2,025 research outputs found
Early Dark Energy Cosmologies
We propose a novel parameterization of the dark energy density. It is
particularly well suited to describe a non-negligible contribution of dark
energy at early times and contains only three parameters, which are all
physically meaningful: the fractional dark energy density today, the equation
of state today and the fractional dark energy density at early times. As we
parameterize Omega_d(a) directly instead of the equation of state, we can give
analytic expressions for the Hubble parameter, the conformal horizon today and
at last scattering, the sound horizon at last scattering, the acoustic scale as
well as the luminosity distance. For an equation of state today w_0 < -1, our
model crosses the cosmological constant boundary. We perform numerical studies
to constrain the parameters of our model by using Cosmic Microwave Background,
Large Scale Structure and Supernovae Ia data. At 95% confidence, we find that
the fractional dark energy density at early times Omega_early < 0.06. This
bound tightens considerably to Omega_early < 0.04 when the latest Boomerang
data is included. We find that both the gold sample of Riess et. al. and the
SNLS data by Astier et. al. when combined with CMB and LSS data mildly prefer
w_0 < -1, but are well compatible with a cosmological constant.Comment: 6 pages, 3 figures; references added, matches published versio
Orbifold Models in M-Theory
Among orbifold compactifications of -theory, we examine
models containing the particle physics Standard Model in four-dimensional
spacetimes, which appear as fixed subspaces of the ten-dimensional spacetimes
at each end of the interval, , spanning the
dimension. Using the projection to break the gauge symmetry in each
of the four-planes and a limiting relation to corresponding heterotic string
compactifications, we discuss the restrictions on the possible resulting gauge
field and matter spectra. In particular, some of the states are non-local: they
connect two four-dimensional Worlds across the dimension.
We illustrate our programmable calculations of the matter field spectrum,
including the anomalous U(1) factor which satisfies a universal Green-Schwarz
relation, discuss a Dynkin diagram technique to showcase a model with
gauge symmetry, and discuss generalizations to
higher order orbifolds.Comment: 23 pages, 2 figures, 4 tables; LaTeX 3 time
Arithmetic Spacetime Geometry from String Theory
An arithmetic framework to string compactification is described. The approach
is exemplified by formulating a strategy that allows to construct geometric
compactifications from exactly solvable theories at . It is shown that the
conformal field theoretic characters can be derived from the geometry of
spacetime, and that the geometry is uniquely determined by the two-dimensional
field theory on the world sheet. The modular forms that appear in these
constructions admit complex multiplication, and allow an interpretation as
generalized McKay-Thompson series associated to the Mathieu and Conway groups.
This leads to a string motivated notion of arithmetic moonshine.Comment: 36 page
Bose-Einstein condensate dark matter phase transition from finite temperature symmetry breaking of Klein-Gordon fields
In this paper the thermal evolution of scalar field dark matter particles at
finite cosmological temperatures is studied. Starting with a real scalar field
in a thermal bath and using the one loop quantum corrections potential, we
rewrite Klein-Gordon's (KG) equation in its hydrodynamical representation and
study the phase transition of this scalar field due to a Z_2 symmetry breaking
of its potential. A very general version of a nonlinear Schr\"odinger equation
is obtained. When introducing Madelung's representation, the continuity and
momentum equations for a non-ideal SFDM fluid are formulated, and the
cosmological scenario with the SFDM described in analogy to an imperfect fluid
is then considered where dissipative contributions are obtained in a natural
way.Additional terms appear compared to those obtained in the classical version
commonly used to describe the \LambdaCDM model, i.e., the ideal fluid. The
equations and parameters that characterize the physical properties of the
system such as its energy, momentum and viscous flow are related to the
temperature of the system, scale factor, Hubble's expansion parameter and the
matter energy density. Finally, some details on how galaxy halos and smaller
structures might be able to form by condensation of this SF are given.Comment: Substantial changes have been made to the paper, following the
referees recommendations. 16 pages. Published in Classical and Quantum
Gravit
Non-uniform doping across the Fermi surface of NbS intercalates
Magnetic ordering of the first row transition metal intercalates of NbS
due to coupling between the conduction electrons and the intercalated ions has
been explained in terms of Fermi surface nesting. We use angle-resolved
photoelectron spectroscopy to investigate the Fermi surface topology and the
valence band structure of the quasi-two-dimensional layer compounds
MnNbS and NiNbS. Charge transfer from the intercalant
species to the host layer leads to non-uniform, pocket selective doping of the
Fermi surface. The implication of our results on the nesting properties are
discussed
Radiative transfer effects in primordial hydrogen recombination
The calculation of a highly accurate cosmological recombination history has
been the object of particular attention recently, as it constitutes the major
