1,413 research outputs found
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
and , where is a metric independent
density, also curvature, curvature square terms, first order formalism and for
scale invariance a dilaton field 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 relevant for the non singular
origin of the Universe and , describing our present
Universe. Surprisingly, avoidance of singularities and stability as imply a positive but small vacuum energy as . 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 and , where is a metric independent density, used as an additional
measure of integration. Also curvature, curvature square terms and for scale
invariance a dilaton field 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 relevant for the non singular origin of the Universe,
followed by an inflationary phase and , 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 , 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
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 Agrà ria
Carles Jordà i Fages neix a Figueres el 1883 i mor a Pont de Molins el 1935. És l’hereu d’una famÃlia 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 cà rrecs polÃtics 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 polÃtica i serà un dels fundadors d’Acció Catalana. Per la seva afiliació polÃtica, 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 polÃtica, però, sobretot, accentuarà la seva faceta de dirigent agrari des de la Unió Sindical Agrà ria, que se situarà en una difÃcil aposta, entre la patronal agrà ria 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 democrà cia 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 Agrà ria, 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
caracterÃstica 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 (Galà pagos, 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 complementà ria, 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
Contribución al conocimiento de los laboulbeniales (Ascomycotina) ibéricos, IV. Género Laboulbenia: adiciones y correcciones
Contribution to the knowledge or Iberian Laboulbeniales (Ascomycotina), IV. Genus
Laboulbenia: additions and corrections. Four species of Laboulbenia are here reported as new for the
Iberian fungus flora: Laboulbenia benjaminii Balazuc, Laboulbenia calathi T. Majewski, Laboulbenia
Etrusca Speg. and Laboulbenia luxurians Peyr. These species are illustrated, described and discussed. Also,
Laboulbenia nebriae subsp. maghrebiana Balazuc is included in the list of taxa of the genus as a correction
of a previous record of Laboulbenia nebriae Peyr. s. str.Se citan cuatro especies del género Laboulbenia, nuevas para la flora micológica ibérica:
Laboulbenia benjaminii Balazuc, Laboulbenia calathi T. Majewski, Laboulbenia
Etrusca Speg. y Laboulbenia luxurians Peyr. Estas especies son ilustradas, descritas y comentadas. Asimismo, Laboulbenia nebriae subsp. maghrebiana es incorporada a la lista de taxones del género a resultas de la corrección de una cita previa de Laboulbenia nebriae Peyr. s. str
Fongs nematòfags: Harposporium anguillulae (Lohde) Karling a la PenÃnsula Ibèrica
El fong endoparà sit nematòfag Harposporium anguillulae (Lohde) Karling fou obtingut de mostres de fems d'ovella recol·lectades en un erm a Catalunya. Aquest fong és primera citació per a la PenÃnsula Ibèrica.Nematophagous fuogi: Harposporium anguillulae (Lohde) Karling in the Iberian PenÃnsula. The endoparasitic nematophagous fungus Harposporium anguillulae (Lohde) Karling was
obtained from samples of lamb dung collected in a waste field in Catalonia. This fungus is newly reported from Iberian Peninsula
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