719 research outputs found
k-Inflation
It is shown that a large class of higher-order (i.e. non-quadratic) scalar
kinetic terms can, without the help of potential terms, drive an inflationary
evolution starting from rather generic initial conditions. In many models, this
kinetically driven inflation (or "k-inflation" for short) rolls slowly from a
high-curvature initial phase, down to a low-curvature phase and can exit
inflation to end up being radiation-dominated, in a naturally graceful manner.
We hope that this novel inflation mechanism might be useful in suggesting new
ways of reconciling the string dilaton with inflation.Comment: LaTeX, 20 pages including 3 figures. Submitted to Phys. Lett.
Essentials of k-essence
We recently introduced the concept of "k-essence" as a dynamical solution for
explaining naturally why the universe has entered an epoch of accelerated
expansion at a late stage of its evolution. The solution avoids fine-tuning of
parameters and anthropic arguments. Instead, k-essence is based on the idea of
a dynamical attractor solution which causes it to act as a cosmological
constant only at the onset of matter-domination. Consequently, k-essence
overtakes the matter density and induces cosmic acceleration at about the
present epoch. In this paper, we present the basic theory of k-essence and
dynamical attractors based on evolving scalar fields with non-linear kinetic
energy terms in the action. We present guidelines for constructing concrete
examples and show that there are two classes of solutions, one in which cosmic
acceleration continues forever and one in which the acceleration has finite
duration.Comment: 14 pages, 11 figure
Where does Cosmological Perturbation Theory Break Down?
We apply the effective field theory approach to the coupled metric-inflaton
system, in order to investigate the impact of higher dimension operators on the
spectrum of scalar and tensor perturbations in the short-wavelength regime. In
both cases, effective corrections at tree-level become important when the
Hubble parameter is of the order of the Planck mass, or when the physical wave
number of a cosmological perturbation mode approaches the square of the Planck
mass divided by the Hubble constant. Thus, the cut-off length below which
conventional cosmological perturbation theory does not apply is likely to be
much smaller than the Planck length. This has implications for the
observability of "trans-Planckian" effects in the spectrum of primordial
perturbations.Comment: 25 pages, uses FeynM
Einstein-Cartan gravity with scalar-fermion interactions
In this paper, we have considered the g-essence and its particular cases,
k-essence and f-essence, within the framework of the Einstein-Cartan theory. We
have shown that a single fermionic field can give rise to the accelerated
expansion within the Einstein-Cartan theory. The exact analytical solution of
the Einstein-Cartan-Dirac equations is found. This solution describes the
accelerated expansion of the Universe with the equation of state parameter
as in the case of CDM model.Comment: 6 pages, title is change
Near Scale Invariance with Modified Dispersion Relations
We describe a novel mechanism to seed a nearly scale invariant spectrum of
adiabatic perturbations during a non-inflationary stage. It relies on a
modified dispersion relation that contains higher powers of the spatial
momentum of matter perturbations. We implement this idea in the context of a
massless scalar field in an otherwise perfectly homogeneous universe. The
couplings of the field to background scalars and tensors give rise to the
required modification of its dispersion relation, and the couplings of the
scalar to matter result in an adiabatic primordial spectrum. This work is meant
to explicitly illustrate that it is possible to seed nearly scale invariant
primordial spectra without inflation, within a conventional expansion history.Comment: 7 pages and no figures. Uses RevTeX
Creating Statistically Anisotropic and Inhomogeneous Perturbations
In almost all structure formation models, primordial perturbations are
created within a homogeneous and isotropic universe, like the one we observe.
Because their ensemble averages inherit the symmetries of the spacetime in
which they are seeded, cosmological perturbations then happen to be
statistically isotropic and homogeneous. Certain anomalies in the cosmic
microwave background on the other hand suggest that perturbations do not
satisfy these statistical properties, thereby challenging perhaps our
understanding of structure formation. In this article we relax this tension. We
show that if the universe contains an appropriate triad of scalar fields with
spatially constant but non-zero gradients, it is possible to generate
statistically anisotropic and inhomogeneous primordial perturbations, even
though the energy momentum tensor of the triad itself is invariant under
translations and rotations.Comment: 20 pages, 1 figure. Uses RevTeX
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
Supernova data may be unable to distinguish between quintessence and k-essence
We consider the efficacy of using luminosity distance measurements of deep
redshift supernovae to discriminate between two forms of dark energy,
quintessence (a scalar field with canonical kinetic terms rolling down a
potential) and k-essence (a scalar field whose cosmic evolution is driven
entirely by non-linear kinetic terms). The primary phenomenological distinction
between the two types of models that can be quantified by supernova searches
(at least in principle) is that the equation of state of
quintessence is falling today while that of k-essence is rising. By simulating
possible datasets that SNAP could obtain, we show that even if the mass
density is known exactly, an ambiguity remains that may not allow a
definitive distinction to be made between the two types of theories.Comment: Version to appear in PL
A Dynamical Solution to the Problem of a Small Cosmological Constant and Late-time Cosmic Acceleration
Increasing evidence suggests that most of the energy density of the universe
consists of a dark energy component with negative pressure, a ``cosmological
constant" that causes the cosmic expansion to accelerate. In this paper, we
address the puzzle of why this component comes to dominate the universe only
recently rather than at some much earlier epoch. We present a class of theories
based on an evolving scalar field where the explanation is based entirely on
internal dynamical properties of the solutions. In the theories we consider,
the dynamics causes the scalar field to lock automatically into a negative
pressure state at the onset of matter-domination such that the present epoch is
the earliest possible time, consistent with nucleosynthesis restrictions, when
it can start to dominate.Comment: 5 pages, 3 figure
Initial Conditions for Vector Inflation
Recently, a model of inflation using non-minimally coupled massive vector
fields has been proposed. For a particular choice of non-minimal coupling
parameter and for a flat FRW model, the model is reduced to the model of
chaotic inflation with massive scalar field. We study the effect of non-zero
curvature of the universe on the onset of vector inflation. We find that in a
curved universe the dynamics of vector inflation can be different from chaotic
inflation, and the fraction of the initial conditions leading to inflationary
solutions is reduced compared with the chaotic inflation case.Comment: 12 pages, 5 figures, version to be published in JCA
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