274 research outputs found
Quintessence and variation of the fine structure constant in the CMBR
We study dependence of the CMB temperature anisotropy spectrum on the value
of the fine structure constant and the equation of state of the dark
energy component of the total density of the universe. We find that bounds
imposed on the variation of from the analysis of currently available
CMB data sets can be significantly relaxed if one also allows for a change in
the equation of state.Comment: 5 pages, 3 figures. Several references added and a few minor typos
corrected in the revised versio
Measuring in the Early Universe: CMB Temperature, Large-Scale Structure and Fisher Matrix Analysis
We extend our recent work on the effects of a time-varying fine-structure
constant in the cosmic microwave background, by providing a thorough
analysis of the degeneracies between and the other cosmological
parameters, and discussing ways to break these with both existing and/or
forthcoming data. In particular, we present the state-of-the-art CMB
constraints on , through a combined analysis of the BOOMERanG, MAXIMA
and DASI datasets. We also present a novel discussion of the constraints on
coming from large-scale structure observations, focusing in particular
on the power spectrum from the 2dF survey. Our results are consistent with no
variation in from the epoch of recombination to the present day, and
restrict any such (relative) variation to be less than about 4%. We show that
the forthcoming MAP and (particularly) Planck experiments will be able to break
most of the currently existing degeneracies between and other
parameters, and measure to better than percent accuracy.Comment: 11 pages in RevTex4 format. Low-quality figures to comply with arXiv
restrictions (better ones available from the authors). v2: Updated Oklo
discussion, plus other cosmetic changes. Version to appear in Phys Rev
Geometry and cosmological perturbations in the bulk inflaton model
We consider a braneworld inflation model driven by the dynamics of a scalar
field living in the 5-dimensional bulk, the so-called ``bulk inflaton model'',
and investigate the geometry in the bulk and large scale cosmological
perturbations on the brane. The bulk gravitational effects on the brane are
described by a projection of the 5-dimensional Weyl tensor, which we denote by
. Focusing on a tachionic potential model, we take a perturbative
approach in the anti-de Sitter (AdS) background with a single de Sitter
brane. We first formulate the evolution equations for in the bulk.
Next, applying them to the case of a spatially homogeneous brane, we obtain two
different integral expressions for . One of them reduces to the
expression obtained previously when evaluated on the brane. The other is a new
expression that may be useful for analyzing the bulk geometry. Then we consider
superhorizon scale cosmological perturbations and evaluate the bulk effects
onto the brane. In the limit , where is the Hubble parameter
on the brane and is the bulk curvature radius, we find that the
effective theory on the brane is identical to the 4-dimensional Einstein-scalar
theory with a simple rescaling of the potential even under the presence of
inhomogeneities. % atleast on super-Hubble horizon scales. In particular, it is
found that the anticipated non-trivial bulk effect due to the spatially
anisotropic part of may appear only at %second order in the low
energy expansion, i.e., at .Comment: 21 pages including 6 pages for several appendixes, no figure
Early Dark Energy Cosmologies
We propose a novel parameterization of the dark energy density. It is
particularly well suited to describe a non-negligible contribution of dark
energy at early times and contains only three parameters, which are all
physically meaningful: the fractional dark energy density today, the equation
of state today and the fractional dark energy density at early times. As we
parameterize Omega_d(a) directly instead of the equation of state, we can give
analytic expressions for the Hubble parameter, the conformal horizon today and
at last scattering, the sound horizon at last scattering, the acoustic scale as
well as the luminosity distance. For an equation of state today w_0 < -1, our
model crosses the cosmological constant boundary. We perform numerical studies
to constrain the parameters of our model by using Cosmic Microwave Background,
Large Scale Structure and Supernovae Ia data. At 95% confidence, we find that
the fractional dark energy density at early times Omega_early < 0.06. This
bound tightens considerably to Omega_early < 0.04 when the latest Boomerang
data is included. We find that both the gold sample of Riess et. al. and the
SNLS data by Astier et. al. when combined with CMB and LSS data mildly prefer
w_0 < -1, but are well compatible with a cosmological constant.Comment: 6 pages, 3 figures; references added, matches published versio
Brane-World Cosmology of Modulus Stabilization with a Bulk Scalar Field
We point out that the potential of Goldberger and Wise for stabilizing the
distance between two 3-branes, separated from each other along an extra
dimension with a warp factor, has a metastable minimum when the branes are
infinitely separated. The classical evolution of the radion (brane separation)
will place it in this false minimum for generic initial conditions. In
particular, inflation could do this if the expansion rate is sufficiently
large. We present a simplified version of the Goldberger-Wise mechanism in
which the radion potential can be computed exactly, and we calculate the rate
of thermal transitions to the true minimum, showing that model parameters can
be chosen to ensure that the universe reaches the desired final state.
