Non Singular Origin of the Universe and the Cosmological Constant Problem (CCP)

We consider a non singular origin for the Universe starting from an Einstein static Universe in the framework of a theory which uses two volume elements $\sqrt{-{g}}d^{4}x$ and $\Phi d^{4}x$, where $\Phi$ is a metric independent density, also curvature, curvature square terms, first order formalism and for scale invariance a dilaton field $\phi$ are considered in the action. In the Einstein frame we also add a cosmological term that parametrizes the zero point fluctuations. The resulting effective potential for the dilaton contains two flat regions, for $\phi \rightarrow \infty$ relevant for the non singular origin of the Universe and $\phi \rightarrow -\infty$, describing our present Universe. Surprisingly, avoidance of singularities and stability as $\phi \rightarrow \infty$ imply a positive but small vacuum energy as $\phi \rightarrow -\infty$. Zero vacuum energy density for the present universe is the "threshold" for universe creation.Comment: awarded an honorable mention in the Gravity Research Foundation 2011 Awards for Essays in Gravitation for 201

Non Singular Origin of the Universe and its Present Vacuum Energy Density

We consider a non singular origin for the Universe starting from an Einstein static Universe, the so called "emergent universe" scenario, in the framework of a theory which uses two volume elements $\sqrt{-{g}}d^{4}x$ and $\Phi d^{4}x$, where $\Phi$ is a metric independent density, used as an additional measure of integration. Also curvature, curvature square terms and for scale invariance a dilaton field $\phi$ are considered in the action. The first order formalism is applied. The integration of the equations of motion associated with the new measure gives rise to the spontaneous symmetry breaking (S.S.B) of scale invariance (S.I.). After S.S.B. of S.I., it is found that a non trivial potential for the dilaton is generated. In the Einstein frame we also add a cosmological term that parametrizes the zero point fluctuations. The resulting effective potential for the dilaton contains two flat regions, for $\phi \rightarrow \infty$ relevant for the non singular origin of the Universe, followed by an inflationary phase and $\phi \rightarrow -\infty$, describing our present Universe. The dynamics of the scalar field becomes non linear and these non linearities are instrumental in the stability of some of the emergent universe solutions, which exists for a parameter range of values of the vacuum energy in $\phi \rightarrow -\infty$, which must be positive but not very big, avoiding the extreme fine tuning required to keep the vacuum energy density of the present universe small. Zero vacuum energy density for the present universe defines the threshold for the creation of the universe.Comment: 28 pages, short version of this paper awarded an honorable mention by the Gravity Research Foundation, 2011, accepted for publication in International Journal of Modern Physics

Large-System Analysis of Multiuser Detection with an Unknown Number of Users: A High-SNR Approach

We analyze multiuser detection under the assumption that the number of users accessing the channel is unknown by the receiver. In this environment, users' activity must be estimated along with any other parameters such as data, power, and location. Our main goal is to determine the performance loss caused by the need for estimating the identities of active users, which are not known a priori. To prevent a loss of optimality, we assume that identities and data are estimated jointly, rather than in two separate steps. We examine the performance of multiuser detectors when the number of potential users is large. Statistical-physics methodologies are used to determine the macroscopic performance of the detector in terms of its multiuser efficiency. Special attention is paid to the fixed-point equation whose solution yields the multiuser efficiency of the optimal (maximum a posteriori) detector in the large signal-to-noise ratio regime. Our analysis yields closed-form approximate bounds to the minimum mean-squared error in this regime. These illustrate the set of solutions of the fixed-point equation, and their relationship with the maximum system load. Next, we study the maximum load that the detector can support for a given quality of service (specified by error probability).Comment: to appear in IEEE Transactions on Information Theor

Atom laser dynamics in a tight-waveguide

We study the transient dynamics that arise during the formation of an atom laser beam in a tight waveguide. During the time evolution the density profile develops a series of wiggles which are related to the diffraction in time phenomenon. The apodization of matter waves, which relies on the use of smooth aperture functions, allows to suppress such oscillations in a time interval, after which there is a revival of the diffraction in time. The revival time scale is directly related to the inverse of the harmonic trap frequency for the atom reservoir.Comment: 6 pages, 5 figures, to be published in the Proceedings of the 395th WE-Heraeus Seminar on "Time Dependent Phenomena in Quantum Mechanics ", organized by T. Kramer and M. Kleber (Blaubeuren, Germany, September 2007

Carles Jordà i Fages (1883-1935). Dirigent d’Acció Catalana i de la Unió Sindical Agrària

