451 research outputs found
Some impacts of quintessence models on cosmic structure formation
Some physical imprints of quintessence scalar fields on dark matter (DM)
clustering are illustrated, and a comparison with the concordance model
is highlighted. First, we estimate the cosmological parameters
for two quintessence models, based on scalar fields rolling down the
Ratra-Peebles or Sugra potential, by a statistical analysis of the Hubble
diagram of type Ia supernovae. Then, the effect of these realistic dark energy
models on large-scale DM clustering is established through N-body simulations.
Various effects like large-scale distribution of DM, cluster mass function and
halos internal velocities are illustrated. It is found that realistic dark
energy models lead to quite different DM clustering, due to a combination of
the variation of the equation of state and differences in the cosmological
parameters, even at . This conclusion contradicts other works in the
recent litterature and the importance of considering more realistic models in
studying the impact of quintessence on structure formation is highlighted.Comment: 9 pages, 5 figures, in "Albert Einstein Century International
Conference", Paris, 18-22 July 2005, AIP Proceedings Conference 861, Pages
858-866, Edited by Jean-Michel ALIMI & Andre FUZF
An Awesome Hypothesis for Dark Energy : The Abnormally Weighting Energy
We introduce the Abnormally Weighting Energy (AWE) hypothesis in which dark
energy (DE) is presented as a consequence of the violation of the weak
equivalence principle (WEP) at cosmological scales by some dark sector. Indeed,
this implies a violation of the strong equivalence principle (SEP) for ordinary
matter and consequent cosmic acceleration in the observable frame as well as
variation of the gravitational constant. The consequent DE mechanism build upon
the AWE hypothesis (i) does not require a violation of the strong energy
condition , (ii) assumes rather non-negligible direct couplings
to the gravitational scalar field (iii) offers a natural convergence mechanism
toward general relativity (iv) accounts fairly for supernovae data from various
couplings and equations of state of the dark sector as well as density
parameters very close to the ones of the concordance model .
Finally (v), this AWE mechanism typically ends up with an Einstein-de Sitter
expansion regime once the attractor is reached.Comment: 3 pages, 1 figure, prepared for the Proceedings of the 11th Marcel
Grossmann Conference, held in Berlin, Germany, July 200
The Abnormally Weighting Energy Hypothesis: The origin of the cosmic acceleration
We generalize tensor-scalar theories of gravitation by the introduction of an
abnormally weighting type of energy. This theory of tensor-scalar anomalous
gravity is based on a relaxation of the weak equivalence principle that is now
restricted to ordinary visible matter only. As a consequence, the convergence
mechanism toward general relativity is modified and produces naturally cosmic
acceleration as an inescapable gravitational feedback induced by the
mass-variation of some invisible sector. The cosmological implications of this
new theoretical framework are studied. This glimpses at an enticing new
symmetry between the visible and invisible sectors, namely that the scalar
charges of visible and invisible matter are exactly opposite.Comment: 10 pages, 4 figures, to appear in the AIP proceedings of the
'Invisible Universe International Conference', UNESCO-Paris, June 29-July 3,
200
Non-Abelian Einstein-Born-Infeld-Dilaton Cosmology
The non-abelian Einstein-Born-Infeld-Dilaton theory, which rules the dynamics
of tensor-scalar gravitation coupled to a -valued gauge field ruled by
Born-Infeld lagrangian, is studied in a cosmological framework. The microscopic
energy exchange between the gauge field and the dilaton which results from a
non-universality of the coupling to gravity modifies the usual behaviour of
tensor-scalar theories coupled to matter fluids. General cosmological
evolutions are derived for different couplings to gravitation and a comparison
to universal coupling is highlighted. Evidences of cosmic acceleration are
presented when the evolution is interpreted in the Jordan physical frame of a
matter respecting the weak equivalence principle. The importance for the
mechanism of cosmic acceleration of the dynamics of the Born-Infeld gauge
field, the attraction role of the matter fluid and the non-universality of the
gravitational couplings is briefly outlined.Comment: 31 pages, 9 figures, minor changes, accepted for publication in Phys.
Rev. D1
Convergence of Scalar-Tensor theories toward General Relativity and Primordial Nucleosynthesis
In this paper, we analyze the conditions for convergence toward General
Relativity of scalar-tensor gravity theories defined by an arbitrary coupling
function (in the Einstein frame). We show that, in general, the
evolution of the scalar field is governed by two opposite mechanisms:
an attraction mechanism which tends to drive scalar-tensor models toward
Einstein's theory, and a repulsion mechanism which has the contrary effect. The
attraction mechanism dominates the recent epochs of the universe evolution if,
and only if, the scalar field and its derivative satisfy certain boundary
conditions. Since these conditions for convergence toward general relativity
depend on the particular scalar-tensor theory used to describe the universe
evolution, the nucleosynthesis bounds on the present value of the coupling
function, , strongly differ from some theories to others. For
example, in theories defined by analytical
estimates lead to very stringent nucleosynthesis bounds on
(). By contrast, in scalar-tensor theories defined by
much larger limits on () are
found.Comment: 20 Pages, 3 Figures, accepted for publication in Class. and Quantum
Gravit
Elliptic solutions of generalized Brans-Dicke gravity with a non-universal coupling
We study a model of the generalized Brans-Dicke gravity presented in both the
Jordan and in the Einstein frames, which are conformally related. We show that
the scalar field equations in the Einstein frame are reduced to the geodesics
equations on the target space of the nonlinear sigma-model. The analytical
solutions in elliptical functions are obtained when the conformal couplings are
given by reciprocal exponential functions. The behavior of the scale factor in
the Jordan frame is studied using numerical computations. For certain
parameters the solutions can describe an accelerated expansion. We also derive
an analytical approximation in exponential functions.Comment: 24 pages, 3 figures; v2: typos fixed, few remarks and references
added; version to appear in EPJ
Multi-objective portfolio optimization of mutual funds under downside risk measure using fuzzy theory
Mutual fund is one of the most popular techniques for many people to invest their funds where a professional fund manager invests people's funds based on some special predefined objectives; therefore, performance evaluation of mutual funds is an important problem. This paper proposes a multi-objective portfolio optimization to offer asset allocation. The proposed model clusters mutual funds with two methods based on six characteristics including rate of return, variance, semivariance, turnover rate, Treynor index and Sharpe index. Semivariance is used as a downside risk measure. The proposed model of this paper uses fuzzy variables for return rate and semivariance. A multi-objective fuzzy mean-semivariance portfolio optimization model is implemented and fuzzy programming technique is adopted to solve the resulted problem. The proposed model of this paper has gathered the information of mutual fund traded on Nasdaq from 2007 to 2009 and Pareto optimal solutions are obtained considering different weights for objective functions. The results of asset allocation, rate of return and risk of each cluster are also determined and they are compared with the results of two clustering methods
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