81 research outputs found
Dynamics of perfect fluid Unified Dark Energy models
In this paper we show that a \emph{one-to-one} correspondence exists between
any dark energy model and an equivalent (from a cosmological point of view, in
the absence of perturbations) quartessence model in which dark matter and dark
energy are described by a single perfect fluid. We further show that if the
density fluctuations are small, the evolution of the sound speed squared,
, is fully coupled to the evolution of the scale factor and that the
transition from the dark matter to the dark energy dominated epoch is faster
(slower) than in a standard CDM model if (). In
particular, we show that the mapping of the simplest quintessence scenario with
constant into a unified dark energy model requires
) contrasting to the Chaplygin
gas scenario where one has . However, we show that non-linear
effects severely complicate the analysis, in particular rendering linear
results invalid even on large cosmological scales. Although a detailed analysis
of non-linear effects requires solving the full Einstein field equations, some
general properties can be understood in simple terms. In particular, we find
that in the context of Chaplygin gas models the transition from the dark matter
to the dark energy dominated era may be anticipated with respect to linear
expectations leading to a background evolution similar to that of standard
CDM models. On the other hand, in models with the expected
transition from the decelerating to the accelerating phase may never happen.Comment: 5 page
Large scale structure and the generalised Chaplygin gas as dark energy
The growth of large scale structure is studied in a universe containing both
cold dark matter (CDM) and generalized Chaplygin gas (GCg). GCg is assumed to
contribute only to the background evolution of the universe while the CDM
component collapses and forms structures. We present some new analytical as
well as numerical results for linear and non-linear growth in such model. The
model passes the standard cosmological distance test without the need of a
cosmological constant (LCDM). But we find that the scenario is severely
constrained by current observations of large scale structure. Any small
deviations of the GCg parameters away from the standard Lambda dominated
cosmology (LCDM) produces substantial suppression for the growth of structures.Comment: 6 pages, matches version accepted for publication in Phys.Rev.D (in
press
WMAP and the Generalized Chaplygin Gas
We compare the WMAP temperature power spectrum and SNIa data to models with a
generalized Chaplygin gas as dark energy. The generalized Chaplygin gas is a
component with an exotic equation of state, p_X=-A/\rho^\alpha_X (a polytropic
gas with negative constant and exponent). Our main result is that, restricting
to a flat universe and to adiabatic pressure perturbations for the generalized
Chaplygin gas, the constraints at 95% CL to the present equation of state w_X =
p_X / \rho_X and to the parameter \alpha are -1\leq w_X < -0.8, 0 \leq \alpha
<0.2, respectively. Moreover, we show that a Chaplygin gas (\alpha =1) as a
candidate for dark energy is ruled out by our analysis at more than the 99.99%
CL. A generalized Chaplygin gas as a unified dark matter candidate
(\Omega_{CDM}=0) appears much less likely than as a dark energy model, although
its \chi^2 is only two sigma away from the expected value.Comment: 10 pages, 8 figures, results clarifie
The Limit of the Generalized Chaplygin Gas Scenario
We explicitly demonstrate that, contrary to recent claims, the dynamics of a
generalized Chaplygin gas model with an equation of state (where is
a positive constant) is equivalent to that of a standard CDM model to
first order in the metric perturbations. We further argue that the analogy
between the two models goes well beyond linear theory and conclude that they
cannot be distinguished based on gravity alone.Comment: 6 pages, to appear in JCA
Large-scale instability in interacting dark energy and dark matter fluids
If dark energy interacts with dark matter, this gives a new approach to the
coincidence problem. But interacting dark energy models can suffer from
pathologies. We consider the case where the dark energy is modelled as a fluid
with constant equation of state parameter w. Non-interacting constant-w models
are well behaved in the background and in the perturbed universe. But the
combination of constant w and a simple interaction with dark matter leads to an
instability in the dark sector perturbations at early times: the curvature
perturbation blows up on super-Hubble scales. Our results underline how
important it is to carefully analyze the relativistic perturbations when
considering models of coupled dark energy. The instability that we find has
been missed in some previous work where the perturbations were not consistently
treated. The unstable mode dominates even if adiabatic initial conditions are
used. The instability also arises regardless of how weak the coupling is. This
non-adiabatic instability is different from previously discovered adiabatic