Finiteness of bulk scalar field brane potentials can have an important impact
on the nucleation rate, and it can also significantly increase the predicted
mass of the radion.Comment: 12 pages, 6 figure
Braneworld Tensor Anisotropies in the CMB
Cosmic microwave background (CMB) observations provide in principle a
high-precision test of models which are motivated by M theory. We set out the
framework of a program to compute the tensor anisotropies in the CMB that are
generated in braneworld models. In the simplest approximation, we show the
braneworld imprint as a correction to the power spectra for standard
temperature and polarization anisotropies.Comment: Minor corrections and references added. Accepted for publication in
Phys. Rev.
Precision Primordial He Measurement with CMB Experiments
Big bang nucleosynthesis (BBN) and the cosmic microwave background (CMB) are
two major pillars of cosmology. Standard BBN accurately predicts the primordial
light element abundances (He, D, He and Li), depending on one
parameter, the baryon density. Light element observations are used as a
baryometers. The CMB anisotropies also contain information about the content of
the universe which allows an important consistency check on the Big Bang model.
In addition CMB observations now have sufficient accuracy to not only determine
the total baryon density, but also resolve its principal constituents, H and
He. We present a global analysis of all recent CMB data, with special
emphasis on the concordance with BBN theory and light element observations. We
find and
(fraction of baryon mass as He) using CMB data alone, in agreement with
He abundance observations. With this concordance established we show that
the inclusion of BBN theory priors significantly reduces the volume of
parameter space. In this case, we find
and . We also find that the inclusion of deuterium
abundance observations reduces the and ranges by a factor
of 2. Further light element observations and CMB anisotropy experiments
will refine this concordance and sharpen BBN and the CMB as tools for precision
cosmology.Comment: 7 pages, 3 color figures made minor changes to bring inline with
journal versio
What Can WMAP Tell Us About The Very Early Universe? New Physics as an Explanation of Suppressed Large Scale Power and Running Spectral Index
The Wilkinson Microwave Anisotropy Probe microwave background data may be
giving us clues about new physics at the transition from a ``stringy'' epoch of
the universe to the standard Friedmann Robertson Walker description. Deviations
on large angular scales of the data, as compared to theoretical expectations,
as well as running of the spectral index of density perturbations, can be
explained by new physics whose scale is set by the height of an inflationary
potential. As examples of possible signatures for this new physics, we study
the cosmic microwave background spectrum for two string inspired models: 1)
modifications to the Friedmann equations and 2) velocity dependent potentials.
The suppression of low ``l'' modes in the microwave background data arises due
to the new physics. In addition, the spectral index is red (n<1) on small
scales and blue (n>1) on large scales, in agreement with data.Comment: 18 pages, 2 figures, submitted for publication in Physical Review D,
references added in this versio
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
Apraxia and motor dysfunction in corticobasal syndrome
Background: Corticobasal syndrome (CBS) is characterized by multifaceted motor system dysfunction and cognitive disturbance; distinctive clinical features include limb apraxia and visuospatial dysfunction. Transcranial magnetic stimulation (TMS) has been used to study motor system dysfunction in CBS, but the relationship of TMS parameters to clinical features has not been studied. The present study explored several hypotheses; firstly, that limb apraxia may be partly due to visuospatial impairment in CBS. Secondly, that motor system dysfunction can be demonstrated in CBS, using threshold-tracking TMS, and is linked to limb apraxia. Finally, that atrophy of the primary motor cortex, studied using voxel-based morphometry analysis (VBM), is associated with motor system dysfunction and limb apraxia in CBS. Methods: Imitation of meaningful and meaningless hand gestures was graded to assess limb apraxia, while cognitive performance was assessed using the Addenbrooke's Cognitive Examination - Revised (ACE-R), with particular emphasis placed on the visuospatial subtask. Patients underwent TMS, to assess cortical function, and VBM. Results: In total, 17 patients with CBS (7 male, 10 female; mean age 64.4+/2 6.6 years) were studied and compared to 17 matched control subjects. Of the CBS patients, 23.5% had a relatively inexcitable motor cortex, with evidence of cortical dysfunction in the remaining 76.5% patients. Reduced resting motor threshold, and visuospatial performance, correlated with limb apraxia. Patients with a resting motor threshold <50% performed significantly worse on the visuospatial sub-task of the ACE-R than other CBS patients. Cortical function correlated with atrophy of the primary and pre-motor cortices, and the thalamus, while apraxia correlated with atrophy of the pre-motor and parietal cortices. Conclusions: Cortical dysfunction appears to underlie the core clinical features of CBS, and is associated with atrophy of the primary motor and pre-motor cortices, as well as the thalamus, while apraxia correlates with pre-motor and parietal atrophy
- …