104 research outputs found
Stability of accelerating cosmology in two scalar-tensor theory: Little Rip versus de Sitter
We develop the general reconstruction scheme in two scalar model. The
quintom-like theory which may describe (different) non-singular Little Rip or
de Sitter cosmology is reconstructed. (In)stability of such dark energy
cosmologies as well as the flow to fixed points is studied. The stability of
Little Rip universe which leads to dissolution of bound objects sometime in
future indicates that no classical transition to de Sitter space occurs.Comment: LaTeX 27 pages, 12 figures, version appeared in Entrop
Modified gravity as realistic candidate for dark energy, inflation and dark matter
We review the unification of early-time inflation with late-time acceleration
in several local modified gravity models which pass Solar System and
cosmological tests. It is also demonstrated that account of non-local
gravitational corrections to the action does not destroy the possibility of
such unification. Dark matter effect is caused by composite graviton degree of
freedom in such models.Comment: LaTeX 6 pages, based on the talk given at DSU08 conference, June 200
Modified f(R) gravity unifying R^m inflation with \LambdaCDM epoch
We consider modified gravity which may unify early-time
inflation with late-time CDM epoch. It is shown that such model passes
the local tests (Newton law, stability of Earth-like gravitational solution,
very heavy mass for additional scalar degree of freedom) and suggests the
realistic alternative for General Relativity. Various scenarios for future
evolution of CDM era are discussed.Comment: LaTeX 10 pages, version to appear in PR
Properties of singularities in (phantom) dark energy universe
The properties of future singularities are investigated in the universe
dominated by dark energy including the phantom-type fluid. We classify the
finite-time singularities into four classes and explicitly present the models
which give rise to these singularities by assuming the form of the equation of
state of dark energy. We show the existence of a stable fixed point with an
equation of state and numerically confirm that this is actually a
late-time attractor in the phantom-dominated universe. We also construct a
phantom dark energy scenario coupled to dark matter that reproduces singular
behaviors of the Big Rip type for the energy density and the curvature of the
universe. The effect of quantum corrections coming from conformal anomaly can
be important when the curvature grows large, which typically moderates the
finite-time singularities.Comment: 17 pages, 6 figures, references are added, version to appear in
Physical Review
Covariant power-counting renormalizable gravity: Lorentz symmetry breaking and accelerating early-time FRW universe
We continue the study of covariant power-counting renormalizable gravity
constrained by scalar Lagrange multiplier. Lorentz symmetry breaking is
investigated in such a theory in comparison with the one in ghost condensation
model. Covariant power-counting renormalizable vector gravity which is
invariant under gauge symmetry is proposed. Several forms of vector
Lagrange multiplier in this theory are discussed. It is shown that covariant
scalar/vector gravity under consideration may have power-law or de Sitter
accelerating cosmological solution corresponding to inflationary era. Simplest
black hole solution is obtained and dispersion relations for graviton are
presented.Comment: LaTeX, 13 page
Gauss-Bonnet Chameleon Mechanism of Dark Energy
As a model of the current accelerated expansion of the universe, we consider
a model of the scalar-Einstein-Gauss-Bonnet gravity. This model includes the
propagating scalar modes, which might give a large correction to the Newton
law. In order to avoid this problem, we propose an extension of the Chameleon
mechanism where the scalar mode becomes massive due to the coupling with the
Gauss-Bonnet term. Since the Gauss-Bonnet invariant does not vanish near the
earth or in the Solar System, even in the vacuum, the scalar mode is massive
even in the vacuum and the correction to the Newton law could be small. We also
discuss about the possibility that the model could describe simultaneously the
inflation in the early universe, in addition to the current accelerated
expansion.Comment: LaTeX 11 pages, no figur
Critical gravity with a scalar field in four dimensions
We consider the critical gravity theory with a scalar field in four
dimensions. We find that this theory has the solution corresponding to the de
Sitter (dS), anti-de Sitter (AdS), and Minkowski background depending on
whether the action includes the cosmological term or not. The Minkowski
background is the solution which cannot be obtained in the model without a
scalar field. At the critical point, we show that the Abbott-Deser (AD) mass of
the Schwarzschild-de Sitter (SdS) black hole and the energy for the massless
graviton vanish, whose situation is not changed from the model without a scalar
field.Comment: 6 page
Inhomogeneous Equation of State of the Universe: Phantom Era, Future Singularity and Crossing the Phantom Barrier
The dark energy universe equation of state (EOS) with inhomogeneous,Hubble
parameter dependent term is considered. The motivation to introduce such a term
comes from time-dependent viscosity considerations and modifications of general
relativity. For several explicit examples of such EOS it is demonstrated how
the type of future singularity changes, how the phantom epoch emerges and how
crossing of phantom barrier occurs. Similar cosmological regimes are considered
for the universe with two interacting fluids and for universe with implicit
EOS. For instance, the crossing of phantom barrier is realized in easier way,
thanks to the presence of inhomogeneous term. The thermodynamical dark energy
model is presented where the universe entropy may be positive even at phantom
era as a result of crossing of w=-1 barrier.Comment: LaTeX, 13 pages, refs. added, version to appear in PR
Is brane cosmology predictable?
The creation of the inflationary brane universe in 5d bulk Einstein and
Einstein-Gauss-Bonnet gravity is considered. We demonstrate that emerging
universe is ambigious due to arbitrary function dependence of the junction
conditions (or freedom in the choice of boundary terms). We argue that some
fundamental physical principle (which may be related with AdS/CFT
correspondence) is necessary in order to fix the 4d geometry in unique way.Comment: LaTeX file, 4 page
The future evolution and finite-time singularities in -gravity unifying the inflation and cosmic acceleration
We study the future evolution of quintessence/phantom dominated epoch in
modified -gravity which unifies the early-time inflation with late-time
acceleration and which is consistent with observational tests. Using the
reconstruction technique it is demonstrated that there are models where any
known (Big Rip, II, III or IV Type) singularity may classically occur. From
another side, in Einstein frame (scalar-tensor description) only IV Type
singularity occurs. Near the singularity the classical description breaks up,
it is demonstrated that quantum effects act against the singularity and may
prevent its appearance. The realistic -gravity which is future
singularity free is proposed. We point out that additional modification of any
-gravity by the terms relevant at the early universe is possible, in such
a way that future singularity does not occur even classically.Comment: 16 pages, discussion is enlarged, version to appear in PR
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