1,099 research outputs found
Liberating the Inflaton from Primordial Spectrum Constraints
I discuss a mechanism that renders the spectral index of the primordial
spectrum and the inflationary stage independent of each other. If a scalar
field acquires an appropriate time-dependent mass, it is possible to generate
an adiabatic, Gaussian scale invariant spectrum of density perturbations during
any stage of inflation. As an illustration, I present a simple model where the
time-dependent mass arises from the coupling of the inflaton to a second
scalar. The mechanism I propose might help to implement a successful
inflationary scenario in particle physics theories that do not yield slow-roll
potentials.Comment: 7 two-column pages, 1 figure. Uses RevTeX
A dynamical dark energy model with a given luminosity distance
It is assumed that the current cosmic acceleration is driven by a scalar
field, the Lagrangian of which is a function of the kinetic term only, and that
the luminosity distance is a given function of the red-shift. Upon comparison
with Baryon Acoustic Oscillations (BAOs) and Cosmic Microwave Background (CMB)
data the parameters of the models are determined, and then the time evolution
of the scalar field is determined by the dynamics using the cosmological
equations. We find that the solution is very different than the corresponding
solution when the non-relativistic matter is ignored, and that the universe
enters the acceleration era at larger red-shift compared to the standard
model.Comment: 4 pages, 3 figures, accepted for publication in GER
Bayesian Limits on Primordial Isotropy Breaking
It is often assumed that primordial perturbations are statistically
isotropic, which implies, among other properties, that their power spectrum is
invariant under rotations. In this article, we test this assumption by placing
model-independent bounds on deviations from rotational invariance of the
primordial spectrum. Using five-year WMAP cosmic microwave anisotropy maps, we
set limits on the overall norm and the amplitude of individual components of
the primordial spectrum quadrupole. We find that there is no significant
evidence for primordial isotropy breaking, and that an eventually non-vanishing
quadrupole has to be subdominant.Comment: 6 double-column pages, 2 figues and 2 tables. Uses REVTeX
Haloes of k-Essence
We study gravitationally bound static and spherically symmetric
configurations of k-essence fields. In particular, we investigate whether these
configurations can reproduce the properties of dark matter haloes. The classes
of Lagrangians we consider lead to non-isotropic fluids with barotropic and
polytropic equations of state. The latter include microscopic realizations of
the often-considered Chaplygin gases, which we find can cluster into dark
matter halo-like objects with flat rotation curves, while exhibiting a dark
energy-like negative pressure on cosmological scales. We complement our studies
with a series of formal general results about the stability and initial value
formulation of non-canonical scalar field theories, and we also discuss a new
class of de Sitter solutions with spacelike field gradients.Comment: 34pages, single column double spacing, 7 figures, 3 Tables, RevTex4.
Additional references and minor clarifications. To be submitted to JCA
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.
Why should primordial perturbations be in a vacuum state?
In order to calculate the power spectrum generated during a stage of
inflation, we have to specify the quantum state of the inflaton perturbations,
which is conventionally assumed to be the Bunch-Davies vacuum. We argue that
this choice is justified only if the interactions of cosmological perturbations
are strong enough to drive excited states toward the vacuum. We quantify this
efficiency by calculating the decay probabilities of excited states to leading
order in the slow-roll expansion in canonical single-field inflationary models.
These probabilities are suppressed by a slow-roll parameter and the squared
Planck mass, and enhanced by ultraviolet and infrared cut-offs. For natural
choices of these scales decays are unlikely, and, hence, the choice of the
Bunch-Davies vacuum as the state of the primordial perturbations does not
appear to be warranted.Comment: 25 pages, 1 figure. Uses RevTeX
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