussian fluctuations generated during inflation. This lower normalization of $\sigma_8$ results from the mild non-Gaussianity on cluster scales, where the one point probability distribution function is well approximated by a $\chi^2$ distribution. We use our estimate of $\sigma_8$ to constrain the string linear energy density $\mu$ and show that it is consistent with the COBE normalization.Comment: 5 pages, 2 figure

Cosmic string loops and large-scale structure

We investigate the contribution made by small loops from a cosmic string network as seeds for large-scale structure formation. We show that cosmic string loops are highly correlated with the long-string network on large scales and therefore contribute significantly to the power spectrum of density perturbations if the average loop lifetime is comparable to or above one Hubble time. This effect further improves the large-scale bias problem previously identified in earlier studies of cosmic string models.Comment: 5 pages, 5 figure

Effects of Inflation on a Cosmic String Loop Population

We study the evolution of simple cosmic string loop solutions in an inflationary universe. We show, for the particular case of circular loops, that periodic solutions do exist in a de Sitter universe, below a critical loop radius $R_c H=1/2$. On the other hand, larger loops freeze in comoving coordinates, and we explicitly show that they can survive more $e$-foldings of inflation than point-like objects. We discuss the implications of these findings for the survival of realistic cosmic string loops during inflation, and for the general characteristics of post-inflationary cosmic string networks. We also consider the analogous solutions for domain walls, in which case the critical radius is $R_c H=2/3$.Comment: 5 pages, 5 figures, accepted for publication in Physical Review

Matter power spectrum for the generalized Chaplygin gas model: The relativistic case

The generalized Chaplygin gas (GCG) model is the prototype of a unified model of dark energy (DE) and dark matter (DM). It is characterized by equation-of-state (EoS) parameters $A$ and $\alpha$. We use a statistical analysis of the 2dFGRS data to constrain these parameters. In particular, we find that very small (close to zero) and very large values ($\alpha\gg 1$) of the equation-of-state parameter $\alpha$ are preferred. To test the validity of this type of unification of the dark sector we admit the existence of a separate DM component in addition to the Chaplygin gas and calculate the probability distribution for the fractional contributions of both components to the total energy density. This analysis favors a model for which the Universe is nearly entirely made up of the separate DM component with an almost negligible Chaplygin gas part. This confirms the results of a previous Newtonian analysis.Comment: Latex file, 8 pages, 15 figures in eps forma

Mechanisms of pathogenic transmission in agroforests: the role of leaf functional traits in shade coffee systems

Coffee leaf rust (CLR), a pathogenic disease that results in premature leaf fall and significant yield loss, has become a major concern in coffee-growing regions, especially in Central and South America, after recent intense epidemics. CLR is caused by the basidiomycete fungus Hemileia vastatrix Berk. et Br., an obligate parasite, which infects new leaves via dispersed urediospores. While there is evidence of biochemical responses of susceptible Coffea varieties to H. vastatrix invasion, these responses occur too late rendering them ineffective. An important system component that has not been thoroughly investigated is the leaf morphology of both coffee plants and shade trees, which may have a potential coordinated role in CLR incidence and severity on a plot-scale. We hypothesize that (1) coffee leaves with enhanced toughness traits, reduced stomatal density, and greater leaf inclination angle will negatively affect CLR; and (2) shade trees with closed canopy architecture and lower leaf inclination angles in the lower canopy stratum will strongly reduce rain throughfall kinetic energy (TKE) thus moderating H. vastatrix invasion. Preliminary results suggest that coffee leaf traits have variable effects on CLR severity, differing between shade management systems, which are similarly reflected in the TKE variability within the shade tree stratum. These preliminary findings advance our understanding of non-chemical resistance measures available in agroecological systems, which may lead to more sustainable and more economical options to battle CLR and pathogen invasions in general. (Texte intégral

Localized D-dimensional global k-defects

We explicitly demonstrate the existence of static global defect solutions of arbitrary dimensionality whose energy does not diverge at spatial infinity, by considering maximally symmetric solutions described by an action with non-standard kinetic terms in a D+1 dimensional Minkowski space-time. We analytically determine the defect profile both at small and large distances from the defect centre. We verify the stability of such solutions and discuss possible implications of our findings, in particular for dark matter and charge fractionalization in graphene.Comment: 6 pages, published versio

Problemas de homogeneización en la Ingenieria. Una experimentación numérica

Se expone brevemente el problema matemático de homogeneización, en sus facetas: homogeneización en el dominio y en el contorno. Con respecto a esta última, se introducen los conceptos de talla crítica y problema límite. Los resultados obtenidos se aplican a un caso particular representado por un cilindro con condiciones de borde periódicas. Se comprueba la validez del desarrollo teórico mediante una experimentación numérica utilizando elementos finitos 3-D observándose la velocidad de convergencia que se obtiene en relación con el caso límite. Finalmente se comenta la posibilidad de utilizar la técnica de homogeneización para obtener unos criterios hacia un diseño eficiente y óptimo de los resultados de las barras corrugadas en el hormigón armado.Peer Reviewe

Topological defects: A problem for cyclic universes?

We study the behaviour of cosmic string networks in contracting universes, and discuss some of their possible consequences. We note that there is a fundamental time asymmetry between defect network evolution for an expanding universe and a contracting universe. A string network with negligible loop production and small-scale structure will asymptotically behave during the collapse phase as a radiation fluid. In realistic networks these two effects are important, making this solution only approximate. We derive new scaling solutions describing this effect, and test them against high-resolution numerical simulations. A string network in a contracting universe, together with the gravitational radiation background it has generated, can significantly affect the dynamics of the universe both locally and globally. The network can be an important source of radiation, entropy and inhomogeneity. We discuss the possible implications of these findings for bouncing and cyclic cosmological models.Comment: 11 RevTeX 4 pages, 6 figures; version to appear in Phys. Rev.

Generalized Chaplygin Gas in a modified gravity approach

We study the generalized Chaplygin gas (GCG) scenario in a modified gravity approach. That is, we impose that our universe has a pure dust configuration, and allow for a modification of gravity that yields a GCG specific scale factor evolution. Moreover, assuming that this new hypothetical gravity theory obeys a generalization of Birkhoff's law, we determine the Schwarzschild-like metric in this new modified gravity. We also study the large scale structure formation in this model. Both the linear and non-linear growth are studied together with the growth of the velocity fluctuation in the linear perturbation theory. We compare our results with those corresponding to the $\Lambda$CDM model and discuss possible distinguishable features.Comment: 13 pages and 4 figures. Final version to appear in PR