179,769 research outputs found
The excitation spectrum of rotating strings with masses at the ends
We compute the spectrum of excitations of the rotating Nambu-Goto string with
masses at the ends. We find interesting quasi-massless modes in the limit of
slow rotation and comment on the nontrivial relation between world-sheet and
target space energy.Comment: 13 pages, 1 figure, v2: Typos corrected, v3: fina
An evolutionary model for simple ecosystems
In this review some simple models of asexual populations evolving on smooth
landscapes are studied. The basic model is based on a cellular automaton, which
is analyzed here in the spatial mean-field limit. Firstly, the evolution on a
fixed fitness landscape is considered. The correspondence between the time
evolution of the population and equilibrium properties of a statistical
mechanics system is investigated, finding the limits for which this mapping
holds. The mutational meltdown, Eigen's error threshold and Muller's ratchet
phenomena are studied in the framework of a simplified model. Finally, the
shape of a quasi-species and the condition of coexistence of multiple species
in a static fitness landscape are analyzed. In the second part, these results
are applied to the study of the coexistence of quasi-species in the presence of
competition, obtaining the conditions for a robust speciation effect in asexual
populations.Comment: 36 pages, including 16 figures, to appear in Annual Review of
Computational Physics, D. Stauffer (ed.), World Scientific, Singapor
Higher harmonics and ac transport from time dependent density functional theory
We report on dynamical quantum transport simulations for realistic molecular
devices based on an approximate formulation of time-dependent Density
Functional Theory with open boundary conditions. The method allows for the
computation of various properties of junctions that are driven by alternating
bias voltages. Besides the ac conductance for hexene connected to gold leads
via thiol anchoring groups, we also investigate higher harmonics in the current
for a benzenedithiol device. Comparison to a classical quasi-static model
reveals that quantum effects may become important already for small ac bias and
that the full dynamical simulations exhibit a much lower number of higher
harmonics. Current rectification is also briefly discussed.Comment: submitted to J. Comp. Elec. (special issue
Quasi-Metric Relativity
This is a survey of a new type of relativistic space-time framework; the
so-called quasi-metric framework. The basic geometric structure underlying
quasi-metric relativity is quasi-metric space-time; this is defined as a
4-dimensional differentiable manifold equipped with two
one-parameter families and of Lorentzian
4-metrics parametrized by a global time function . The metric family is found from field equations, whereas the metric family is used to propagate sources and to compare predictions to experiments. A
linear and symmetric affine connection compatible with the family
is defined, giving rise to equations of motion.
Furthermore a quasi-metric theory of gravity, including field equations and
local conservation laws, is presented. Just as for General Relativity, the
field equations accommodate two independent propagating dynamical degrees of
freedom. On the other hand, the particular structure of quasi-metric geometry
allows only a partial coupling of space-time geometry to the active
stress-energy tensor. Besides, the field equations are defined from projections
of physical and geometrical tensors with respect to a ``preferred'' foliation
of quasi-metric space-time into spatial hypersurfaces. The dynamical nature of
this foliation makes the field equations unsuitable for a standard
PPN-analysis. This implies that the experimental status of the theory is not
completely clear at this point in time. The theory seems to be consistent with
a number of cosmological observations and it satisfies all the classical solar
system tests, though. Moreover, in its non-metric sector the new theory has
experimental support where General Relativity fails or is irrelevant.Comment: 39 pages, no figures, LaTeX; v2: some points clarified; v3:
connection changed; v4: extended and local conservation laws changed; v5:
major revision; v6: accepted for publication in G&C; v7: must have
non-universal gravitational coupling; v8: rewritten with fully coupled
theory; v9: major revision (fully coupled theory abandoned
Vacuum solutions of the gravitational field equations in the brane world model
We consider some classes of solutions of the static, spherically symmetric
gravitational field equations in the vacuum in the brane world scenario, in
which our Universe is a three-brane embedded in a higher dimensional
space-time. The vacuum field equations on the brane are reduced to a system of
two ordinary differential equations, which describe all the geometric
properties of the vacuum as functions of the dark pressure and dark radiation
terms (the projections of the Weyl curvature of the bulk, generating non-local
brane stresses). Several classes of exact solutions of the vacuum gravitational
field equations on the brane are derived. In the particular case of a vanishing
dark pressure the integration of the field equations can be reduced to the
integration of an Abel type equation. A perturbative procedure, based on the
iterative solution of an integral equation, is also developed for this case.
Brane vacuums with particular symmetries are investigated by using Lie group
techniques. In the case of a static vacuum brane admitting a one-parameter
group of conformal motions the exact solution of the field equations can be
found, with the functional form of the dark radiation and pressure terms
uniquely fixed by the symmetry. The requirement of the invariance of the field
equations with respect to the quasi-homologous group of transformations also
imposes a unique, linear proportionality relation between the dark energy and
dark pressure. A homology theorem for the static, spherically symmetric
gravitational field equations in the vacuum on the brane is also proven.Comment: 13 pages, no figures, to appear in PR
Large Scale Structure Formation of Normal Branch in DGP Brane World Model
In this paper, we study the large scale structure formation of the normal
branch in DGP model (Dvail, Gabadadze and Porrati brane world model) by
applying the scaling method developed by Sawicki, Song and Hu for solving the
coupled perturbed equations of motion of on-brane and off-brane. There is
detectable departure of perturbed gravitational potential from LCDM even at the
minimal deviation of the effective equation of state w_eff below -1. The
modified perturbed gravitational potential weakens the integrated Sachs-Wolfe
effect which is strengthened in the self-accelerating branch DGP model.
Additionally, we discuss the validity of the scaling solution in the de Sitter
limit at late times.Comment: 6 pages, 2 figure
Extra dimensions and the strong CP problem
In higher-dimensional theories such as Brane World models with
quasi-localized non-Abelian gauge fields the vacuum structure turns out to be
trivial. Since the gauge theory behaves at large distances as a
-dimensional and thus the topology of the infinity is that of of
rather than , the set of gauge mappings are homotopically
trivial and the CP-violating -term vanishes on the brane world-volume.
As well there are no contributions to the -term from the
higher-dimensional solitonic configurations. In this way, the strong CP problem
is absent in the models with quasi-localized gluons.Comment: 7 pp., latex, no figure
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