Carles Jordà i Fages neix a Figueres el 1883 i mor a Pont de Molins el 1935. És l’hereu d’una família d’hisendats que aviat diversificarà les seves inversions, de manera decidida, en el negoci bancari, la qual cosa permetrà que ell i les seves germanes es traslladin a estudiar a Barcelona. Estudiarà enginyeria i entrarà en contacte amb els nuclis catalanistes de la Lliga que el portaran a ser elegit per a càrrecs polítics a la Mancomunitat de Catalunya i a l’Ajuntament de Barcelona, on arribarà a ser alcalde accidental. Unes inversions amb mals resultats truncaran la seva vida barcelonina i el 1921 retornarà a l’Empordà. On manté la seva presència política i serà un dels fundadors d’Acció Catalana. Per la seva afiliació política, durant la dictadura de Primo de Rivera serà perseguit i exiliat a Casp. De retorn a l’Empordà, impulsa el cooperativisme agrari. Amb la II República mantindrà la seva adscripció política, però, sobretot, accentuarà la seva faceta de dirigent agrari des de la Unió Sindical Agrària, que se situarà en una difícil aposta, entre la patronal agrària i el sindicalisme rabassaire, tot buscant fórmules de concòrdia i entesa en moments de radicalització. Morirà en plena maduresa el 1935. La seva memòria es perdrà durant el franquisme i la democràcia restaurada.Carles Jordà I Fages was born in Figueres in 1883 and died in Pont de Molins in 1935. He was the heir of a family of landowners who early on determined to diversify their investments in banking businesses, among others. This allowed him and his sisters to move to Barcelona for their studies. He studied engineering and came into contact with the Catalan centres of the Lliga which led to his election to political offices during theMancomunitat de Catalonia and at the city council of Barcelona, where he became acting mayor. Some bad investments led to the end of his residence in Barcelona and he returned to the Empordà in 1921 where he maintained his political presence and became one of the founders of Acció Catalana. During the Primo de Rivera dictatorship he was persecuted for his political alliances and exiled to Casp. Upon return to the Empordà, he promoted agrarian cooperatives. With the Second Republic, he maintained his political subscription but especially accentuated his role as agrarian leader with the Unió Sindical Agrària, which, with a difficult commitment, acted between the agrarian landowners’ union and workers' syndicalism by trying to find peaceful ways and understanding in times of radicalization. He died at a prime age in 1935 while developing his role as mediator. His legacy was forgotten during the Franco regime and the restored democracy

Sobre alguns fongs rars recol·lectats en insectes vius

Durant l'estudi dels fongs entomòfags en els darrers anys s'han recol·lectat tota una sèrie de taxons poc freqüents, pertanyents a diferents grups taxonòmics i que presenten com a característica comuna l'excepcional reducció dels seus tal·lus. Aquestes mostres de fongs pertanyen als gèneres Amphoromorpha, Chantransiopsis, Entomocosma, Hormiscium i Thaxteriola; tots aquests taxons són discutits i il·lustrats. Chantransiopsis és trobat per primer cop a l'Archipiélago de Colón (Galàpagos, Equador). Les mostres d'Entomocosma i d'Hormiscium signifiquen segones citacions mundials, ja que aquests gèneres només eren coneguts per les seves respectives descripcions originals. Thaxteriola és primera citació ibèrica. A més, s'inclouen dues espècies no descrites. Com a informació complementària, es resumeixen totes les dades conegudes sobre aquests fongs, i altres espècies relacionades, que creixen sobre insectes vius.About some rare fungi collected on living insects. During the study of entomogenous fungi in the past years, several unusual fungi belonging to different taxonomic groups have been collected. These fungi show exceptionally reduced thalli as a common characteristic. The samples here included are forms of fungi belonging to the genera Amphoromorpha, Chantransiopsis, Entomocosma, Hormiscium and Thaxteriola; all these taxa are discussed and illustrated. Chantransiopsis is newly recorded from Galapagos Islands (Equador). Entomocosma and Hormiscium are second world records because they were only known from their original descriptions. Thaxteriola is a first Iberian record. Also, two undescribed species are included. As a complementary information, data on these and related fungi growing on living insects are summarized

Luther-Emery Phase and Atomic-Density Waves in a Trapped Fermion Gas

The Luther-Emery liquid is a state of matter that is predicted to occur in one-dimensional systems of interacting fermions and is characterized by a gapless charge spectrum and a gapped spin spectrum. In this Letter we discuss a realization of the Luther-Emery phase in a trapped cold-atom gas. We study by means of the density-matrix renormalization-group technique a two-component atomic Fermi gas with attractive interactions subject to parabolic trapping inside an optical lattice. We demonstrate how this system exhibits compound phases characterized by the coexistence of spin pairing and atomic-density waves. A smooth crossover occurs with increasing magnitude of the atom-atom attraction to a state in which tightly bound spin-singlet dimers occupy the center of the trap. The existence of atomic-density waves could be detected in the elastic contribution to the light-scattering diffraction pattern.Comment: 10 pages, 3 figures, 1 Table, submitted to Phys. Rev. on July 25th 200