81,318 research outputs found
On the stability of spherically symmetric spacetimes in metric f(R) gravity
We consider stability properties of spherically symmetric spacetimes of stars
in metric f(R) gravity. We stress that these not only depend on the particular
model, but also on the specific physical configuration. Typically
configurations giving the desired are strongly
constrained, while those corresponding to are
less affected. Furthermore, even when the former are found strictly stable in
time, the domain of acceptable static spherical solutions typically shrinks to
a point in the phase space. Unless a physical reason to prefer such a
particular configuration can be found, this poses a naturalness problem for the
currently known metric f(R) models for accelerating expansion of the Universe.Comment: Published version, 9 pages, 3 figure
Ligand design for site-selective installation of Pd and Pt centers to generate homo- and heteropolymetallic motifs
The modular synthesis of a series of nitrogen-rich polydentate ligands that feature a common pincer-type framework is reported. These ligands allow for site-selective installation of palladium and platinum to give rise to bi- and trimetallic complexes that have d^(8)βd^(8) interactions
Activity Level-Predation Risk Tradeoff in a Tadpole Guild: Implications for Community Organization Along the Hydroperiod Gradient
Increasing activity levels permit greater food intake for use towards growth and reproduction, consequently increasing predation risk via increased detection. Larval anurans are models to examine activity level-predation risk tradeoffs, as they occupy a variety of lentic habitats that impose constraints on the distribution and abundance of species. Ephemeral ponds have a low abundance and diversity of predators and as a result tadpole species tend to have high foraging rates for rapid development. Permanent ponds generally possess a greater diversity and abundance of predators and tadpoles inhabiting these locations tend to have low activity rates or chemical defenses to minimize predation risk. The objective of this research was to examine how interspecific variation in activity level and response to predation risk, corresponds to the distributions of tadpole species along the hydroperiod gradient. Furthermore, we examined the intraspecific variation in activity level among the species. We conducted a series of laboratory experiments in which we quantified baseline activity patterns and the change in activity after the addition of a predator or exposure to alarm cues, for 12 species of larval anurans native to East Texas. Species that maintained a high activity level generally occupied ephemeral ponds and species that maintained low activity levels generally occupied permanent ponds. Only one species (Gastrophryne carolinensis) decreased their activity level in the presence of predator cues or conspecific alarm cues. These results highlight this tradeoff can have consequences on the life histories of multiple species, providing insight into how it affects the organization of ecological communities
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