Why all these prejudices against a constant?

The expansion of the observed universe appears to be accelerating. A simple explanation of this phenomenon is provided by the non-vanishing of the cosmological constant in the Einstein equations. Arguments are commonly presented to the effect that this simple explanation is not viable or not sufficient, and therefore we are facing the "great mystery" of the "nature of a dark energy". We argue that these arguments are unconvincing, or ill-founded.Comment: 9 pages, 4 figure

A note on the geometrical interpretation of quantum groups and non-commutative spaces in gravity

Quantum groups and non-commutative spaces have been repeatedly utilized in approaches to quantum gravity. They provide a mathematically elegant cut-off, often interpreted as related to the Planck-scale quantum uncertainty in position. We consider here a different geometrical interpretation of this cut-off, where the relevant non-commutative space is the space of directions around any spacetime point. The limitations in angular resolution expresses the finiteness of the angular size of a Planck-scale minimal surface at a maximum distance 1/\sqrt{Lambda} related the cosmological constant Lambda. This yields a simple geometrical interpretation for the relation between the quantum deformation parameter q=exp[i Lambda l_{Planck}^2 ] and the cosmological constant, and resolves a difficulty of more conventional interpretations of the physical geometry described by quantum groups or fuzzy spaces.Comment: 2 pages, 1 figur

Face amplitude of spinfoam quantum gravity

The structure of the boundary Hilbert-space and the condition that amplitudes behave appropriately under compositions determine the face amplitude of a spinfoam theory. In quantum gravity the face amplitude turns out to be simpler than originally thought.Comment: 5 pages, 2 figure

Towards Spinfoam Cosmology

We compute the transition amplitude between coherent quantum-states of geometry peaked on homogeneous isotropic metrics. We use the holomorphic representations of loop quantum gravity and the Kaminski-Kisielowski-Lewandowski generalization of the new vertex, and work at first order in the vertex expansion, second order in the graph (multipole) expansion, and first order in 1/volume. We show that the resulting amplitude is in the kernel of a differential operator whose classical limit is the canonical hamiltonian of a Friedmann-Robertson-Walker cosmology. This result is an indication that the dynamics of loop quantum gravity defined by the new vertex yields the Friedmann equation in the appropriate limit.Comment: 8 page

Monte Carlo simulations of dusty spiral galaxies: extinction and polarization properties

We present Monte Carlo simulations of dusty spiral galaxies, modelled as bulge + disk systems, aimed to study their extinction and polarization properties. The extinction parameters (absorption and scattering) of dust grains are calculated from Mie's theory for a full distribution of sizes and materials; the radiation transfer is carried on for the four Stokes parameters. Photometric and polarimetric maps of galaxies of different optical depths, inclinations and bulge-to-total ratios have been produced in the B and I bandpasses. As expected, the effect of scattering is to reduce substantially the extinction for a given optical depth, in particular for what concerns the obscuration of bright bulge cores. For the same reason, scattering reduces also the reddening, as evaluated from B-I maps. On the other hand the bluing directly due to forward scattering is hardly appreciable. Radial color gradients are often found. A comparison with ``sandwich'' models shows that they fail dramatically to reproduce the extinction - optical depth relation. The degree of linear polarization produced by scattering is usually of the order of a few percent; it increases with optical depth, and with inclination (less than 80 degrees). The polarization pattern is always perpendicular to the major axis, unless the dust distribution is drastically modified. There is little local correlation between extinction and polarization degree and there is a trend of increasing polarization from the B to the I band. We discuss implications and relevance of the results for studies of the structure and morphology of spiral galaxies and of their interstellar medium.Comment: 43 pages, 13 Postscript figures, Latex AAS manuscript substyle, Ap. J. Accepte

Non-tachyonic open descendants of the 0B string theory

We use the crosscap constraint to construct open descendants of the 0B string compactified on $T^6 /Z_3$ and on $T^4/Z_2$ free of tachyons both in the closed and in the open unoriented sectors. In four dimensions the construction results in a Chan-Paton gauge group $U(8)\otimes U(12)\otimes U(12)$ with three generations of chiral fermions in the representations $(\overline{8},1,\overline{12}) +(8,12,1)+(1,\overline{66},1)+(1,1,66)$.Comment: 13 pages, Latex. Minor corrections. Final version to appear on Phys.Lett.

Understanding the language of unexpected: a critical and experimental approach to enhance the productive defect

No processamento industrial, os erros e marcas de produção são eliminados porque vistos como aspetos que diminuem o valor do objeto, quando visíveis. Esses fatores considerados comummente como ‘defeitos’, no entanto, evidenciam a narrativa do processo produtivo, fazendo, portanto, parte da história do objeto. Partindo do estado de design e produção de assuntos sobre produtos industriais e artesanais, a tese pretende investigar e analisar novos métodos de projeto relacionados à experimentação de forma capaz de deixar traços e significados capazes de permanecer impressos em a superfície e a memória do objeto e daqueles que o usam. Em particular, tem como objetivo trazer o valor do processo de imperfeição em objetos durante a fase executiva. Através de um processo experimental de Auto produção, este projeto quer dizer, através do uso de material cerâmico, a mutação da forma de um objeto através da deterioração do molde investigando, ao mesmo tempo, a estética processual. O objetivo é o de desenhar uma diretriz específica útil para a análise crítica das tecnologias de produção.In industrial processing errors are eliminated because considered as defect, if visible, as elements that decrease the value of the object. These imperfections are commonly considered ‘defects’; however, they reveal the narrative of the production process and part of the story of the object. From the design, the craftsmanship, and the industrial process as a whole, the thesis intends to investigate and analyse new design methods related to experimenting shapes, that are able to leave traces and meanings related to the object’s past for those who use it. In particular, it aims to bring the value of imperfection into objects. This project wants to convey to the consumer, that through the use of ceramic material, the change in shape is the result of the mould’s deterioration over time. The aim is to draw a useful specific guideline for the Technology production

Discrete Deformations in Type I Vacua

We study supersymmetric orientifolds where the world-sheet parity transformation is combined with a conjugation of some compact complex coordinates. We investigate their T-duality relation to standard orientifolds and discuss the origin of continuous and discrete moduli. In contrast to standard orientifolds, the antisymmetric tensor describes a continuous deformation, while the off-diagonal part of the metric is frozen to quantized values and is responsible for the rank reduction of the gauge group. We also give a geometrical interpretation of some recently constructed six-dimensional permutational orientifolds.Comment: 12 pages, TeX, harvmac, 4 figures, eq.(3.15) change