331 research outputs found
Topological defects: A problem for cyclic universes?
We study the behaviour of cosmic string networks in contracting universes,
and discuss some of their possible consequences. We note that there is a
fundamental time asymmetry between defect network evolution for an expanding
universe and a contracting universe. A string network with negligible loop
production and small-scale structure will asymptotically behave during the
collapse phase as a radiation fluid. In realistic networks these two effects
are important, making this solution only approximate. We derive new scaling
solutions describing this effect, and test them against high-resolution
numerical simulations. A string network in a contracting universe, together
with the gravitational radiation background it has generated, can significantly
affect the dynamics of the universe both locally and globally. The network can
be an important source of radiation, entropy and inhomogeneity. We discuss the
possible implications of these findings for bouncing and cyclic cosmological
models.Comment: 11 RevTeX 4 pages, 6 figures; version to appear in Phys. Rev.
Theoretical Study of Electron-Phonon Interaction in ZrB2 and TaB2
Using full-potential, density-functional-based methods we have studied
electron-phonon interaction in ZrB2 and TaB2 in P6/mmm crystal structure. Our
results for phonon density of states and Eliashberg function show that the
electron-phonon coupling in ZrB2 is much weaker than in TaB2. In particular, we
find that the average electron-phonon coupling constant \lambda is equal to
0.14 for ZrB2 and 0.72 for TaB2. The solutions of the isotropic Eliashberg gap
equation indicate no superconductivity for ZrB2 but a superconducting
transition temperature Tc of around 12 K for TaB2 with \mu* ~0.16.Comment: Increased q points from 12 to 28, added 3 figs and a section on
convergence analysi
Chemical telemetry of OH observed to measure interstellar magnetic fields
We present models for the chemistry in gas moving towards the ionization
front of an HII region. When it is far from the ionization front, the gas is
highly depleted of elements more massive than helium. However, as it approaches
the ionization front, ices are destroyed and species formed on the grain
surfaces are injected into the gas phase. Photodissociation removes gas phase
molecular species as the gas flows towards the ionization front. We identify
models for which the OH column densities are comparable to those measured in
observations undertaken to study the magnetic fields in star forming regions
and give results for the column densities of other species that should be
abundant if the observed OH arises through a combination of the liberation of
H2O from surfaces and photodissociation. They include CH3OH, H2CO, and H2S.
Observations of these other species may help establish the nature of the OH
spatial distribution in the clouds, which is important for the interpretation
of the magnetic field results.Comment: 11 pages, 2 figures, accepted by Astrophysics and Space Scienc
Selectivity and functional diversity in arbuscular mycorrhizas of co-occurring fungi and plants from a temperate deciduous woodland
1 The arbuscular mycorrhizal (AM) fungi colonizing plants at a woodland site in North Yorkshire (UK) have been characterized from the roots of five plant species (Rubus fruticosus agg. L., Epilobium angustifolium L., Acer pseudoplatanus L., Ajuga reptans L. and Glechoma hederacea L.), and identified using small-subunit rRNA (SSUrRNA) gene amplification and sequencing. 2 Interactions between five plant species from the site and four co-occurring glomalean fungi were investigated in artificial one-to-one AM symbioses. Three of the fungi were isolated from the site; the fourth was a culture genetically similar to a taxon found at the site. Phosphorus uptake and growth responses were compared with non-mycorrhizal controls. 3 Individual fungi colonized each plant with different spatial distribution and intensity. Some did not colonize at all, indicating incompatibility under the conditions used in the experiments. 4 Glomus hoi consistently occupied a large proportion of root systems and outperformed the other fungi, improving P uptake and enhancing the growth of four out of the five plant species. Only G. hoi colonized and increased P uptake in Acer pseudoplatanus, the host plant with which it associates almost exclusively under field conditions. Colonization of all plant species by Scutellospora dipurpurescens was sparse, and beneficial to only one of the host plants (Teucrium scorodonia). Archaeospora trappei and Glomus sp. UY1225 had variable effects on the host plants, conferring a range of P uptake and growth benefits on Lysimachia nummularia and T. scorodonia, increasing P uptake whilst not affecting biomass in Ajuga reptans and Glechoma hederacea, and failing to form mycorrhizas with A. pseudoplatanus. 5 These experimental mycorrhizas show that root colonization, symbiont compatibility and plant performance vary with each fungus-plant combination, even when the plants and fungi naturally co-exist. 6 We provide evidence of physical and functional selectivity in AM. The small number of described AM fungal species (154) has been ascribed to their supposed lack of host specificity, but if the selectivity we have observed is the general rule, then we may predict that many more, probably hard-to-culture glomalean species await discovery, or that members of species as currently perceived may be physiologically or functionally distinct
Fermionic massive modes along cosmic strings
The influence on cosmic string dynamics of fermionic massive bound states
propagating in the vortex, and getting their mass only from coupling to the
string forming Higgs field, is studied. Such massive fermionic currents are
numerically found to exist for a wide range of model parameters and seen to
modify drastically the usual string dynamics coming from the zero mode currents
alone. In particular, by means of a quantization procedure, a new equation of
state describing cosmic strings with any kind of fermionic current, massive or
massless, is derived and found to involve, at least, one state parameter per
trapped fermion species. This equation of state exhibits transitions from
subsonic to supersonic regimes while the massive modes are filled.Comment: 27 pages, 15 figures, uses ReVTeX. Shortened version, accepted for
publication in Phys. Rev.
