Is Dark Energy an illusion?

Much evidence has accumulated that within the context of general relativistic Friedmann-Robertson-Walker (FRW) cosmology there must exist a new, and gravitationally repulsive, substance in the Universe. The effect of this new type of energy density on the expansion of the Universe is to cause its acceleration, and the name that is given to it is ‘Dark Energy’. To say whether or not Dark Energy really exists, however, requires a definite model for the Universe. That is, to be sure of the existence of Dark Energy, and the cosmological acceleration it causes, we must first be sure of the cosmological model we are using to interpret our observations. This is the subject of the present contribution, which will concentrate on the observational status of the Copernican Principle, which is at the heart of the FRW model. In particular, we will outline recent progress that has been made toward answering the question ‘can the observations usually requiring the existence of Dark Energy be accounted for without introducing any new and exotic types of energy density, if we are prepared to give up some of the assumptions of the standard cosmological model?’, or, alternatively, ‘is Dark Energy an illusion?’.     

Is equilibrium always an entropy maximum?

A systematic development is given of the view that in the case of systems with long-range forces and which are therefore nonextensive (in some sense) some thermodynamic results do not hold. Among these is the relationU – TS + p = N and the Gibbs-Duhem equation. If a search for an equilibrium state is made by maximization of the entropy one may obtain misleading results because superadditivity may be violated. The considerations are worked out for a simple gas model, but they are relevant to black hole thermodynamics. Rather general conclusions can be drawn which transcend special systems.Based on a seminar given at the University of Florida, April 1983.On leave of absence from the Faculty of Mathematical Studies, University, Southampton S09 5NH, England.     

Electric flow through an ideal ferromagnet–superconductor junction

It is investigated the possibility of controlling the electric flow through a ferromagnet–superconductor junction by spin polarization, within a simple, ideal model of a perfect ferromagnetic–superconductor junction. The ferromagnetic and superconducting properties as well as the Andreev reflection are briefly reviewed and the electrical resistance of the junction is computed both in the diffusive and ballistic regime for the ferromagnetic sample. It is shown that the resistance of the junction increases with increasing magnetization, including both positive or negative jumps on passing from the ballistic to the diffusive regime.     

Fröhlich electron-phonon interaction in CdSxSe1-xnanocrystals

We demonstrate that the polar electron-phonon coupling in CdS_xSe_1-x nanocrystals increases with decreasing crystallite size. This result is supported by the size dependence of the Huang-Rhys parameter measured by absorption and photoluminescence spectroscopy and by microprobe resonant Raman measurements which allow us to determine the size dependence of the electron-LO phonon coupling and the relative strength between Fröhlich and deformation potential Raman polarizabilities.     

Is There an Optimal Substrate Geometry for Wetting?

Within a 1+1-dimensional SOS type model with a periodic rough substrate, we show that the differential wall tension, which governs wetting, has a maximum as a function of a certain aspect ratio of the substrate. This result is based on a low-temperature expansion leading, in a first approximation, to Wenzel's law for the wall tension and allowing us to study the corrections to this law. It implies that the contact angle is minimum for a substrate with the corresponding aspect ratio. Our results are in agreement and explain recent numerical simulations.     

Metastable π junction between an s(±)-wave and an s-wave superconductor

We examine a contact between a superconductor whose order parameter changes sign across the Brillioun zone, and an ordinary, uniform-sign superconductor. Within a Ginzburg-Landau-type model, we find that if the barrier between the two superconductors is not too high, the frustration of the Josephson coupling between different portions of the Fermi surface across the contact can lead to surprising consequences. These include time-reversal symmetry breaking at the interface and unusual energy-phase relations with multiple local minima. We propose this mechanism as a possible explanation for the half-integer flux quantum transitions in composite niobium-iron pnictide superconducting loops, which were discovered in recent experiments [C.-T. Chen et al., Nature Phys. 6, 260 (2010).].     

Is Phosphorescence Lifetime an Indicator of Angiogenesis in Cortical Sarcoma?

In this paper, the oxygen dependent phosphorescence quenching method is proposed to study the correlation between the phosphorescence lifetime and microvessel density in tumors. In the implementation, the S180 transplanted tumor in the mouse is used for the collection of the time-resolved phosphorescence, the tumor microvessel density is measured by immunohistochemical examination of FVIII, the correlation between microvessel density and phosphorescence lifetime is analyzed by multiple regression method. The results show the phosphorescence decay constant measured in tumors is enlarged in the tumor progression. Furthermore, the relative total microvessel area is positively correlative with the phosphorescence lifetime, which is estimated by a two dimension regression equation. It is concluded phosphorescence lifetime is a valuable indicator of angiogenesis during the tumor development.     

Is multiscaling an artifact in the stochastically forced Burgers equation?

We study turbulence in the one-dimensional Burgers equation with a white-in-time, Gaussian random force that has a Fourier-space spectrum approximately 1/k, where k is the wave number. From very high-resolution numerical simulations, in the limit of vanishing viscosity, we find evidence for multiscaling of velocity structure functions which cannot be falsified by standard tests. We find a new artifact in which logarithmic corrections can appear disguised as anomalous scaling and conclude that bifractal scaling is likely.     

Microscopic theory of electron-phonon interaction and superconductivity of BaPb1−xBixO3

Electron-phonon coupling coefficients in BaPb_1−xBi_xO₃ are calculated microscopically on the basis of the tight-binding band model which utilizes energy band structures obtained with the use of the self-consistent LAPW method by Mattheiss and Hamann. Assuming the rigid-band model we calculate, as a function of Bi concentration x, the dimension-l less electron-phonon coupling constant λ in McMillan's strong-coupling theory of superconductor. We obtain large values of λ such as λ>1 for x>0.1. The origin of this large electron-phonon coupling is ascribed to: (i) the hybridization between 2pσ orbitals of 0 atoms and 6s orbitals of Pb or Bi atoms is large in the conduction band, (ii) this σ-bonding is strongly affected by a vibrational motion of 0 atoms in the direction connecting the 0 and Pb (Bi) atoms, and (iii) the mass of 0 atoms is light. We also evaluate superconducting transition temperature T_c as a function of x on the basis of the McMillan-Allen-Dynes equation. The results explain semi-quantitatively the observed x-dependence of T_c.     

Pseudogap and Superconducting Gap in SmFeAs（O1-xFx） Superconductor from Photoemission Spectroscopy

High resolution photoemission measurements are carried out on non-superconducting SmOFeAs paxent compound and superconducting SmFeAs（O1-xFx） （x = 0.12, and 0.15） compounds. The momentum-integrated spectra exhibit a clear Fermi cutoff that shows little leading-edge shift in the superconducting state. A robust feature at 13 meV is identified in all these samples. Spectral weight suppression near IF, F with decreasing temperature is observed in both undoped and doped samples that points to a possible existence of a pseudogap in these Fe-based compounds.