804 research outputs found
The IR-Completion of Gravity: What happens at Hubble Scales?
We have recently proposed an "Ultra-Strong" version of the Equivalence
Principle (EP) that is not satisfied by standard semiclassical gravity. In the
theory that we are conjecturing, the vacuum expectation value of the (bare)
energy momentum tensor is exactly the same as in flat space: quartically
divergent with the cut-off and with no spacetime dependent (subleading) ter ms.
The presence of such terms seems in fact related to some known difficulties,
such as the black hole information loss and the cosmological constant problem.
Since the terms that we want to get rid of are subleading in the high-momentum
expansion, we attempt to explore the conjectured theory by "IR-completing" GR.
We consider a scalar field in a flat FRW Universe and isolate the first
IR-correction to its Fourier modes operators that kills the quadratic (next to
leading) time dependent divergence of the stress energy tensor VEV. Analogously
to other modifications of field operators that have been proposed in the
literature (typically in the UV), the present approach seems to suggest a
breakdown (here, in the IR, at large distances) of the metric manifold
description. We show that corrections to GR are in fact very tiny, become
effective at distances comparable to the inverse curvature and do not contain
any adjustable parameter. Finally, we derive some cosmological implications. By
studying the consistency of the canonical commutation relations, we infer a
correction to the distance between two comoving observers, which grows as the
scale factor only when small compared to the Hubble length, but gets relevant
corrections otherwise. The corrections to cosmological distance measures are
also calculable and, for a spatially flat matter dominated Universe, go in the
direction of an effective positive acceleration.Comment: 27 pages, 2 figures. Final version, references adde
String-inspired cosmology: Late time transition from scaling matter era to dark energy universe caused by a Gauss-Bonnet coupling
The Gauss-Bonnet (GB) curvature invariant coupled to a scalar field
can lead to an exit from a scaling matter-dominated epoch to a late-time
accelerated expansion, which is attractive to alleviate the coincident problem
of dark energy. We derive the condition for the existence of cosmological
scaling solutions in the presence of the GB coupling for a general scalar-field
Lagrangian density , where is a kinematic
term of the scalar field. The GB coupling and the Lagrangian density are
restricted to be in the form and , respectively, where is a constant and is an
arbitrary function. We also derive fixed points for such a scaling Lagrangian
with a GB coupling and clarify the conditions
under which the scaling matter era is followed by a de-Sitter solution which
can appear in the presence of the GB coupling. Among scaling models proposed in
the current literature, we find that the models which allow such a cosmological
evolution are an ordinary scalar field with an exponential potential and a
tachyon field with an inverse square potential, although the latter requires a
coupling between dark energy and dark matter.Comment: 18 pages, 4 figures, version to appear in JCA
Minimal Scales from an Extended Hilbert Space
We consider an extension of the conventional quantum Heisenberg algebra,
assuming that coordinates as well as momenta fulfil nontrivial commutation
relations. As a consequence, a minimal length and a minimal mass scale are
implemented. Our commutators do not depend on positions and momenta and we
provide an extension of the coordinate coherent state approach to
Noncommutative Geometry. We explore, as toy model, the corresponding quantum
field theory in a (2+1)-dimensional spacetime. Then we investigate the more
realistic case of a (3+1)-dimensional spacetime, foliated into noncommutative
planes. As a result, we obtain propagators, which are finite in the ultraviolet
as well as the infrared regime.Comment: 16 pages, version which matches that published on CQ
What is needed of a tachyon if it is to be the dark energy?
We study a dark energy scenario in the presence of a tachyon field
with potential and a barotropic perfect fluid. The cosmological
dynamics crucially depends on the asymptotic behavior of the quantity
. If is a constant, which corresponds to
an inverse square potential , there exists one
stable critical point that gives an acceleration of the universe at late times.
