1,444 research outputs found
Modified cosmology from extended entropy with varying exponent
We present a modified cosmological scenario that arises from the application
of non-extensive thermodynamics with varying exponent. We extract the modified
Friedmann equations, which contain new terms quantified by the non-extensive
exponent, possessing standard CDM cosmology as a subcase. Concerning
the universe evolution at late times we obtain an effective dark energy sector,
and we show that we can acquire the usual thermal history, with the successive
sequence of matter and dark-energy epochs, with the effective dark-energy
equation-of-state parameter being in the quintessence or in the phantom regime.
The interesting feature of the scenario is that the above behaviors can be
obtained even if the explicit cosmological constant is set to zero, namely they
arise purely from the extra terms. Additionally, we confront the model with
Supernovae type Ia and Hubble parameter observational data, and we show that
the agreement is very good. Concerning the early-time universe we obtain
inflationary de Sitter solutions, which are driven by an effective cosmological
constant that includes the new terms of non-extensive thermodynamics. This
effective screening can provide a description of both inflation and late-time
acceleration with the same parameter choices, which is a significant advantage.Comment: 10 pages, 2 figures, to appear in Eur.Phys.J.
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
String versus Einstein frame in an AdS/CFT induced quantum dilatonic brane-world universe
AdS/CFT induced quantum dilatonic brane-world where 4d boundary is flat or de
Sitter (inflationary) or Anti-de Sitter brane is considered. The classical
brane tension is fixed but boundary QFT produces the effective brane tension
via the account of corresponding conformal anomaly induced effective action.
This results in inducing of brane-worlds in accordance with AdS/CFT set-up as
warped compactification. The explicit, independent construction of quantum
induced dilatonic brane-worlds in two frames: string and Einstein one is done.
It is demonstrated their complete equivalency for all quantum cosmological
brane-worlds under discussion, including several examples of classical
brane-world black holes. This is different from quantum corrected 4d dilatonic
gravity where de Sitter solution exists in Einstein but not in Jordan (string)
frame. The role of quantum corrections on massive graviton perturbations around
Anti-de Sitter brane is briefly discussed.Comment: LaTeX file, 24 pages, minor changes to match with published versio
Phantom phase power-law solution in gravity
Power-law solutions for gravity coupled with perfect fluid have been
studied for spatially flat universe. It is shown that despite the matter
dominated and accelerating power-law solutions, the power-law solution exists
for an special form of when this universe enters a Phantom phase.Comment: 10 pages, Published online in Astrophysics and Space Scienc
Dark energy generated from a (super)string effective action with higher order curvature corrections and a dynamical dilaton
We investigate the possibility of a dark energy universe emerging from an
action with higher-order string loop corrections to Einstein gravity in the
presence of a massless dilaton. These curvature corrections (up to order)
are different depending upon the type of (super)string model which is
considered. We find in fact that Type II, heterotic, and bosonic strings
respond differently to dark energy. A dark energy solution is shown to exist in
the case of the bosonic string, while the other two theories do not lead to
realistic dark energy universes. Detailed analysis of the dynamical stability
of the de-Sitter solution is presented for the case of a bosonic string. A
general prescription for the construction of a de-Sitter solution for the
low-energy (super)string effective action is also indicated. Beyond the
low-energy (super)string effective action, when the higher-curvature correction
coefficients depend on the dilaton, the reconstruction of the theory from the
universe expansion history is done with a corresponding prescription for the
scalar potentials.Comment: 15 pages, 7 eps figures, minor corrections, published versio
f(R,T) gravity
We consider f(R,T) modified theories of gravity, where the gravitational
Lagrangian is given by an arbitrary function of the Ricci scalar R and of the
trace of the stress-energy tensor T. We obtain the gravitational field
equations in the metric formalism, as well as the equations of motion for test
particles, which follow from the covariant divergence of the stress-energy
tensor. Generally, the gravitational field equations depend on the nature of
the matter source. The field equations of several particular models,
corresponding to some explicit forms of the function f(R,T), are also
presented. An important case, which is analyzed in detail, is represented by
scalar field models. We write down the action and briefly consider the
cosmological implications of the models, where is
the trace of the stress-energy tensor of a self-interacting scalar field. The
equations of motion of the test particles are also obtained from a variational
principle. The motion of massive test particles is non-geodesic, and takes
place in the presence of an extra force orthogonal to the four-velocity. The
Newtonian limit of the equation of motion is further analyzed. Finally, we
provide a constraint on the magnitude of the extra-acceleration by analyzing
the perihelion precession of the planet Mercury in the framework of the present
model.Comment: 14 pages. V2: minor corrections, to appear in PR
Accelerating universe from F(T) gravity
It is shown that the acceleration of the universe can be understood by
considering a F(T) gravity models. For these F(T) gravity models, a variant of
the accelerating cosmology reconstruction program is developed. Some explicit
examples of F(T) are reconstructed from the background FRW expansion history.Comment: 13 pages, references adde
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