308 research outputs found
Enhancing the tensor-to-scalar ratio in simple inflation
We show that in theories with a nontrivial kinetic term the contribution of
the gravitational waves to the CMB fluctuations can be substantially larger
than that is naively expected in simple inflationary models. This increase of
the tensor-to-scalar perturbation ratio leads to a larger B-component of the
CMB polarization, thus making the prospects for future detection much more
promising. The other important consequence of the considered model is a higher
energy scale of inflation and hence higher reheating temperature compared to a
simple inflation.Comment: 9 pages, 1 figure and references are added, discussion is slightly
extended, published versio
Comment on superluminality in general relativity
General relativity provides an appropriate framework for addressing the issue
of sub- or superluminality as an apparent effect. Even though a massless
particle travels on the light cone, its average velocity over a finite path
measured by different observers is not necessarily equal to the velocity of
light, as a consequence of the time dilation or contraction in gravitational
fields. This phenomenon occurs in either direction (increase or depletion)
irrespectively of the details and strength of the gravitational interaction.
Hence, it does not intrinsically guarantee superluminality, even when the
gravitational field is reinforced.Comment: 6 page
Simple generalizations of Anti-de Sitter space-time
We consider new cosmological solutions which generalize the cosmological
patch of the Anti-de Sitter (AdS) space-time, allowing for fluids with
equations of state such that . We use them to derive the associated
full manifolds. We find that these solutions can all be embedded in flat
five-dimensional space-time with signature, revealing deformed
hyperboloids. The topology and causal-structure of these spaces is therefore
unchanged, and closed time-like curves are identified, before a covering space
is considered. However the structure of Killing vector fields is entirely
different and so we may expect a different structure of Killing horizons in
these solutions.Comment: 6 Pages, 5 Figures, Corrections and additions made for publication in
Journal of Classical and Quantum Gravit
Suppressing Quantum Fluctuations in Classicalization
We study vacuum quantum fluctuations of simple Nambu-Goldstone bosons -
derivatively coupled single scalar-field theories possessing shift-symmetry in
field space. We argue that quantum fluctuations of the interacting field can be
drastically suppressed with respect to the free-field case. Moreover, the
power-spectrum of these fluctuations can soften to become red for sufficiently
small scales. In quasiclassical approximation, we demonstrate that this
suppression can only occur for those theories that admit such classical static
backgrounds around which small perturbations propagate faster than light. Thus,
a quasiclassical softening of quantum fluctuations is only possible for
theories which classicalize instead of having a usual Lorentz invariant and
local Wilsonian UV- completion. We illustrate our analysis by estimating the
quantum fluctuations for the DBI-like theories.Comment: 6 pages, no figures, published version, more general discussion of
uncertainty relation in QFT, improved and more general derivation of the main
resul
Crossing Phantom Boundary in Modified Gravity : Jordan Frame vs Einstein Frame
We study capability of gravity models to allow crossing the phantom
boundary in both Jordan and Einstein conformal frames. In Einstein frame, these
models are equivalent to Einstein gravity together with a scalar field
minimally coupled to gravity. This scalar degree of freedom appears as a
quintessence field with a coupling with the matter sector. We investigate
evolution of the equation of sate parameter for some cosmologically viable
gravity models in both conformal frames. This investigation (beyond mere
theoretical arguments) acts as an operational tool to distinguish physical
status of the two conformal frames. It shows that the two conformal frames have
not the same physical status.Comment: 13 Pages, 4 figs., To appear in Gravitation and Cosmolog
Stable phantom-divide crossing in two scalar models with matter
We construct cosmological models with two scalar fields, which has the
structure as in the ghost condensation model or k-essence model. The models can
describe the stable phantom crossing, which should be contrasted with one
scalar tensor models, where the infinite instability occurs at the crossing the
phantom divide. We give a general formulation of the reconstruction in terms of
the e-foldings N by including the matter although in the previous two scalar
models, which are extensions of the scalar tensor model, it was difficult to
give a formulation of the reconstruction when we include matters. In the
formulation of the reconstruction, we start with a model with some arbitrary
functions, and find the functions which generates the history in the expansion
of the universe. We also give general arguments for the stabilities of the
models and the reconstructed solution. The viability of a model is also
investigated by comparing the observational data.Comment: 12 pages, 1 figur
Stationary Configurations Imply Shift Symmetry: No Bondi Accretion for Quintessence / k-Essence
In this paper we show that, for general scalar fields, stationary
configurations are possible for shift symmetric theories only. This symmetry
with respect to constant translations in field space should either be manifest
in the original field variables or reveal itself after an appropriate field
redefinition. In particular this result implies that neither k-Essence nor
Quintessence can have exact steady state / Bondi accretion onto Black Holes. We
also discuss the role of field redefinitions in k-Essence theories. Here we
study the transformation properties of observables and other variables in
k-Essence and emphasize which of them are covariant under field redefinitions.
Finally we find that stationary field configurations are necessarily linear in
Killing time, provided that shift symmetry is realized in terms of these field
variables.Comment: 8 page
On Power Law Inflation in DBI Models
Inflationary models in string theory which identify the inflaton with an open
string modulus lead to effective field theories with non-canonical kinetic
terms: Dirac-Born-Infeld scalar field theories. In the case of a -brane
moving in an AdS throat with a quadratic scalar field potential DBI kinetic
terms allow a novel realization of power law inflation. This note adresses the
question of whether this behaviour is special to this particular choice of
throat geometry and potential. The answer is that for any throat geometry one
can explicitly find a potential which leads to power law inflation. This
generalizes the well known fact that an exponential potential gives power law
inflation in the case of canonical kinetic terms.Comment: References and comments adde
Scalar-tensor cosmology with R^{-1} curvature correction by Noether Symmetry
We discuss scalar-tensor cosmology with an extra correction by the
Noether Symmetry Approach. The existence of such a symmetry selects the forms
of the coupling , of the potential and allows to obtain
physically interesting exact cosmological solutions.Comment: 7 page
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