When asymptotically, we can have a viable dark energy scenario
in which the system approaches an ``instantaneous'' critical point that
dynamically changes with . If approaches infinity
asymptotically, the universe does not exhibit an acceleration at late times. In
this case, however, we find an interesting possibility that a transient
acceleration occurs in a regime where is smaller than of order
unity.Comment: 11 pages and 3 figures, minor clarifications added; final version to
appear in PR
On compatibility of string effective action with an accelerating universe
In this paper, we fully investigate the cosmological effects of the moduli
dependent one-loop corrections to the gravitational couplings of the string
effective action to explain the cosmic acceleration problem in early (and/or
late) universe. These corrections comprise a Gauss-Bonnet (GB) invariant
multiplied by universal non-trivial functions of the common modulus
and the dilaton . The model exhibits several features of cosmological
interest, including the transition between deceleration and acceleration
phases. By considering some phenomenologically motivated ansatzs for one of the
scalars and/or the scale factor (of the universe), we also construct a number
of interesting inflationary potentials. In all examples under consideration, we
find that the model leads only to a standard inflation () when the
numerical coefficient associated with modulus-GB coupling is positive,
while the model can lead also to a non-standard inflation (), if
is negative. In the absence of (or trivial) coupling between the GB term and
the scalars, there is no crossing between the phases, while
this is possible with non-trivial GB couplings, even for constant dilaton phase
of the standard picture. Within our model, after a sufficient amount of e-folds
of expansion, the rolling of both fields and can be small. In
turn, any possible violation of equivalence principle or deviations from the
standard general relativity may be small enough to easily satisfy all
astrophysical and cosmological constraints.Comment: 30 pages, 8 figures; v2 significant changes in notations, appendix
and refs added; v3 significant revisions, refs added; v4 appendix extended,
new refs, published versio
Coupled Quintessence and Phantom Based On a Dilaton
Based on dilatonic dark energy model, we consider two cases: dilaton field
with positive kinetic energy(coupled quintessence) and with negative kinetic
energy(phantom). In the two cases, we investigate the existence of attractor
solutions which correspond to an equation of state parameter and a
cosmic density parameter . We find that the coupled term
between matter and dilaton can't affect the existence of attractor solutions.
In the Mexican hat potential, the attractor behaviors, the evolution of state
parameter and cosmic density parameter , are shown
mathematically. Finally, we show the effect of coupling term on the evolution
of and with
respect to numerically.Comment: 9 pages, 11 figures, some references and Journal-ref adde
Aspects of Scalar Field Dynamics in Gauss-Bonnet Brane Worlds
The Einstein-Gauss-Bonnet equations projected from the bulk to brane lead to
a complicated Friedmann equation which simplifies to in the
asymptotic regimes. The Randall-Sundrum (RS) scenario corresponds to
whereas & give rise to high energy Gauss-Bonnet (GB) regime and
the standard GR respectively. Amazingly, while evolving from RS regime to high
energy GB limit, one passes through a GR like region which has important
implications for brane world inflation. For tachyon GB inflation with
potentials investigated in this paper, the scalar to
tensor ratio of perturbations is maximum around the RS region and is
generally suppressed in the high energy regime for the positive values of .
The ratio is very low for at all energy scales relative to GB inflation
with ordinary scalar field. The models based upon tachyon inflation with
polynomial type of potentials with generic positive values of turn out to
be in the observational contour bound at all energy scales varying
from GR to high energy GB limit. The spectral index improves for the
lower values of and approaches its scale invariant limit for in the
high energy GB regime. The ratio also remains small for large negative
values of , however, difference arises for models close to scale invariance
limit. In this case, the tensor to scale ratio is large in the GB regime
whereas it is suppressed in the intermediate region between RS and GB. Within
the frame work of patch cosmologies governed by , the behavior
of ordinary scalar field near cosmological singularity and the nature of
scaling solutions are distinguished for the values of .Comment: 15 pages, 10 eps figures; appendix on various scales in GB brane
world included and references updated; final version to appear in PR
UV stable, Lorentz-violating dark energy with transient phantom era
Phantom fields with negative kinetic energy are often plagued by the vacuum
quantum instability in the ultraviolet region. We present a Lorentz-violating
dark energy model free from this problem and show that the crossing of the
cosmological constant boundary w=-1 to the phantom equation of state is
realized before reaching a de Sitter attractor. Another interesting feature is
a peculiar time-dependence of the effective Newton's constant; the magnitude of
this effect is naturally small but may be close to experimental limits. We also
derive momentum scales of instabilities at which tachyons or ghosts appear in
the infrared region around the present Hubble scale and clarify the conditions
under which tachyonic instabilities do not spoil homogeneity of the
present/future Universe.Comment: 22 pages, 7 figures; Presentation modified substantially, results and
conclusions unchanged. Journal versio
Dilatonic ghost condensate as dark energy
We explore a dark energy model with a ghost scalar field in the context of
the runaway dilaton scenario in low-energy effective string theory. We address
the problem of vacuum stability by implementing higher-order derivative terms
and show that a cosmologically viable model of ``phantomized'' dark energy can
be constructed without violating the stability of quantum fluctuations. We also
analytically derive the condition under which cosmological scaling solutions
exist starting from a general Lagrangian including the phantom type scalar
field. We apply this method to the case where the dilaton is coupled to
non-relativistic dark matter and find that the system tends to become quantum
mechanically unstable when a constant coupling is always present. Nevertheless,
it is possible to obtain a viable cosmological solution in which the energy
density of the dilaton eventually approaches the present value of dark energy
provided that the coupling rapidly grows during the transition to the scalar
field dominated era.Comment: 26 pages, 6 figure
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
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