299 research outputs found
Evidence for horizon-scale power from CMB polarization
The CMB temperature power spectrum offers ambiguous evidence for the
existence of horizon-scale power in the primordial power spectrum due to
uncertainties in spatial curvature and the physics of cosmic acceleration as
well as the observed low quadrupole. Current polarization data from WMAP
provide evidence for horizon-scale power that is robust to these uncertainties.
Polarization on the largest scales arises mainly from scattering at z<6 when
the universe is fully ionized, making the evidence robust to ionization history
variations at higher redshifts as well. A cutoff in the power spectrum is
limited to C=k_C/10^{-4} Mpc^{-1}<5.2 (95% CL) by polarization, only slightly
weaker than joint temperature and polarization constraints in flat LCDM
(C<4.2). Planck should improve the polarization limit to C<3.6 for any model of
the acceleration epoch and ionization history as well as provide tests for
foreground and systematic contamination.Comment: 4 pages, 2 figures; submitted to Phys. Rev. D (Rapid Communications).
Code for modified reionization in CAMB and CosmoMC available at
http://background.uchicago.edu/camb_rpc
The Maximum B-mode Polarization of the Cosmic Microwave Background from Inhomogeneous Reionization
We compute the B-mode polarization power spectrum of the CMB from an epoch of
inhomogeneous reionization, using a simple model in which HII regions are
represented by ionized spherical bubbles with a log normal distribution of
sizes whose clustering properties are determined by large-scale structure. Both
the global ionization fraction and the characteristic radius of HII regions are
allowed to be free functions of redshift. Models that would produce substantial
contamination to degree scale gravitational wave B-mode measurements have power
that is dominated by the shot noise of the bubbles. Rare bubbles of >100 Mpc at
z>20 can produce signals that in fact exceed the B-modes from gravitational
lensing and are comparable to the maximal allowed signal of gravitational waves
(~0.1uK) while still being consistent with global constraints on the total
optical depth. Even bubbles down to 20 Mpc at z~15, or 40 Mpc at z~10 can be
relevant (0.01uK) once the lensing signal is removed either statistically or
directly. However, currently favored theoretical models that have ionization
bubbles that only grow to such sizes at the very end of a fairly prompt and
late reionization produce signals which are at most at these levels.Comment: 14 pages, 11 figures; published in ApJ; corrected Fig. 4 and updated
reference
Figures of merit for present and future dark energy probes
We compare current and forecasted constraints on dynamical dark energy models
from Type Ia supernovae and the cosmic microwave background using figures of
merit based on the volume of the allowed dark energy parameter space. For a
two-parameter dark energy equation of state that varies linearly with the scale
factor, and assuming a flat universe, the area of the error ellipse can be
reduced by a factor of ~10 relative to current constraints by future
space-based supernova data and CMB measurements from the Planck satellite. If
the dark energy equation of state is described by a more general basis of
principal components, the expected improvement in volume-based figures of merit
is much greater. While the forecasted precision for any single parameter is
only a factor of 2-5 smaller than current uncertainties, the constraints on
dark energy models bounded by -1<w<1 improve for approximately 6 independent
dark energy parameters resulting in a reduction of the total allowed volume of
principal component parameter space by a factor of ~100. Typical quintessence
models can be adequately described by just 2-3 of these parameters even given
the precision of future data, leading to a more modest but still significant
improvement. In addition to advances in supernova and CMB data, percent-level
measurement of absolute distance and/or the expansion rate is required to
ensure that dark energy constraints remain robust to variations in spatial
curvature.Comment: 9 pages, 7 figures; submitted to Phys. Rev.
Model-independent constraints on reionization from large-scale CMB polarization
On large angular scales, the polarization of the CMB contains information
about the evolution of the average ionization during the epoch of reionization.
