2,938 research outputs found
Cosmologies with variable parameters and dynamical cosmon: implications on the cosmic coincidence problem
Dynamical dark energy (DE) has been proposed to explain various aspects of
the cosmological constant (CC) problem(s). For example, it is very difficult to
accept that a strictly constant Lambda-term constitutes the ultimate
explanation for the DE in our Universe. It is also hard to acquiesce in the
idea that we accidentally happen to live in an epoch where the CC contributes
an energy density value right in the ballpark of the rapidly diluting matter
density. It should perhaps be more plausible to conceive that the vacuum
energy, is actually a dynamical quantity as the Universe itself. More
generally, we could even entertain the possibility that the total DE is in fact
a mixture of vacuum energy and other dynamical components (e.g. fields, higher
order terms in the effective action etc) which can be represented collectively
by an effective entity X (dubbed the ``cosmon''). The ``cosmon'', therefore,
acts as a dynamical DE component different from the vacuum energy. While it can
actually behave phantom-like by itself, the overall DE fluid may effectively
appear as standard quintessence, or even mimic at present an almost exact CC
behavior. Thanks to the versatility of such cosmic fluid we can show that a
composite DE system of this sort (``LXCDM'') may have a key to resolving the
mysterious coincidence problem.Comment: LaTeX, 13 pages, 5 figure
What is there in the black box of dark energy: variable cosmological parameters or multiple (interacting) components?
The coincidence problems and other dynamical features of dark energy are
studied in cosmological models with variable cosmological parameters and in
models with the composite dark energy. It is found that many of the problems
usually considered to be cosmological coincidences can be explained or
significantly alleviated in the aforementioned models.Comment: 6 pages, 1 figure, talk given at IRGAC2006 (Barcelona, July 11-15,
2006), to appear in J. Phys.
A proposal on how to apply the balanced scorecard to the collective investment managerial firms
lity. Consequently these firms need to reformulate their strategic forecast for the next years. This need arises, among other factors, from the collapse of global financial markets, progressive liberalizations in tax and company regulation and the evolution in information technology. Collective investments are very appealing for small investors, as they allow them to benefit from the skills of a professional management, as well as to spread different risks with a diversified portfolio in the different mutual funds. Nevertheless in moments of crisis such as the world is experiencing now, the great volatility of markets and capital losses clearly offset the attractiveness of the portfolio management. The starting point is to take into account the new scenario now developing in collective investment management firms: increasing competence, less captive clients, a more independent sales force, and decrease in their P&L account. With this background we propose to use a planning tool that, in the last ten years, has proven itself to be a great help in improving the management of the firms. The aim is to identify their core activities in order to define a better strategy, gain new clients, and improve the quality of their services. The final purpose is to increase their return on investment. Taking into account all the above mentioned reasons we have considered the collective investment management firms in Spain a good field to apply Balanced Scorecard as a device to improve management control . In our research paper, we propose a balanced scorecard model, exploring three possible strategic scenarios, together with an operating simulation of this device with a five years´scope for these type of firms. To obtain this model we have used dynamic systems theory and other information technology instruments.
Effective growth of matter density fluctuations in the running LCDM and LXCDM models
We investigate the matter density fluctuations \delta\rho/\rho for two dark
energy (DE) models in the literature in which the cosmological term \Lambda is
a running parameter. In the first model, the running LCDM model, matter and DE
exchange energy, whereas in the second model, the LXCDM model, the total DE and
matter components are conserved separately. The LXCDM model was proposed as an
interesting solution to the cosmic coincidence problem. It includes an extra
dynamical component, the "cosmon" X, which interacts with the running \Lambda,
but not with matter. In our analysis we make use of the current value of the
linear bias parameter, b^2(0)= P_{GG}/P_{MM}, where P_{MM} ~
(\delta\rho/\rho)^2 is the present matter power spectrum and P_{GG} is the
galaxy fluctuation power spectrum. The former can be computed within a given
model, and the latter is found from the observed LSS data (at small z) obtained
by the 2dF galaxy redshift survey. It is found that b^2(0)=1 within a 10%
accuracy for the standard LCDM model. Adopting this limit for any DE model and
using a method based on the effective equation of state for the DE, we can set
a limit on the growth of matter density perturbations for the running LCDM
model, the solution of which is known. This provides a good test of the
procedure, which we then apply to the LXCDM model in order to determine the
physical region of parameter space, compatible with the LSS data. In this
region, the LXCDM model is consistent with known observations and provides at
the same time a viable solution to the cosmic coincidence problem.Comment: LaTeX, 38 pages, 8 figures. Version accepted in JCA
