500 research outputs found
Revisiting the asymptotic dynamics of General Relativity on AdS
The dual dynamics of Einstein gravity on AdS supplemented with boundary
conditions of KdV-type is identified. It corresponds to a two-dimensional field
theory at the boundary, described by a novel action principle whose field
equations are given by two copies of the "potential modified KdV equation". The
asymptotic symmetries then transmute into the global Noether symmetries of the
dual action, giving rise to an infinite set of commuting conserved charges,
implying the integrability of the system. Noteworthy, the theory at the
boundary is non-relativistic and possesses anisotropic scaling of Lifshitz
type.Comment: 18 page
Reaction Kinetics in the Production of Pd Nanoparticles in Reverse Microemulsions. Effect on Particle Size
In the synthesis of metallic nanoparticles in microemulsions, we hypothesized
that particle size is mainly controlled by the reaction rate. Thus, the changes
observed on the particle sizes as reaction conditions, such as concentrations,
temperature, type of surfactant used, etc., are varied should not be correlated
directly to the modification of those conditions but indirectly to the changes
they produce on the reaction rates. By means of time resolved UV-vis
spectroscopy, we measured the reaction rates in the production of Pd
nanoparticles inside microemulsions at different reactant concentrations,
keeping all the other parameters constant. The measured reaction rates were
then correlated with the particle sizes measured by transmission electron
microscopy (TEM). We found that nanoparticle size increases linearly as the
reaction rates increases, independently of the actual reactant concentrations.
We proposed that the kinetics is controlled mainly by the diffusion of the
reducing agent through the surfactant monolayer covering the microemulsion
membrane. With this model, we predicted that particle size should depend
indirectly, via the reaction kinetics, on the micelle radius (v0 ~ r^-3), the
water volume (v0~vw^3) and the total microemulsion volume (v0~vT^-3), and
temperature (Arrhenius). Some of these predictions were explored in this
article
Selection bias due to conditioning on a collider
Effect estimates may be biased when the study design or the data analysis is conditional on a collider—a variable that is caused by two other variables. Causal directed acyclic graphs are a helpful tool to identify colliders that may introduce selection bias in observational research.S
Revisiting the g-null paradox
The parametric g-formula is an approach to estimating causal effects of
sustained treatment strategies from observational data. An often cited
limitation of the parametric g-formula is the g-null paradox: a phenomenon in
which model misspecification in the parametric g-formula is guaranteed under
the conditions that motivate its use (i.e., when identifiability conditions
hold and measured time-varying confounders are affected by past treatment).
Many users of the parametric g-formula know they must acknowledge the g-null
paradox as a limitation when reporting results but still require clarity on its
meaning and implications. Here we revisit the g-null paradox to clarify its
role in causal inference studies. In doing so, we present analytic examples and
a simulation-based illustration of the bias of parametric g-formula estimates
under the conditions associated with this paradox. Our results highlight the
importance of avoiding overly parsimonious models for the components of the
g-formula when using this method
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