Interpretation of the polarization spectrum usually requires the assumption of
a fixed functional form for the evolution, e.g. instantaneous reionization. We
develop a model-independent method where a small set of principal components
completely encapsulate the effects of reionization on the large-angle E-mode
polarization for any reionization history within an adjustable range in
redshift. Using Markov Chain Monte Carlo methods, we apply this approach to
both the 3-year WMAP data and simulated future data. WMAP data constrain two
principal components of the reionization history, approximately corresponding
to the total optical depth and the difference between the contributions to the
optical depth at high and low redshifts. The optical depth is consistent with
the constraint found in previous analyses of WMAP data that assume
instantaneous reionization, with only slightly larger uncertainty due to the
expanded set of models. Using the principal component approach, WMAP data also
place a 95% CL upper limit of 0.08 on the contribution to the optical depth
from redshifts z>20. With improvements in polarization sensitivity and
foreground modeling, approximately five of the principal components can
ultimately be measured. Constraints on the principal components, which probe
the entire reionization history, can test models of reionization, provide
model-independent constraints on the optical depth, and detect signatures of
high-redshift reionization.Comment: 14 pages, 13 figures; submitted to Ap
Hiding dark energy transitions at low redshift
We show that it is both observationally allowable and theoretically possible
to have large fluctuations in the dark energy equation of state as long as they
occur at ultra-low redshifts z<0.02. These fluctuations would masquerade as a
local transition in the Hubble rate of a few percent or less and escape even
future, high precision, high redshift measurements of the expansion history and
structure. Scalar field models that exhibit this behavior have a sharp feature
in the potential that the field traverses within a fraction of an e-fold of the
present. The equation of state parameter can become arbitrarily large if a
sharp dip or bump in the potential causes the kinetic and potential energy of
the field to both be large and have opposite sign. While canonical scalar field
models can decrease the expansion rate at low redshift, increasing the local
expansion rate requires a non-canonical kinetic term for the scalar field.Comment: 4 pages, 2 figures; submitted to Phys. Rev. D (Brief Report
Testing dark energy paradigms with weak gravitational lensing
Any theory invoked to explain cosmic acceleration predicts consistency
relations between the expansion history, structure growth, and all related
observables. Currently there exist high-quality measurements of the expansion
history from Type Ia supernovae, the cosmic microwave background temperature
and polarization spectra, and baryon acoustic oscillations. We can use
constraints from these datasets to predict what future probes of structure
growth should observe. We apply this method to predict what range of cosmic
shear power spectra would be expected if we lived in a LambdaCDM universe, with
or without spatial curvature, and what results would be inconsistent and
therefore falsify the model. Though predictions are relaxed if one allows for
an arbitrary quintessence equation of state , we find that any
observation that rules out LambdaCDM due to excess lensing will also rule out
all quintessence models, with or without early dark energy. We further explore
how uncertainties in the nonlinear matter power spectrum, e.g. from approximate
fitting formulas such as Halofit, warm dark matter, or baryons, impact these
limits.Comment: 12 pages, 11 figures, submitted to PR
Testing flatness of the universe with probes of cosmic distances and growth
When using distance measurements to probe spatial curvature, the geometric
degeneracy between curvature and dark energy in the distance-redshift relation
typically requires either making strong assumptions about the dark energy
evolution or sacrificing precision in a more model-independent approach.
Measurements of the redshift evolution of the linear growth of perturbations
can break the geometric degeneracy, providing curvature constraints that are
both precise and model-independent. Future supernova, CMB, and cluster data
have the potential to measure the curvature with an accuracy of
sigma(Omega_K)=0.002, without specifying a particular dark energy
phenomenology. In combination with distance measurements, the evolution of the
growth function at low redshifts provides the strongest curvature constraint if
the high-redshift universe is well approximated as being purely matter
dominated. However, in the presence of early dark energy or massive neutrinos,
the precision in curvature is reduced due to additional degeneracies, and
precise normalization of the growth function relative to recombination is
important for obtaining accurate constraints. Curvature limits from distances
and growth compare favorably to other approaches to curvature estimation
proposed in the literature, providing either greater accuracy or greater
freedom from dark energy modeling assumptions, and are complementary due to the
use of independent data sets. Model-independent estimates of curvature are
critical for both testing inflation and obtaining unbiased constraints on dark
energy parameters.Comment: 23 pages, 11 figures; submitted to Phys. Rev.
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