169 research outputs found
Statefinder diagnosis in a non-flat universe and the holographic model of dark energy
In this paper, we study the holographic dark energy model in non-flat
universe from the statefinder viewpoint. We plot the evolutionary trajectories
of the holographic dark energy model for different values of the parameter
as well as for different contributions of spatial curvature, in the statefinder
parameter-planes. The statefinder diagrams characterize the properties of the
holographic dark energy and show the discrimination between this scenario and
other dark energy models. As we show, the contributions of the spatial
curvature in the model can be diagnosed out explicitly by the statefinder
diagrams. Furthermore, we also investigate the holographic dark energy model in
the plane, which can provide us with a useful dynamical diagnosis
complement to the statefinder geometrical diagnosis.Comment: 16 pages, 4 figures; final versio
Cosmological Scaling Solutions of Multiple Tachyon Fields with Inverse Square Potentials
We investigate cosmological dynamics of multiple tachyon fields with inverse
square potentials. A phase-space analysis of the spatially flat FRW models
shows that there exists power-law cosmological scaling solutions. We study the
stability of the solutions and find that the potential-kinetic-scaling solution
is a global attractor. However, in the presence of a barotropic fluid the
solution is an attractor only in one region of the parameter space and the
tracking solution is an attractor in the other region. We briefly discuss the
physical consequences of these results.Comment: 10 pages, 1 figure, LaTeX2
Cosmological Evolution of Interacting Phantom Energy with Dark Matter
We investigate the cosmological evolution of an interacting phantom energy
model in which the phantom field has interaction with the dark matter. We
discuss the existence and stability of scaling solutions for two types of
specific interactions. One is motivated by the conformal transformation in
string theory and the other is motivated by analogy with dissipation. In the
former case, there exist no scaling solutions. In the latter case, there exist
stable scaling solutions, which may give a phenomenological solution of the
coincidence problem. Furthermore, the universe either accelerates forever or
ends with a singularity, which is determined by not only the model parameters
but also the initial velocity of the phantom field.Comment: 7 pages, 11 figures, RevTe
Cosmological evolution of interacting phantom (quintessence) model in Loop Quantum Gravity
The dynamics of interacting dark energy model in loop quantum cosmology (LQC)
is studied in this paper. The dark energy has a constant equation of state
and interacts with dark matter through a form . We
find for quintessence model () the cosmological evolution in LQC is the
same as that in classical Einstein cosmology; whereas for phantom dark energy
(), although there are the same critical points in LQC and classical
Einstein cosmology, loop quantum effect reduces significantly the parameter
spacetime () required by stability. If parameters and satisfy
the conditions that the critical points are existent and stable, the universe
will enter an era dominated by dark energy and dark matter with a constant
energy ratio between them, and accelerate forever; otherwise it will enter an
oscillatory regime. Comparing our results with the observations we find at
confidence level the universe will accelerate forever.Comment: 15 pages, 8 figures, to appear in JCA
Crossing the Phantom Divide: Theoretical Implications and Observational Status
If the dark energy equation of state parameter w(z) crosses the phantom
divide line w=-1 (or equivalently if the expression d(H^2(z))/dz - 3\Omega_m
H_0^2 (1+z)^2 changes sign) at recent redshifts, then there are two possible
cosmological implications: Either the dark energy consists of multiple
components with at least one non-canonical phantom component or general
relativity needs to be extended to a more general theory on cosmological
scales. The former possibility requires the existence of a phantom component
which has been shown to suffer from serious theoretical problems and
instabilities. Therefore, the later possibility is the simplest realistic
theoretical framework in which such a crossing can be realized. After providing
a pedagogical description of various dark energy observational probes, we use a
set of such probes (including the Gold SnIa sample, the first year SNLS
dataset, the 3-year WMAP CMB shift parameter, the SDSS baryon acoustic
oscillations peak (BAO), the X-ray gas mass fraction in clusters and the linear
growth rate of perturbations at z=0.15 as obtained from the 2dF galaxy redshift
survey) to investigate the priors required for cosmological observations to
favor crossing of the phantom divide. We find that a low \Omega_m prior
(0.2<\Omega_m <0.25) leads, for most observational probes (except of the SNLS
data), to an increased probability (mild trend) for phantom divide crossing. An
interesting degeneracy of the ISW effect in the CMB perturbation spectrum is
also pointed out.Comment: Accepted in JCAP (to appear). Comments added, typos corrected. 19
pages (revtex), 8 figures. The numerical analysis files (Mathematica +
Fortran) with instructions are available at
http://leandros.physics.uoi.gr/pdl-cross/pdl-cross.htm . The ppt file of a
relevant talk may be downloaded from
http://leandros.physics.uoi.gr/pdl-cross/pdl2006.pp
Coupled dark energy: Towards a general description of the dynamics
In dark energy models of scalar-field coupled to a barotropic perfect fluid,
the existence of cosmological scaling solutions restricts the Lagrangian of the
field \vp to p=X g(Xe^{\lambda \vp}), where X=-g^{\mu\nu} \partial_\mu \vp
\partial_\nu \vp /2, is a constant and is an arbitrary function.
We derive general evolution equations in an autonomous form for this Lagrangian
and investigate the stability of fixed points for several different dark energy
models--(i) ordinary (phantom) field, (ii) dilatonic ghost condensate, and
(iii) (phantom) tachyon. We find the existence of scalar-field dominant fixed
points (\Omega_\vp=1) with an accelerated expansion in all models
irrespective of the presence of the coupling between dark energy and dark
matter. These fixed points are always classically stable for a phantom field,
implying that the universe is eventually dominated by the energy density of a
scalar field if phantom is responsible for dark energy. When the equation of
state w_\vp for the field \vp is larger than -1, we find that scaling
solutions are stable if the scalar-field dominant solution is unstable, and
vice versa. Therefore in this case the final attractor is either a scaling
solution with constant \Omega_\vp satisfying 0<\Omega_\vp<1 or a
scalar-field dominant solution with \Omega_\vp=1.Comment: 21 pages, 5 figures; minor clarifications added, typos corrected and
references updated; final version to appear in JCA
Spatiotemporal variation in alpine grassland phenology in the Qinghai-Tibetan Plateau from 1999 to 2009
- …