61,449 research outputs found
Coupled quintessence with a possible transient accelerating phase
We discuss some cosmological consequences of a general model of coupled
quintessence in which the phenomenological coupling between the cold dark
matter and dark energy is a function of the cosmic scale factor .
This class of models presents cosmological solutions in which the Universe is
currently dominated by an exotic component, but will eventually be dominated by
cold dark matter in the future. This dynamical behavior is considerably
different from the standard CDM evolution, and may alleviate some
conflicts in reconciling the idea of the dark energy-dominated universe with
observables in String/M-theory. Finally, we investigate some observational
features of this model and discuss some constraints on its parameters from
current SNe Ia, BAO and CMB data.Comment: 7 pages, 5 figures, LaTe
A possible analogy between the dynamics of a skydiver and a scalar field: cosmological consequences
The cosmological consequences of a slow rolling scalar field with constant
kinetic term in analogy to the vertical movement of a skydiver after reaching
terminal velocity are investigated. It is shown that the terminal scalar field
hypothesis is quite realistic. In this approach, the scalar field potential is
given by a quadratic function of the field. This model provides solutions in
which the Universe was dominated in the past by a mixture of baryons and dark
matter, is currently accelerating (as indicated by type Ia supernovae data),
but will be followed by a contraction phase. The theoretical predictions of
this model are consistent with current observations, therefore, a terminal
scalar field is a viable candidate to dark energy.Comment: Accepted for publicatio
Geometry and Topology of Random 2-complexes
We study random 2-dimensional complexes in the Linial - Meshulam model and
find torsion in their fundamental groups at various regimes. We find a simple
algorithmically testable criterion for a subcomplex of a random 2-complex to be
aspherical; this implies that any aspherical subcomplex of a random 2-complex
satisfies the Whitehead conjecture. We use inequalities for Cheeger constants
and systoles of simplicial surfaces to analyse spheres and projective planes
lying in random 2-complexes. Our proofs exploit the strong hyperbolicity
property of random 2-complexes.Comment: 37 page
The asphericity of random 2-dimensional complexes
We study random 2-dimensional complexes in the Linial - Meshulam model and
prove that for the probability parameter satisfying a
random 2-complex
contains several pairwise disjoint tetrahedra such that the 2-complex
obtained by removing any face from each of these tetrahedra is aspherical.
Moreover, we prove that the obtained complex satisfies the Whitehead
conjecture, i.e. any subcomplex is aspherical. This implies that
is homotopy equivalent to a wedge where is a
2-dimensional aspherical simplicial complex. We also show that under the
assumptions c/n30<\epsilon<1/47Zp\pi_1(Y)$ of a random 2-complex equals 2.Comment: 11 pages, 2 figure
3D MHD simulation of flare supra-arcade downflows in a turbulent current sheet medium
Supra-arcade downflows (SADs) are sunward, generally dark, plasma density
depletions originated above posteruption flare arcades. In this paper using 3D
MHD simulations we investigate if the SAD cavities can be produced by a direct
combination of the tearing mode and Kelvin-Helmholtz instabilities leading to a
turbulent current sheet (CS) medium or if the current sheet is merely the
background where SADs are produced triggered by an impulsive deposition of
energy. We find that to give account of the observational dark lane structures
an addition of local energy, provided by a reconnection event, is required. We
suggest that there may be a closed relation between characteristic SAD sizes
and CS widths that must be satisfied to obtain an observable SAD.Comment: 12 pages, 10 figures. Accepted for publication in The Astrophysical
Journa
MHD simulations of coronal supra-arcade downflows including anisotropic thermal conduction
Coronal supra-arcade downflows (SADs) are observed as dark trails descending
towards hot turbulent fan shaped regions. Due to the large temperature values,
and gradients in these fan regions the thermal conduction should be very
efficient. While several models have been proposed to explain the triggering
and the evolution of SADs, none of these scenarios address a systematic
consideration of thermal conduction. Thus, we accomplish this task numerically
simulating the evolution of SADs within this framework. That is, SADs are
conceived as voided (subdense) cavities formed by non-linear waves triggered by
downflowing bursty localized reconnection events in a perturbed hot fan. We
generate a properly turbulent fan, obtained by a stirring force that permits
control of the energy and vorticity input in the medium where SADs develop. We
include anisotropic thermal conduction and consider plasma properties
consistent with observations. Our aim is to study if it is possible to prevent
SADs to vanish by thermal diffusion. We find that this will be the case,
depending on the turbulence parameters. In particular, if the magnetic field
lines are able to envelope the voided cavities, thermally isolating them from
the hot environment. Velocity shear perturbations that are able to generate
instabilities of the Kelvin-Helmholtz type help to produce magnetic islands,
extending the life-time of SADs
Terahertz emitters and detectors based on carbon nanotubes
We formulate and justify several proposals utilizing the unique electronic
properties of different types of carbon nanotubes for a broad range of
applications to THz optoelectronics, including THz generation by hot electrons
in quasi-metallic nanotubes, frequency multiplication in chiral-nanotube-based
superlattices controlled by a transverse electric field, and THz radiation
detection and emission by armchair nanotubes in a strong magnetic field.Comment: Keynote lecture at Nanomodeling II Conference (Annual SPIE Meeting,
San Diego, August 2006). A slightly shorter version will be published in
Proceedings of SPI
A note on quadrangular embedding of Abelian Cayley Graphs
The genus graphs have been studied by many authors, but just a few results
concerning in special cases: Planar, Toroidal, Complete, Bipartite and
Cartesian Product of Bipartite. We present here a derive general lower bound
for the genus of a abelian Cayley graph and construct a family of circulant
graphs which reach this bound
Dynamical and observational analysis of interacting models
We investigate the dynamical behaviour of a general class of interacting
models in the dark sector in which the phenomenological coupling between cold
dark matter and dark energy is a power law of the cosmic scale factor. From
numerical simulations we show that, in this background, dark energy always
dominates the current composition cosmic. This behaviour may alleviate
substantially the coincidence problem. By using current type Ia supernovae,
baryonic acoustic oscillations and cosmic microwave background data, we perform
a joint statistical analysis and obtain constraints on free parameters of this
class of model
On the Letelier spacetime with quintessence: solution, thermodynamics and Hawking radiation
We obtain the solution corresponding to a static and spherically symmetric
black hole with a cloud of strings (Letelier spacetime) immersed in a
quintessential fluid. We discuss some aspects of its thermodynamics. We also
present a discussion about Hawking radiation of particles, in the background
under consideration
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