The evolution of density perturbations in f(R) gravity

We give a rigorous and mathematically well defined presentation of the Covariant and Gauge Invariant theory of scalar perturbations of a Friedmann-Lemaitre-Robertson-Walker universe for Fourth Order Gravity, where the matter is described by a perfect fluid with a barotropic equation of state. The general perturbations equations are applied to a simple background solution of R^n gravity. We obtain exact solutions of the perturbations equations for scales much bigger than the Hubble radius. These solutions have a number of interesting features. In particular, we find that for all values of n there is always a growing mode for the density contrast, even if the universe undergoes an accelerated expansion. Such a behaviour does not occur in standard General Relativity, where as soon as Dark Energy dominates, the density contrast experiences an unrelenting decay. This peculiarity is sufficiently novel to warrant further investigation on fourth order gravity models.Comment: 21 pages, 2 figures, typos corrected, submitted to PR

Cosmological dynamics of exponential gravity

We present a detailed investigation of the cosmological dynamics based on $\exp (-R/{\Lambda})$ gravity. We apply the dynamical system approach to both the vacuum and matter cases and obtain exact solutions and their stability in the finite and asymptotic regimes. The results show that cosmic histories exist which admit a double de-Sitter phase which could be useful for describing the early and the late-time accelerating universe.Comment: 17 pages LaTeX, 3 figure

Presence of Dalbulus maidis (Hemiptera: Cicadellidae) and of Spiroplasma kunkelii in the temperate region of Argentina

"Corn stunt" is one of the main corn (Zea mays L.) diseases in the Americas and Dalbulus maidis (DeLong & Wolcott) is the key vector of the pathogen Spiroplasma kunkelii Whitcomb. In Argentina, the corn-producing area is in the temperate region, where vector and pathogen prevalence levels are unknown. In this study, the prevalence and distribution of D. maidis and S. kunkelii in the temperate region of Argentina and D. maidis overwintering ability in this region were determined. Surveys were conducted in 2005-2006 and 2006-2007 seasons to determine D. maidis and S. kunkelii presence, and in winter 2006 to determine the vector overwintering ability. The highest S. kunkelii prevalence and incidence levels were found in the transition area from the temperate to the subtropical region, related to the highest D. maidis prevalence and insects sampled per location. D. maidis adults were found in volunteer corn plants and spontaneous vegetation in autumn and winter months, which were inoculative for the pathogen S. kunkelii. This overwintering ability was related to detection of D. maidis insects in corn crops at early growth stages in the following growing season. This work emphasizes that corn stunt disease is present in the temperate region of Argentina, and this highlights the need to develop proper agronomic practices like monitoring insect vector populations and controlling voluntary plants. This study also indicates that further research is needed to understand the potential yield reduction caused by this pathogen on symptomless plants and population dynamics of the insect vector.Facultad de Ciencias Naturales y Muse

The dark side of the moon: severe therapy-resistant asthma in children

Problematic severe asthma is the term used to describe children whose asthma is not responsive to standard therapy with high-dose inhaled corticosteroids and additional controllers. These children need to be assessed by a step-wise systematic protocol in order to confirm the diagnosis, evaluate co-morbidities, assess the adherence to treatment, and finally evaluate the basic management. More than half of these children have “difficult-to-treat asthma”, which improves if the basic management is correct. Children whose asthma remains uncontrolled despite resolution of any reversible factors are termed “severe therapy-resistant” asthmatics; for them, an individualised treatment plan is developed after a detailed and invasive protocol of investigation. Therapeutic options for these patients can be divided into medications used in lower doses for children with less severe asthma, and those used in other pediatric diseases but not for asthma. Most treatments are unlicensed and there is a lack of high-quality evidence. Children with recurrent severe exacerbations, in particular in the context of good baseline asthma control, are particularly difficult to treat, and there is no evidence on which therapeutic option to recommend. International collaborations, using standard protocols of investigation, are needed to better understand mechanisms of severe therapy-resistant asthma and to deliver evidence-based treatments in the future

Physical interactions between marine phytoplankton and PET plastics in seawater.