Equation of state of cosmic strings with fermionic current-carriers
The relevant characteristic features, including energy per unit length and
tension, of a cosmic string carrying massless fermionic currents in the
framework of the Witten model in the neutral limit are derived through
quantization of the spinor fields along the string. The construction of a Fock
space is performed by means of a separation between longitudinal modes and the
so-called transverse zero energy solutions of the Dirac equation in the vortex.
As a result, quantization leads to a set of naturally defined state parameters
which are the number densities of particles and anti-particles trapped in the
cosmic string. It is seen that the usual one-parameter formalism for describing
the macroscopic dynamics of current-carrying vortices is not sufficient in the
case of fermionic carriers.Comment: 30 pages, 15 figures, uses ReVTeX, equation of state corrected,
comments and references added. Accepted for publication in Phys. Rev.
Cosmic Microwave Background Anisotropies from Scaling Seeds: Global Defect Models
We investigate the global texture model of structure formation in cosmogonies
with non-zero cosmological constant for different values of the Hubble
parameter. We find that the absence of significant acoustic peaks and little
power on large scales are robust predictions of these models. However, from a
careful comparison with data we conclude that at present we cannot safely
reject the model on the grounds of present CMB data. Exclusion by means of
galaxy correlation data requires assumptions on biasing and statistics. New,
very stringent constraints come from peculiar velocities.
Investigating the large-N limit, we argue that our main conclusions apply to
all global O(N) models of structure formation.Comment: LaTeX file with RevTex, 27 pages, 23 eps figs., submitted to Phys.
Rev. D. A version with higher quality images can be found at
http://mykonos.unige.ch/~kunz/download/lam.tar.gz for the LaTeX archive and
at http://mykonos.unige.ch/~kunz/download/lam.ps.gz for the compiled
PostScript fil
Deviation From \Lambda CDM With Cosmic Strings Networks
In this work, we consider a network of cosmic strings to explain possible
deviation from \Lambda CDM behaviour. We use different observational data to
constrain the model and show that a small but non zero contribution from the
string network is allowed by the observational data which can result in a
reasonable departure from \Lambda CDM evolution. But by calculating the
Bayesian Evidence, we show that the present data still strongly favour the
concordance \Lambda CDM model irrespective of the choice of the prior.Comment: 15 Pages, Latex Style, 4 eps figures, Revised Version, Accepted for
publication in European Physical Journal
Observational Constraints on Chaplygin Quartessence: Background Results
We derive the constraints set by several experiments on the quartessence
Chaplygin model (QCM). In this scenario, a single fluid component drives the
Universe from a nonrelativistic matter-dominated phase to an accelerated
expansion phase behaving, first, like dark matter and in a more recent epoch
like dark energy. We consider current data from SNIa experiments, statistics of
gravitational lensing, FR IIb radio galaxies, and x-ray gas mass fraction in
galaxy clusters. We investigate the constraints from this data set on flat
Chaplygin quartessence cosmologies. The observables considered here are
dependent essentially on the background geometry, and not on the specific form
of the QCM fluctuations. We obtain the confidence region on the two parameters
of the model from a combined analysis of all the above tests. We find that the
best-fit occurs close to the CDM limit (). The standard
Chaplygin quartessence () is also allowed by the data, but only at
the level.Comment: Replaced to match the published version, references update
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