Cosmologies with a time dependent vacuum
The idea that the cosmological term, Lambda, should be a time dependent
quantity in cosmology is a most natural one. It is difficult to conceive an
expanding universe with a strictly constant vacuum energy density, namely one
that has remained immutable since the origin of time. A smoothly evolving
vacuum energy density that inherits its time-dependence from cosmological
functions, such as the Hubble rate or the scale factor, is not only a
qualitatively more plausible and intuitive idea, but is also suggested by
fundamental physics, in particular by quantum field theory (QFT) in curved
space-time. To implement this notion, is not strictly necessary to resort to ad
hoc scalar fields, as usually done in the literature (e.g. in quintessence
formulations and the like). A "running" Lambda term can be expected on very
similar grounds as one expects (and observes) the running of couplings and
masses with a physical energy scale in QFT. Furthermore, the experimental
evidence that the equation of state of the dark energy could be evolving with
time/redshift (including the possibility that it might currently behave
phantom-like) suggests that a time-variable Lambda term (possibly accompanied
by a variable Newton's gravitational coupling G=G(t)) could account in a
natural way for all these features. Remarkably enough, a class of these models
(the "new cosmon") could even be the clue for solving the old cosmological
constant problem, including the coincidence problem.Comment: LaTeX, 15 pages, 4 figure
Perturbations in the relaxation mechanism for a large cosmological constant
Recently, a mechanism for relaxing a large cosmological constant (CC) has
been proposed [arxiv:0902.2215], which permits solutions with low Hubble rates
at late times without fine-tuning. The setup is implemented in the LXCDM
framework, and we found a reasonable cosmological background evolution similar
to the LCDM model with a fine-tuned CC. In this work we analyse analytically
the perturbations in this relaxation model, and we show that their evolution is
also similar to the LCDM model, especially in the matter era. Some tracking
properties of the vacuum energy are discussed, too.Comment: 18 pages, LaTeX; discussion improved, accepted by CQ
Dark energy: a quantum fossil from the inflationary Universe?
The discovery of dark energy (DE) as the physical cause for the accelerated
expansion of the Universe is the most remarkable experimental finding of modern
cosmology. However, it leads to insurmountable theoretical difficulties from
the point of view of fundamental physics. Inflation, on the other hand,
constitutes another crucial ingredient, which seems necessary to solve other
cosmological conundrums and provides the primeval quantum seeds for structure
formation. One may wonder if there is any deep relationship between these two
paradigms. In this work, we suggest that the existence of the DE in the present
Universe could be linked to the quantum field theoretical mechanism that may
have triggered primordial inflation in the early Universe. This mechanism,
based on quantum conformal symmetry, induces a logarithmic,
asymptotically-free, running of the gravitational coupling. If this evolution
persists in the present Universe, and if matter is conserved, the general
covariance of Einstein's equations demands the existence of dynamical DE in the
form of a running cosmological term whose variation follows a power law of the
redshift.Comment: LaTeX, 14 pages, extended discussion. References added. Accepted in
J. Phys. A: Mathematical and Theoretica
Phase diagram of silica from computer simulation
We evaluate the phase diagram of the ``BKS'' potential [Van Beest, Kramer and
van Santen, Phys. Rev. Lett. 64, 1955 (1990)], a model of silica widely used in
molecular dynamics (MD) simulations. We conduct MD simulations of the liquid,
and three crystals (beta-quartz, coesite and stishovite) over wide ranges of
temperature and density, and evaluate the total Gibbs free energy of each
phase. The phase boundaries are determined by the intersection of these free
energy surfaces. Not unexpectedly for a classical pair potential, our results
reveal quantitative discrepancies between the locations of the BKS and real
silica phase boundaries. At the same time, we find that the topology of the
real phase diagram is reproduced, confirming that the BKS model provides a
satisfactory qualitative description of a silica-like material. We also compare
the phase boundaries with the locations of liquid-state thermodynamic anomalies
identified in previous studies of the BKS model.Comment: 7 pages, 7 figure
Nonlinear analysis of a simple model of temperature evolution in a satellite
We analyse a simple model of the heat transfer to and from a small satellite
orbiting round a solar system planet. Our approach considers the satellite
isothermal, with external heat input from the environment and from internal
energy dissipation, and output to the environment as black-body radiation. The
resulting nonlinear ordinary differential equation for the satellite's
temperature is analysed by qualitative, perturbation and numerical methods,
which show that the temperature approaches a periodic pattern (attracting limit
cycle). This approach can occur in two ways, according to the values of the
parameters: (i) a slow decay towards the limit cycle over a time longer than
the period, or (ii) a fast decay towards the limit cycle over a time shorter
than the period. In the first case, an exactly soluble average equation is
valid. We discuss the consequences of our model for the thermal stability of
satellites.Comment: 13 pages, 4 figures (5 EPS files
- …