Plastics are the most abundant marine debris globally dispersed in the oceans and its production is rising with documented negative impacts in marine ecosystems. However, the chemical-physical and biological interactions occurring between plastic and planktonic communities of different types of microorganisms are poorly understood. In these respects, it is of paramount importance to understand, on a molecular level on the surface, what happens to plastic fragments when dispersed in the ocean and directly interacting with phytoplankton assemblages. This study presents a computer-aided analysis of electron paramagnetic resonance (EPR) spectra of selected spin probes able to enter the phyoplanktonic cell interface and interact with the plastic surface. Two different marine phytoplankton species were analyzed, such as the diatom Skeletonema marinoi and dinoflagellate Lingulodinium polyedrum, in absence and presence of polyethylene terephthalate (PET) fragments in synthetic seawater (ASPM), in order to insitu characterize the interactions occurring between the microalgal cells and plastic surfaces. The analysis was performed at increasing incubation times. The cellular growth and adhesion rates of microalgae in batch culture medium and on the plastic fragments were also evaluated. The data agreed with the EPR results, which showed a significant difference in terms of surface properties between the diatom and dinoflagellate species. Low-polar interactions of lipid aggregates with the plastic surface sites were mainly responsible for the cell-plastic adhesion by S. marinoi, which is exponentially growing on the plastic surface over the incubation time

The evolution of cosmological gravitational waves in f(R) gravity

We give a rigorous and mathematically clear presentation of the Covariant and Gauge Invariant theory of gravitational waves in a perturbed Friedmann-Lemaitre-Robertson-Walker universe for Fourth Order Gravity, where the matter is described by a perfect fluid with a barotropic equation of state. As an example of a consistent analysis of tensor perturbations in Fourth Order Gravity, we apply the formalism to a simple background solution of R^n gravity. We obtain the exact solutions of the perturbation equations for scales much bigger than and smaller than the Hubble radius. It is shown that the evolution of tensor modes is highly sensitive to the choice of n and an interesting new feature arises. During the radiation dominated era, their exist a growing tensor perturbation for nearly all choices of n. This occurs even when the background model is undergoing accelerated expansion as opposed to the case of General Relativity. Consequently, cosmological gravitational wave modes can in principle provide a strong constraint on the theory of gravity independent of other cosmological data sets.Comment: 19 pages, 4 figures; v2: corrected to match version accepted for publication in PR

The phase portrait of a matter bounce in Horava-Lifshitz cosmology

The occurrence of a bounce in FRW cosmology requires modifications of general relativity. An example of such a modification is the recently proposed Horava-Lifshitz theory of gravity, which includes a ``dark radiation'' term with a negative coefficient in the analog of the Friedmann equation. This paper describes a phase space analysis of models of this sort with the aim of determining to what extent bouncing solutions can occur. A simplification, valid in the relevant region, allows a reduction of the dimension of phase space so that visualization in three dimensions is possible. It is found that a bounce is possible, but not generic in models under consideration. Apart from previously known bouncing solutions some new ones are also described. Other interesting solutions found include ones which describe a novel sort of oscillating universes.Comment: 14 pages, 8 figure

Compactifying the state space for alternative theories of gravity

In this paper we address important issues surrounding the choice of variables when performing a dynamical systems analysis of alternative theories of gravity. We discuss the advantages and disadvantages of compactifying the state space, and illustrate this using two examples. We first show how to define a compact state space for the class of LRS Bianchi type I models in $R^n$-gravity and compare to a non--compact expansion--normalised approach. In the second example we consider the flat Friedmann matter subspace of the previous example, and compare the compact analysis to studies where non-compact non--expansion--normalised variables were used. In both examples we comment on the existence of bouncing or recollapsing orbits as well as the existence of static models.Comment: 18 pages, revised to match published versio

Cosmological dynamics of R^n gravity

A detailed analysis of dynamics of cosmological models based on $R^{n}$ gravity is presented. We show that the cosmological equations can be written as a first order autonomous system and analyzed using the standard techniques of dynamical system theory. In absence of perfect fluid matter, we find exact solutions whose behavior and stability are analyzed in terms of the values of the parameter $n$. When matter is introduced, the nature of the (non-minimal) coupling between matter and higher order gravity induces restrictions on the allowed values of $n$. Selecting such intervals of values and following the same procedure used in the vacuum case, we present exact solutions and analyze their stability for a generic value of the parameter $n$. From this analysis emerges the result that for a large set of initial conditions an accelerated expansion is an attractor for the evolution of the $R^n$ cosmology. When matter is present a transient almost-Friedman phase can also be present before the transition to an accelerated expansion.Comment: revised and extended version, 35 pages, 12 tables, 14 figures which are not included and can be found at http://www.mth.uct.ac.za/~peter/R

Axially symmetric solutions in f(R)-gravity

Axially symmetric solutions for f (R)-gravity can be derived starting from exact spherically sym- metric solutions achieved by Noether symmetries. The method takes advantage of a complex coordi- nate transformation previously developed by Newman and Janis in General Relativity. An example is worked out to show the general validity of the approach. The physical properties of the solution are also considered.Comment: 13 pages, 1 figure, to appear in Classical and Quantum Gravity 201