theoretical uncertainty when predicting the angular power spectrum of Cosmic
Microwave Background anisotropies. Lyman transitions, in particular the
Lyman-alpha line, have long been recognized as one of the bottlenecks of
recombination, due to their very low escape probabilities. The Sobolev
approximation does not describe radiative transfer in the vicinity of Lyman
lines to a sufficient degree of accuracy, and several corrections have already
been computed in other works. In this paper, the impact of some previously
ignored radiative transfer effects is calculated. First, the effect of Thomson
scattering in the vicinity of the Lyman-alpha line is evaluated, using a full
redistribution kernel incorporated into a radiative transfer code. The effect
of feedback of distortions generated by the optically thick deuterium
Lyman-alpha line blueward of the hydrogen line is investigated with an analytic
approximation. It is shown that both effects are negligible during cosmological
hydrogen recombination. Secondly, the importance of high-lying, non overlapping
Lyman transitions is assessed. It is shown that escape from lines above
Ly-gamma and frequency diffusion in Ly-beta and higher lines can be neglected
without loss of accuracy. Thirdly, a formalism generalizing the Sobolev
approximation is developed to account for the overlap of the high-lying Lyman
lines, which is shown to lead to negligible changes to the recombination
history. Finally, the possibility of a cosmological hydrogen recombination
maser is investigated. It is shown that there is no such maser in the purely
radiative treatment presented here.Comment: 23 pages, 4 figures, to be submitted to PR
Autopsy in adults with congenital heart disease (ACHD).
The adult congenital heart diseases (ACHD) population is exceeding the pediatric congenital heart diseases (CHD) population and is progressively expanding each year, representing more than 90% of patients with CHD. Of these, about 75% have undergone surgical and/or percutaneous intervention for palliation or correction. Autopsy can be a very challenging procedure in ACHD patients. The approach and protocol to be used may vary depending on whether the pathologists are facing native disease without surgical or percutaneous interventions, but with various degrees of cardiac remodeling, or previously palliated or corrected CHD. Moreover, interventions for the same condition have evolved over the last decades, as has perioperative myocardial preservations and postoperative care, with different long-term sequelae depending on the era in which patients were operated on. Careful clinicopathological correlation is, thus, required to assist the pathologist in performing the autopsy and reaching a diagnosis regarding the cause of death. Due to the heterogeneity of the structural abnormalities, and the wide variety of surgical and interventional procedures, there are no standard methods for dissecting the heart at autopsy. In this paper, we describe the most common types of CHDs that a pathologist could encounter at autopsy, including the various types of surgical and percutaneous procedures and major pathological manifestations. We also propose a practical systematic approach to the autopsy of ACHD patients
Early Dark Energy at High Redshifts: Status and Perspectives
Early dark energy models, for which the contribution to the dark energy
density at high redshifts is not negligible, influence the growth of cosmic
structures and could leave observable signatures that are different from the
standard cosmological constant cold dark matter (CDM) model. In this
paper, we present updated constraints on early dark energy using geometrical
and dynamical probes. From WMAP five-year data, baryon acoustic oscillations
and type Ia supernovae luminosity distances, we obtain an upper limit of the
dark energy density at the last scattering surface (lss), (95% C.L.). When we include higher redshift
observational probes, such as measurements of the linear growth factors,
Gamma-Ray Bursts (GRBs) and Lyman- forest (\lya), this limit improves
significantly and becomes (95%
C.L.). Furthermore, we find that future measurements, based on the
Alcock-Paczy\'nski test using the 21cm neutral hydrogen line, on GRBs and on
the \lya forest, could constrain the behavior of the dark energy component and
distinguish at a high confidence level between early dark energy models and
pure CDM. In this case, the constraints on the amount of early dark
energy at the last scattering surface improve by a factor ten, when compared to
present constraints. We also discuss the impact on the parameter , the
growth rate index, which describes the growth of structures in standard and in
modified gravity models.Comment: 11 pages, 9 figures and 4 table
Access and metro network convergence for flexible end-to-end network design
This paper reports on the architectural, protocol, physical layer, and integrated testbed demonstrations carried out by the DISCUS FP7 consortium in the area of access - metro network convergence. Our architecture modeling results show the vast potential for cost and power savings that node consolidation can bring. The architecture, however, also recognizes the limits of long-reach transmission for low-latency 5G services and proposes ways to address such shortcomings in future projects. The testbed results, which have been conducted end-to-end, across access - metro and core, and have targeted all the layers of the network from the application down to the physical layer, show the practical feasibility of the concepts proposed in the project
- …