696 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
Logarithm of the scale factor as a generalised coordinate in a lagrangian for dark matter and dark energy
A lagrangian for the essence field is set up with canonical kinetic
terms and incorporating the scaling relation of [1]. There are two degrees of
freedom, {\it viz.}, ( is the scale factor) and the
scalar field , and an interaction term involving and .The
Euler-Lagrange equations are solved for and . Using these solutions
quantities of cosmological interest are determined. The energy density
has a constant component which we identify as dark energy and a component
behaving as which we call dark matter. The pressure is {\it
negative} for time and the sound velocity . When dark energy dominates, the deceleration
parameter while in the matter dominated era . The
equation of state parameter is shown to be consistent with
for dark energy domination and during the matter
dominated era we have . Bounds for the parameters of the theory are
estimated from observational data.
Keywords: k-essence models, dark matter, dark energy
PACS No: 98.80.-kComment: 16 pages, latex, paper shortened by 2 pages for journal publicatio
A Phantom Menace? Cosmological consequences of a dark energy component with super-negative equation of state
It is extraordinary that a number of observations indicate that we live in a
spatially flat, low matter density Universe, which is currently undergoing a
period of accelerating expansion. The effort to explain this current state has
focused attention on cosmological models in which the dominant component of the
cosmic energy density has negative pressure, with an equation of state . Remarking that most observations are consistent with models right up to
the or cosmological constant () limit, it is natural to ask
what lies on the other side, at . In this regard, we construct a toy
model of a ``phantom'' energy component which possesses an equation of state
. Such a component is found to be compatible with most classical tests of
cosmology based on current data, including the recent type 1a SNe data as well
as the cosmic microwave background anisotropy and mass power spectrum. If the
future observations continue to allow , then barring unanticipated
systematic effects, the dominant component of the cosmic energy density may be
stranger than anything expected.Comment: update of original version, includes new material, matches version
appearing in Phys. Lett. B, (17 pages, 7 eps figures
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
- …