Trans-Planckian Issues for Inflationary Cosmology

The accelerated expansion of space during the period of cosmological inflation leads to trans-Planckian issues which need to be addressed. Most importantly, the physical wavelength of fluctuations which are studied at the present time by means of cosmological observations may well originate with a wavelength smaller than the Planck length at the beginning of the inflationary phase. Thus, questions arise as to whether the usual predictions of inflationary cosmology are robust considering our ignorance of physics on trans-Planckian scales, and whether the imprints of Planck-scale physics are at the present time observable. These and other related questions are reviewed in this article.Comment: 32 pages, 11 figures; invited review for "Classical and Quantum Gravity

Sparse inverse covariance estimation with the lasso

We consider the problem of estimating sparse graphs by a lasso penalty applied to the inverse covariance matrix. Using a coordinate descent procedure for the lasso, we develop a simple algorithm that is remarkably fast: in the worst cases, it solves a 1000 node problem (~500,000 parameters) in about a minute, and is 50 to 2000 times faster than competing methods. It also provides a conceptual link between the exact problem and the approximation suggested by Meinhausen and Buhlmann (2006). We illustrate the method on some cell-signaling data from proteomics.Comment: submitte

Electron transfer through a single barrier inside a molecule: from strong to weak coupling

In all theoretical treatments of electron transport through single molecules between two metal electrodes, a clear distinction has to be made between a coherent transport regime with a strong coupling throughout the junction and a Coulomb blockade regime in which the molecule is only weakly coupled to both leads. The former case where the tunnelling barrier is considered to be delocalized across the system can be well described with common mean-field techniques based on density functional theory (DFT), while the latter case with its two distinct barriers localized at the interfaces usually requires a multideterminant description. There is a third scenario with just one barrier localized inside the molecule which we investigate here using a variety of quantum-chemical methods by studying partial charge shifts in biphenyl radical ions induced by an electric field at different angles to modulate the coupling and thereby the barrier within the $\pi$-system. We find steps rounded off at the edges in the charge versus field curves for weak and intermediate coupling, whose accurate description requires a correct treatment of both exchange and dynamical correlation effects is essential. We establish that DFT standard functionals fail to reproduce this feature, while a long range corrected hybrid functional fares much better, which makes it a reasonable choice for a proper DFT-based transport description of such single barrier systemsComment: 8 pages, 4 figures; J. Chem. Phys., in print (2012

Massive gravity and the suppression of anisotropies and gravitational waves in a matter-dominated contracting universe

We consider a modified gravity model with a massive graviton, but which nevertheless only propagates two gravitational degrees of freedom and which is free of ghosts. We show that non-singular bouncing cosmological background solutions can be generated. In addition, the mass term for the graviton prevents anisotropies from blowing up in the contracting phase and also suppresses the spectrum of gravitational waves compared to that of the scalar cosmological perturbations. This addresses two of the main problems of the matter bounce scenario.Comment: 20 pages, 2 figure

Bank privatization and performance - emprical evidence from Nigeria

The authors assess the effect of privatization on performance in a panel of Nigerian banks for the period 1990-2001. They find evidence of performance improvement in nine banks that were privatized, which is remarkable given the inhospitable environment for true financial intermediation. Their results also suggest negativeeffects of the continuing minority government ownership on the performance of many Nigerian banks. The authors'results complement aggregate indications of decreasing financial intermediation over the 1990s. Banks that focused on investment in government bonds and non-lending activities enjoyed a relatively higher performance.Municipal Financial Management,Banks&Banking Reform,Financial Crisis Management&Restructuring,Payment Systems&Infrastructure,International Terrorism&Counterterrorism,Banks&Banking Reform,Financial Crisis Management&Restructuring,Municipal Financial Management,Financial Intermediation,Economic Theory&Research