Brane-World Gravity

The observable universe could be a 1+3-surface (the "brane") embedded in a 1+3+\textit{d}-dimensional spacetime (the "bulk"), with Standard Model particles and fields trapped on the brane while gravity is free to access the bulk. At least one of the \textit{d} extra spatial dimensions could be very large relative to the Planck scale, which lowers the fundamental gravity scale, possibly even down to the electroweak ($\sim$ TeV) level. This revolutionary picture arises in the framework of recent developments in M theory. The 1+10-dimensional M theory encompasses the known 1+9-dimensional superstring theories, and is widely considered to be a promising potential route to quantum gravity. At low energies, gravity is localized at the brane and general relativity is recovered, but at high energies gravity "leaks" into the bulk, behaving in a truly higher-dimensional way. This introduces significant changes to gravitational dynamics and perturbations, with interesting and potentially testable implications for high-energy astrophysics, black holes, and cosmology. Brane-world models offer a phenomenological way to test some of the novel predictions and corrections to general relativity that are implied by M theory. This review analyzes the geometry, dynamics and perturbations of simple brane-world models for cosmology and astrophysics, mainly focusing on warped 5-dimensional brane-worlds based on the Randall--Sundrum models. We also cover the simplest brane-world models in which 4-dimensional gravity on the brane is modified at \emph{low} energies -- the 5-dimensional Dvali--Gabadadze--Porrati models. Then we discuss co-dimension two branes in 6-dimensional models.Comment: A major update of Living Reviews in Relativity 7:7 (2004) "Brane-World Gravity", 119 pages, 28 figures, the update contains new material on RS perturbations, including full numerical solutions of gravitational waves and scalar perturbations, on DGP models, and also on 6D models. A published version in Living Reviews in Relativit

π-π stacking tackled with density functional theory

Through comparison with ab initio reference data, we have evaluated the performance of various density functionals for describing π-π interactions as a function of the geometry between two stacked benzenes or benzene analogs, between two stacked DNA bases, and between two stacked Watson–Crick pairs. Our main purpose is to find a robust and computationally efficient density functional to be used specifically and only for describing π-π stacking interactions in DNA and other biological molecules in the framework of our recently developed QM/QM approach "QUILD". In line with previous studies, most standard density functionals recover, at best, only part of the favorable stacking interactions. An exception is the new KT1 functional, which correctly yields bound π-stacked structures. Surprisingly, a similarly good performance is achieved with the computationally very robust and efficient local density approximation (LDA). Furthermore, we show that classical electrostatic interactions determine the shape and depth of the π-π stacking potential energy surface

Magnetic PDMS microparticles for biomedical and energy applications

Polydimethylsiloxane (PDMS) is one of the most widely used polymers in microfluidics. Furthermore, magnetic nanoparticles (MNPs), due their superior thermal properties, are also gaining a great interest among the industry and microfluidic scientific community. In this work, a technique based on a flow focusing principle was used to produce magnetic PDMS microparticles. A microvisualization system composed by digital video cameras and optical lenses was used to control and measure the size of the obtained microparticles. To the best of our knowledge, this is the first work that shows magnetic PDMS microparticles able to be used for both biomedical and energy applications.This work was supported by Fundação para a Ciência e a Tecnologia (FCT) under the strategic grants UID/EMS/04077/2019, UID/EEA/04436/2019 and UID/EMS/00532/2019. The authors are also grateful for the funding of FCT through the projects POCI-01-0145-FEDER-016861, POCI-01-0145-FEDER-028159, NORTE-01-0145-FEDER-029394, NORTE-01-0145-FEDER-030171, funded by COMPETE2020, NORTE2020, PORTUGAL2020, and FEDER, and the PhD grant SFRH/BD/91192/2012. The authors also acknowledge to FCT for partially financing the research under the framework of the project UTAP-EXPL/CTE/0064/2017, financiado no âmbito do Projeto 5665 - Parcerias Internacionais de Ciência e Tecnologia, UT Austin Programme. Partial support from the Spanish Ministry of Science and Education (grant no. DPI2016-78887) and Junta de Extremadura (grants no. GR15014 and IB18005, partially financed by FEDER funds) are gratefully acknowledged too

Influence of the leaving group on the dynamics of a gas-phase SN2 reaction

In addition to nucleophile and solvent, the leaving group has a significant influence on nucleophilic substitution (SN2) reactions. Its role is frequently discussed with respect to reactivity, but its influence on the reaction dynamics remains obscured. Here, we uncover the influence of the leaving group on the gas phase dynamics of SN2 reactions in a combined approach of crossed-beam imaging and dynamics simulations. We have studied the reaction F- + CH3Cl and compared it to F- + CH3I. For the two leaving groups Cl and I we find very similar structures and energetics, but the dynamics show qualitatively different features. Simple scaling of the leaving group mass does not explain these differences. Instead, the relevant impact parameters for the reaction mechanisms are found to be crucial, which is attributed to the relative orientation of the approaching reactants. This effect occurs on short time scales and may also prevail under solution phase conditions