5,369 research outputs found
No realistic wormholes from ghost-free scalar-tensor phantom dark energy
It is proved that no wormholes can be formed in viable scalar-tensor models
of dark energy admitting its phantom-like () behaviour in cosmology,
even in the presence of electric or magnetic fields, if the non-minimal
coupling function is everywhere positive and the scalar field
itself is not a ghost. Some special static, spherically symmetric wormhole
solutions may exist if is allowed to reach zero or to become
negative, so that the effective gravitational constant becomes negative in some
region making the graviton a ghost. If remains non-negative, such solutions
require severe fine tuning and a very peculiar kind of model. If is
allowed, it is argued (and confirmed by previous investigations) that such
solutions are generically unstable under non-static perturbations, the
instability appearing right near transition surfaces to negative .Comment: 8 pages, late
Phantom universe from CPT symmetric QFT
Inspired by the generalization of quantum theory for the case of
non-Hermitian Hamiltonians with CPT symmetry, we construct a simple classical
cosmological scalar field based model describing a smooth transition from
ordinary dark energy to the phantom one
Cold black holes and conformal continuations
We study Einstein gravity minimally coupled to a scalar field in a static,
spherically symmetric space-time in four dimensions. Black hole solutions are
shown to exist for a phantom scalar field whose kinetic energy is negative.
These ``scalar black holes'' have an infinite horizon area and zero Hawking
temperature and are termed ``cold black holes'' (CBHs). The relevant explicit
solutions are well-known in the massless case (the so-called anti-Fisher
solution), and we have found a particular example of a CBH with a nonzero
potential . All CBHs with are shown to behave
near the horizon quite similarly to those with a massless field. The above
solutions can be converted by a conformal transformation to Jordan frames of a
general class of scalar-tensor theories of gravity, but CBH horizons in one
frame are in many cases converted to singularities in the other, which gives
rise to a new type of conformal continuation.Comment: 15 pages, late
Parametrization of dark energy equation of state Revisited
A comparative study of various parametrizations of the dark energy equation
of state is made. Astrophysical constraints from LSS, CMB and BBN are laid down
to test the physical viability and cosmological compatibility of these
parametrizations. A critical evaluation of the 4-index parametrizations reveals
that Hannestad-M\"{o}rtsell as well as Lee parametrizations are simple and
transparent in probing the evolution of the dark energy during the expansion
history of the universe and they satisfy the LSS, CMB and BBN constraints on
the dark energy density parameter for the best fit values.Comment: 11 page
Holographic Dark Energy Scenario and Variable Modified Chaplygin Gas
In this letter, we have considered that the universe is filled with normal
matter and variable modified Chaplygin gas. Also we have considered the
interaction between normal matter and variable modified Chaplygin gas in FRW
universe. Then we have considered a correspondence between the holographic dark
energy density and interacting variable modified Chaplygin gas energy density.
Then we have reconstructed the potential of the scalar field which describes
the variable modified Chaplygin cosmology.Comment: 4 latex pages, no figures, RevTeX styl
Equation of state description of the dark energy transition between quintessence and phantom regimes
The dark energy crossing of the cosmological constant boundary (the
transition between the quintessence and phantom regimes) is described in terms
of the implicitly defined dark energy equation of state. The generalizations of
the models explicitly constructed to exhibit the crossing provide the insight
into the cancellation mechanism which makes the transition possible.Comment: 3 pages, talk given at TAUP200
No evidence for Dark Energy Metamorphosis ?
Recent attempts to fit Type Ia supernova data by modeling the dark energy
density as a truncated Taylor series have suggested the possibility of
metamorphosis, i.e., a rapidly evolving equation of state parameter, w_DE(z).
However, we show that fits using that parametrization have significant
problems: evolution of w_DE(z) is both favoured and in some sense forced, and
the equation of state parameter blows up or diverges in large regions of the
parameter space. To further elucidate these problems we have simulated sets of
supernova data in a Lambda-universe to show that the suggested ``evidence'' for
metamorphosis is also common for w_DE=-1.Comment: 12 pages, 5 figures, Minor revisions to match version accepted in
Journal of Cosmology and Astroparticle Physic
(In)finiteness of Spherically Symmetric Static Perfect Fluids
This work is concerned with the finiteness problem for static, spherically
symmetric perfect fluids in both Newtonian Gravity and General Relativity. We
derive criteria on the barotropic equation of state guaranteeing that the
corresponding perfect fluid solutions possess finite/infinite extent. In the
Newtonian case, for the large class of monotonic equations of state, and in
General Relativity we improve earlier results
New Physics and the Landau Pole
In scalar field theories the Landau pole is an ultraviolet singularity in the
running coupling constant that indicates a mass scale at which the theory
breaks down and new physics must intervene. However, new physics at the pole
will in general affect the running of the low energy coupling constant, which
will in turn affect the location of the pole and the related upper limit
(``triviality'' bound) on the low energy coupling constant. If the new physics
is strongly coupled to the scalar fields these effects can be significant even
though they are power suppressed. We explore the possible range of such effects
by deriving the one loop renormalization group equations for an effective
scalar field theory with a dimension 6 operator representing the low energy
effects of the new physics. As an independent check we also consider a
renormalizable model of the high-scale physics constructed so that its low
energy limit coincides with the effective theory.Comment: 26 pages, 5 figure
Quintessential Phenomena in Higher Dimensional Space Time
The higher dimensional cosmology provides a natural setting to treat, at a
classical level, the cosmological effects of vacuum energy. Here we discuss two
situations where starting with an ordinary matter field without any equation of
state we end up with a Chaplygin type of gas apparently as a consequence of
extra dimensions. In the second case we study the quintessential phenomena in
higher dimensional spacetime with the help of a Chaplygin type of matter field.
The first case suffers from the disqualification that no dimensional reduction
occurs, which is, however, rectified in the second case. Both the models show
the sought after feature of occurrence of \emph{flip} in the rate of expansion.
It is observed that with the increase of dimensions the occurrence of
\emph{flip} is delayed for both the models, more in line with current
observational demands. Interestingly we see that depending on some initial
conditions our model admits QCDM, CDM and also Phantom like evolution
within a unified framework. Our solutions are general in nature in the sense
that when the extra dimensions are switched off the known 4D model is
recovered.Comment: 17 Pages, 7 figure
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