108 research outputs found
From the production of primordial perturbations to the end of inflation
In addition to generating the appropriate perturbation power spectrum, an
inflationary scenario must take into account the need for inflation to end
subsequently. In the context of single-field inflation models where inflation
ends by breaking of the slow-roll condition, we constrain the first and second
derivatives of the inflaton potential using this additional requirement. We
compare this with current observational constraints from the primordial
spectrum and discuss several issues relating to our results.Comment: RevTex4, 6 pages, 7 figures. To match version accepted by PR
Evolution of large-scale perturbations in quintessence models
We carry out a comprehensive study of the dynamics of large-scale
perturbations in quintessence scenarios. We model the contents of the Universe
by a perfect fluid with equation of state w_f and a scalar field Q with
potential V(Q). We are able to reduce the perturbation equations to a system of
four first-order equations. During each of the five main regimes of
quintessence field behaviour, these equations have constant coefficients,
enabling analytic solution of the perturbation evolution by eigenvector
decomposition. We determine these solutions and discuss their main properties.Comment: 5 pages RevTeX4 file with two figures incorporate
Early Tracking Behavior in Small-field Quintessence Models
We study several quintessence models which are singular at Q=0, and use a
simple initial constraint to see when they enter
tracking regime, disregarding the details of inflation. We find it can give
strong constraints for the inverse power-law potential ,
which has to enter tracking regime for . While for the
supergravity model , the constraint is much
weakened. For another kind inverse power-law potential , it exhibits no constraints.Comment: 11 pages,5 figures. Improved versio
K-essential Phantom Energy: Doomsday around the Corner? Revisited
We generalize some of those results reported by Gonz\'{a}lez-D\'{i}az by
further tuning the parameter () which is closely related to the
canonical kinetic term in -essence formalism. The scale factor could
be negative and decreasing within a specific range of (, : the equation-of-state parameter) during the initial
evolutional period.Comment: 1 Figure, 6 page
Observational constraints on thawing quintessence models
We use a dynamical systems approach to study thawing quintessence models,
using a multi-parameter extension of the exponential potential which can
approximate the form of typical thawing potentials. We impose observational
constraints using a compilation of current data, and forecast the tightening of
constraints expected from future dark energy surveys, as well as discussing the
relation of our results to analytical constraints already in the literature.Comment: 6 pages MNRAS style with 8 figures included. Minor updates to match
MNRAS accepted versio
K-essence and the coincidence problem
K-essence has been proposed as a possible means of explaining the coincidence
problem of the Universe beginning to accelerate only at the present epoch. We
carry out a comprehensive dynamical systems analysis of the k-essence models
given so far in the literature. We numerically study the basin of attraction of
the tracker solutions and we highlight the behaviour of the field close to
sound speed divergences. We find that, when written in terms of parameters with
a simple dynamical interpretation, the basins of attraction represent only a
small region of the phase space.Comment: 5 pages RevTeX4 file with two figures incorporated. Minor changes to
match PRD accepted versio
Quintessence as k-essence
Quintessence and k-essence have been proposed as candidates for the dark
energy component of the universe that would be responsible of the currently
observed accelerated expansion. In this paper we investigate the degree of
resemblance between those two theoretical setups, and find that every
quintessence model can be viewed as a k-essence model generated by a kinetic
linear function. In addition, we show the true effects of k-essence begin at
second order in the expansion of the kinetic function in powers of the kinetic
energy.Comment: 14 pages, improved discussion, matches published versio
On A Cosmological Invariant as an Observational Probe in the Early Universe
k-essence scalar field models are usually taken to have lagrangians of the
form with some general function of
. Under certain conditions this lagrangian
in the context of the early universe can take the form of that of an oscillator
with time dependent frequency. The Ermakov invariant for a time dependent
oscillator in a cosmological scenario then leads to an invariant quadratic form
involving the Hubble parameter and the logarithm of the scale factor. In
principle, this invariant can lead to further observational probes for the
early universe. Moreover, if such an invariant can be observationally verified
then the presence of dark energy will also be indirectly confirmed.Comment: 4 pages, Revte
Perturbations in cosmologies with a scalar field and a perfect fluid
We study the properties of cosmological density perturbations in a
multi-component system consisting of a scalar field and a perfect fluid. We
discuss the number of degrees of freedom completely describing the system,
introduce a full set of dynamical gauge-invariant equations in terms of the
curvature and entropy perturbations, and display an efficient formulation of
these equations as a first-order system linked by a fairly sparse matrix. Our
formalism includes spatial gradients, extending previous formulations
restricted to the large-scale limit, and fully accounts for the evolution of an
isocurvature mode intrinsic to the scalar field. We then address the issue of
the adiabatic condition, in particular demonstrating its preservation on large
scales. Finally, we apply our formalism to the quintessence scenario and
clearly underline the importance of initial conditions when considering
late-time perturbations. In particular, we show that entropy perturbations can
still be present when the quintessence field energy density becomes
non-negligible.Comment: RevTex4, 9 pages, 3 figures. Significant additions on the
quintessence scenario (new appendix and additional numerical example).
Conclusions unchanged, but more robus
Estimating Temperature Fluctuations in the Early Universe
A lagrangian for the essence field is constructed for a constant scalar
potential and its form determined when the scale factor was very small compared
to the present epoch but very large compared to the inflationary epoch. This
means that one is already in an expanding and flat universe. The form is
similar to that of an oscillator with time-dependent frequency. Expansion is
naturally built into the theory with the existence of growing classical
solutions of the scale factor. The formalism allows one to estimate
fluctuations of the temperature of the background radiation in these early
stages (compared to the present epoch) of the universe. If the temperature at
time is and at time the temperature is
(), then for small times, the probability for the logarithm of
inverse temperature evolution can be estimated to be given by
where
, is the Planck mass and Planck's constant and the
speed of light has been put equal to unity. There is the further possibility
that a single scalar field may suffice for an inflationary scenario as well as
the dark matter and dark energy realms.Comment: 8 pages, Revtex, title,abstract and format changed for journal
publication,no change in basic results, clarifications and a figure added.
Keywords: physics of the early universe,inflation, dark matter theory, dark
energy theory. PACS: 95.35.+d ; 95.36.+x ; 98.80.Cq ; 98.80.-
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