9 research outputs found
Planck-scale quintessence and the physics of structure formation
In a recent paper we considered the possibility of a scalar field providing
an explanation for the cosmic acceleration. Our model had the interesting
properties of attractor-like behavior and having its parameters of O(1) in
Planck units. Here we discuss the effect of the field on large scale structure
and CMB anisotropies. We show how some versions of our model inspired by
"brane" physics have novel features due to the fact that the scalar field has a
significant role over a wider range of redshifts than for typical "dark energy"
models. One of these features is the additional suppression of the formation of
large scale structure, as compared with cosmological constant models. In light
of the new pressures being placed on cosmological parameters (in particular
H_0) by CMB data, this added suppression allows our "brane" models to give
excellent fits to both CMB and large scale structure data.Comment: 18 pages, 12 figures, submitted to PR
Equation of State of Oscillating Brans-Dicke Scalar and Extra Dimensions
We consider a Brans-Dicke scalar field stabilized by a general power law
potential with power index at a finite equilibrium value. Redshifting
matter induces oscillations of the scalar field around its equilibrium due to
the scalar field coupling to the trace of the energy momentum tensor. If the
stabilizing potential is sufficiently steep these high frequency oscillations
are consistent with observational and experimental constraints for arbitrary
value of the Brans-Dicke parameter . We study analytically and
numerically the equation of state of these high frequency oscillations in terms
of the parameters and and find the corresponding evolution of the
universe scale factor. We find that the equation of state parameter can be
negative and less than -1 but it is not related to the evolution of the scale
factor in the usual way. Nevertheless, accelerating expansion is found for a
certain parameter range. Our analysis applies also to oscillations of the size
of extra dimensions (the radion field) around an equilibrium value. This
duality between self-coupled Brans-Dicke and radion dynamics is applicable for
where D is the number of extra dimensions.Comment: 10 two-column pages, RevTex4, 8 figures. Added clarifying
discussions, new references. Accepted in Phys. Rev. D (to appear
Localized D-dimensional global k-defects
We explicitly demonstrate the existence of static global defect solutions of
arbitrary dimensionality whose energy does not diverge at spatial infinity, by
considering maximally symmetric solutions described by an action with
non-standard kinetic terms in a D+1 dimensional Minkowski space-time. We
analytically determine the defect profile both at small and large distances
from the defect centre. We verify the stability of such solutions and discuss
possible implications of our findings, in particular for dark matter and charge
fractionalization in graphene.Comment: 6 pages, published versio
Early-universe constraints on a Primordial Scaling Field
In the past years 'quintessence' models have been considered which can
produce the accelerated expansion in the universe suggested by recent
astronomical observations. One of the key differences between quintessence and
a cosmological constant is that the energy density in quintessence,
, could be a significant fraction of the overall energy even in
the early universe, while the cosmological constant will be dynamically
relevant only at late times. We use standard Big Bang Nucleosynthesis and the
observed abundances of primordial nuclides to put constraints on
at temperatures near . We point out that current experimental data
does not support the presence of such a field, providing the strong constraint
at C.L. and strengthening previous
results. We also consider the effect a scaling field has on CMB anisotropies
using the recent data from Boomerang and DASI, providing the CMB constraint
at during the radiation dominated epoch.Comment: 5 pages, 4 figures. The revised version includes the new Boomerang
and DASI dat
First-order formalism for dark energy and dust
This work deals with first-order formalism for dark energy and dust in
standard cosmology, for models described by real scalar field in the presence
of dust in spatially flat space. The field dynamics may be standard or
tachyonic, and we show how the equations of motion can be solved by first-order
differential equations. We investigate a model to illustrate how the dustlike
matter may affect the cosmic evolution using this framework.Comment: 5 pages, 1 figure; title changed, new author included, discussions
extended, references added, version to appear in EPJ
A 680,000-person megastudy of nudges to encourage vaccination in pharmacies
Encouraging vaccination is a pressing policy problem. To assess whether text-based reminders can encourage pharmacy vaccination and what kinds of messages work best, we conducted a megastudy. We randomly assigned 689,693 Walmart pharmacy patients to receive one of 22 different text reminders using a variety of different behavioral science principles to nudge flu vaccination or to a business-as-usual control condition that received no messages. We found that the reminder texts that we tested increased pharmacy vaccination rates by an average of 2.0 percentage points, or 6.8%, over a 3-mo follow-up period. The most-effective messages reminded patients that a flu shot was waiting for them and delivered reminders on multiple days. The top-performing intervention included two texts delivered 3 d apart and communicated to patients that a vaccine was “waiting for you.” Neither experts nor lay people anticipated that this would be the best-performing treatment, underscoring the value of simultaneously testing many different nudges in a highly powered megastudy.https://www.pnas.org/content/pnas/119/6/e2115126119.full.pd