instabilities on small scales in the strong-coupling regime.Comment: 15 pages, 5 figures. New reference; published versio
Large-scale instability in interacting dark energy and dark matter fluids
If dark energy interacts with dark matter, this gives a new approach to the
coincidence problem. But interacting dark energy models can suffer from
pathologies. We consider the case where the dark energy is modelled as a fluid
with constant equation of state parameter w. Non-interacting constant-w models
are well behaved in the background and in the perturbed universe. But the
combination of constant w and a simple interaction with dark matter leads to an
instability in the dark sector perturbations at early times: the curvature
perturbation blows up on super-Hubble scales. Our results underline how
important it is to carefully analyze the relativistic perturbations when
considering models of coupled dark energy. The instability that we find has
been missed in some previous work where the perturbations were not consistently
treated. The unstable mode dominates even if adiabatic initial conditions are
used. The instability also arises regardless of how weak the coupling is. This
non-adiabatic instability is different from previously discovered adiabatic
instabilities on small scales in the strong-coupling regime.Comment: 15 pages, 5 figures. New reference; published versio
The onset of the non-linear regime in unified dark matter models
We discuss the onset of the non-linear regime in the context of unified dark
matter models involving a generalised Chaplygin gas. We show that the
transition from dark matter-like to dark energy-like behaviour will never be
smooth. In some regions of space the transition will never take place while in
others it may happen sooner or later than naively expected. As a result the
linear theory used in previous studies may break down late in the matter
dominated era even on large cosmological scales. We study the importance of
this effect showing that its magnitude depends on the exact form of the
equation of state in the low density regime. We expect that our results will be
relevant for other unified dark matter scenarios particularly those where the
quartessence candidate is a perfect fluid.Comment: 5 pages, 4 figure
Extended tachyon field, Chaplygin gas and solvable k-essence cosmologies
We investigate a flat Friedmann-Robertson-Walker spacetime filled with
k-essence and find the set of functions F which generate three different
families of extended tachyon fields and Chaplygin gases. They lead to
accelerated and superaccelerated expanding scenarios.
For any function F, we find the first integral of the k-field equation when
the k-field is driven by an inverse square potential or by a constant one. In
the former, we obtain the general solution of the coupled Einstein-k-field
equations for a linear function F. This model shares the same kinematics of the
background geometry with the ordinary scalar field one driven by an exponential
potential. However, they are dynamically different. For a constant potential,
we introduce a k-field model that exhibits a transition from a power-law phase
to a de Sitter stage, inducing a modified Chaplygin gas.Comment: 24 pages, revised version accepted for publication in Phys. Rev.
Cyclic Nucleotide Phosphodiesterases and Compartmentation in Normal and Diseased Heart
International audienceCyclic nucleotide phosphodiesterases (PDEs) degrade the second messengers cAMP and cGMP, thereby regulating multiple aspects of cardiac function. This highly diverse class of enzymes encoded by 21 genes encompasses 11 families which are not only responsible for the termination of cyclic nucleotide signalling, but are also involved in the generation of dynamic microdomains of cAMP and cGMP controlling specific cell functions in response to various neurohormonal stimuli. In myocardium, the PDE3 and PDE4 families are predominant to degrade cAMP and thereby regulate cardiac excitation-contraction coupling. PDE3 inhibitors are positive inotropes and vasodilators in human, but their use is limited to acute heart failure and intermittent claudication. PDE5 is particularly important to degrade cGMP in vascular smooth muscle, and PDE5 inhibitors are used to treat erectile dysfunction and pulmonary hypertension. However, these drugs do not seem efficient in heart failure with preserved ejection fraction. There is experimental evidence that these PDEs as well as other PDE families including PDE1, PDE2 and PDE9 may play important roles in cardiac diseases such as hypertrophy and heart failure. After a brief presentation of the cyclic nucleotide pathways in cardiac cells and the major characteristics of the PDE superfamily, this chapter will present their role in cyclic nucleotide compartmentation and the current use of PDE inhibitors in cardiac diseases together with the recent research progresses that could lead to a better exploitation of the therapeutic potential of these enzymes in the future
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