655 research outputs found
Dark energy cosmology from higher-order, string-inspired gravity and its reconstruction
In this paper we investigate the cosmological effects of modified gravity
with string curvature corrections added to Einstein-Hilbert action in the
presence of a dynamically evolving scalar field coupled to Riemann invariants.
The scenario exhibits several features of cosmological interest for late
universe. It is shown that higher order stringy corrections can lead to a class
of dark energy models consistent with recent observations. The model can give
rise to quintessence, deSitter or phantom dark energy, in last case without
recourse to negative kinetic energy field. The detailed treatment of
reconstruction program for general scalar-Gauss-Bonnet gravity is presented for
any given cosmology. The explicit examples of reconstructed scalar potentials
are given for accelerated (quintessence, cosmological constant or phantom)
universe. Finally, the relation with modified gravity is established on
classical level and is extended to include third order terms on curvature.Comment: 15 pages, 3 figures, minor changes, published versio
Effective Action of Composite Fields for General Gauge Theories in BLT-Covariant Formalism
The gauge dependence of the effective action of composite fields for general
gauge theories in the framework of the quantization method by Batalin, Lavrov
and Tyutin is studied. The corresponding Ward identities are obtained. The
variation of composite fields effective action is found in terms of new set of
operators depending on composite field. The theorem of the on-shell gauge
fixing independence for the effective action of composite fields in such
formalism is proved. brief discussion of gravitational-vector induced
interaction for Maxwell theory with composite fields is given.Comment: Typos corrected. Latex fil
Classifying and avoiding singularities in the alternative gravity dark energy models
The future finite-time singularities emerging in alternative gravity dark
energy models are classified and studied in Jordan and Einstein frames. It is
shown that such singularity may occur even in flat spacetime for the specific
choice of the effective potential. The conditions for the avoidance of
finite-time singularities are presented and discussed. The problem is reduced
to the study of a scalar field evolving on an effective potential by using the
conformal transformations. Some viable modified gravity models are analyzed in
detail and the way to cure singularity is considered by introducing the
higher-order curvature corrections. These results maybe relevant for the
resolution of the conjectured problem in the relativistic star formation in
such modified gravity where finite-time singularity is also manifested.Comment: 21 pages, 12 figures, published version in PR
Hydrostatic equilibrium and stellar structure in f(R)-gravity
We investigate the hydrostatic equilibrium of stellar structure by taking
into account the modi- fied La\'e-Emden equation coming out from f(R)-gravity.
Such an equation is obtained in metric approach by considering the Newtonian
limit of f(R)-gravity, which gives rise to a modified Poisson equation, and
then introducing a relation between pressure and density with polytropic index
n. The modified equation results an integro-differential equation, which, in
the limit f(R) \rightarrow R, becomes the standard La\'e-Emden equation. We
find the radial profiles of gravitational potential by solving for some values
of n. The comparison of solutions with those coming from General Relativity
shows that they are compatible and physically relevant.Comment: 9 pages, 1 figur
Universal features of the holographic duality: conformal anomaly and brane gravity trapping from 5d AdS Black Hole
We calculate the holographic conformal anomaly and brane Newton potential
when bulk is 5d AdS BH. It is shown that such anomaly is the same as in the
case of pure AdS or (asymptotically) dS bulk spaces, i.e. it is (bulk) metric
independent one. While Newton potential on the static brane in AdS BH is
different from the one in pure AdS space, the gravity trapping still occurs for
two branes system. This indicates to metric independence of gravity
localization.Comment: LaTeX file, 14 page
Dark Energy: Vacuum Fluctuations, the Effective Phantom Phase, and Holography
We aim at the construction of dark energy models without exotic matter but
with a phantom-like equation of state (an effective phantom phase). The first
model we consider is decaying vacuum cosmology where the fluctuations of the
vacuum are taken into account. In this case, the phantom cosmology (with an
effective, observational being less than -1) emerges even for the case
of a real dark energy with a physical equation of state parameter
larger than -1. The second proposal is a generalized holographic model, which
is produced by the presence of an infrared cutoff. It also leads to an
effective phantom phase, which is not a transient one as in the first model.
However, we show that quantum effects are able to prevent its evolution towards
a Big Rip singularity.Comment: 13 pages, 2 figures, revtex4, version to appear in Physical Review
Zeta-function approach to Casimir energy with singular potentials
In the framework of zeta-function approach the Casimir energy for three
simple model system: single delta potential, step function potential and three
delta potentials is analyzed. It is shown that the energy contains
contributions which are peculiar to the potentials. It is suggested to
renormalize the energy using the condition that the energy of infinitely
separated potentials is zero which corresponds to subtraction all terms of
asymptotic expansion of zeta-function. The energy obtained in this way obeys
all physically reasonable conditions. It is finite in the Dirichlet limit and
it may be attractive or repulsive depending on the strength of potential. The
effective action is calculated and it is shown that the surface contribution
appears. The renormalization of the effective action is discussed.Comment: 17 pages, 2 figures, added reference, address correcte
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