11 research outputs found
Complex Lagrangians and phantom cosmology
Motivated by the generalization of quantum theory for the case of
non-Hermitian Hamiltonians with PT symmetry, we show how a classical
cosmological model describes a smooth transition from ordinary dark energy to
the phantom one. The model is based on a classical complex Lagrangian of a
scalar field. Specific symmetry properties analogous to PT in non-Hermitian
quantum mechanics lead to purely real equation of motion.Comment: 11 pages, to be published in J.Phys.A, refs. adde
Extended quintessence, inflation, and stable de Sitter spaces
A new gauge-invariant criterion for stability against inhomogeneous
perturbations of de Sitter space is applied to scenarios of dark energy and
inflation in scalar-tensor gravity. The results extend previous studies.Comment: 16 pages, LaTeX, to appear in Class. Quantum Gra
Reconstruction of scalar potentials in two-field cosmological models
We study the procedure of the reconstruction of phantom-scalar field
potentials in two-field cosmological models. It is shown that while in the
one-field case the chosen cosmological evolution defines uniquely the form of
the scalar potential, in the two-field case one has an infinite number of
possibilities. The classification of a large class of possible potentials is
presented and the dependence of cosmological dynamics on the choice of initial
conditions is investigated qualitatively and numerically for two particular
models.Comment: final version, 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
Dark energy problem: from phantom theory to modified Gauss-Bonnet gravity
The solution of dark energy problem in the models without scalars is
presented. It is shown that late-time accelerating cosmology may be generated
by the ideal fluid with some implicit equation of state. The universe evolution
within modified Gauss-Bonnet gravity is considered. It is demonstrated that
such gravitational approach may predict the (quintessential, cosmological
constant or transient phantom) acceleration of the late-time universe with
natural transiton from deceleration to acceleration (or from non-phantom to
phantom era in the last case).Comment: LaTeX 8 pages, prepared for the Proceedings of QFEXT'05, minor
correctons, references adde
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
Natural Phantom Dark Energy, Wiggling Hubble Parameter and Direct Data
Recent direct data indicate that the parameter may wiggle with
respect to . On the other hand the luminosity distance data of supernovae
flatten the wiggles of because of integration effect. It is expected
that the fitting results can be very different in a model permitting a wiggling
because the data of supernovae is highly degenerated to such a model. As
an example the natural phantom dark energy is investigated in this paper. The
dynamical property of this model is studied. The model is fitted by the direct
data set and the SNLS data set, respectively. And the results are quite
different, as expected. The quantum stability of this model is also shortly
discussed. We find it is a viable model if we treat it as an effective theory
truncated by an upperbound.Comment: 14 pages, 2 figures, discussions on the stability added, conclusions
not change
Cheng-Weyl Vector Field and its Cosmological Application
Weyl's idea on scale invariance was resurrected by Cheng in 1988. The
requirement of local scale invariance leads to a completely new vector field,
which we call the ``Cheng-Weyl vector field''. The Cheng-Weyl vector field
couples only to a scalar field and the gravitational field naturally. It does
not interact with other known matters in the standard model of particle
physics. In the present work, the (generalized) Cheng-Weyl vector field coupled
with the scalar field and its cosmological application are investigated. A
mixture of the scalar field and a so-called ``cosmic triad'' of three mutually
orthogonal Cheng-Weyl vector fields is regarded as the dark energy in the
universe. The cosmological evolution of this ``mixed'' dark energy model is
studied. We find that the effective equation-of-state parameter of the dark
energy can cross the phantom divide in some cases; the first and
second cosmological coincidence problems can be alleviated at the same time in
this model.Comment: 16 pages, revtex4; v2: references added; v3: discussions added, to
appear in JCAP; v4: published versio