1,039 research outputs found
CMB power spectrum parameter degeneracies in the era of precision cosmology
Cosmological parameter constraints from the CMB power spectra alone suffer
several well-known degeneracies. These degeneracies can be broken by numerical
artefacts and also a variety of physical effects that become quantitatively
important with high-accuracy data e.g. from the Planck satellite. We study
degeneracies in models with flat and non-flat spatial sections, non-trivial
dark energy and massive neutrinos, and investigate the importance of various
physical degeneracy-breaking effects. We test the CAMB power spectrum code for
numerical accuracy, and demonstrate that the numerical calculations are
accurate enough for degeneracies to be broken mainly by true physical effects
(the integrated Sachs-Wolfe effect, CMB lensing and geometrical and other
effects through recombination) rather than numerical artefacts. We quantify the
impact of CMB lensing on the power spectra, which inevitably provides
degeneracy-breaking information even without using information in the
non-Gaussianity. Finally we check the numerical accuracy of sample-based
parameter constraints using CAMB and CosmoMC. In an appendix we document recent
changes to CAMB's numerical treatment of massive neutrino perturbations, which
are tested along with other recent improvements by our degeneracy exploration
results.Comment: 27 pages, 28 figures. Latest CAMB version available from
http://camb.info/. Reduced number of figures, plot legend corrected and minor
edits to match published versio
Generation of Curvature Perturbations with Extra Anisotropic Stress
We study the evolution of curvature perturbations and the cosmic microwave
background (CMB) power spectrum in the presence of an hypothesized extra
anisotropic stress which might arise, for example, from the dark radiation term
in brane-world cosmology. We evolve the scalar modes of such perturbations
before and after neutrino decoupling and analyze their effects on the CMB
spectrum. A novel result of this work is that the cancellation of the neutrino
and extra anisotropic stress could lead to a spectrum of residual curvature
perturbations which is similar to the observed CMB power spectrum. This implies
a possible additional consideration in the determination of cosmological
parameters from the CMB analysis.Comment: 13 pages, 2 figures; improved discussio
Analysis of CMB polarization on an incomplete sky
The full sky cosmic microwave background polarization field can be decomposed
into 'electric' and 'magnetic' components. Working in harmonic space we
construct window functions that allow clean separation of the electric and
magnetic modes from observations over only a portion of the sky. Our
construction is exact for azimuthally symmetric patches, but should continue to
perform well for arbitrary patches. From the window functions we obtain
variables that allow for robust estimation of the magnetic component without
risk of contamination from the probably much larger electric signal. For
isotropic, uncorrelated noise the variables have a very simple diagonal noise
correlation, and further analysis using them should be no harder than analysing
the temperature field. For an azimuthally-symmetric patch, such as that
obtained from survey missions when the galactic region is removed, the
exactly-separated variables are fast to compute allowing us to estimate the
magnetic signal that could be detected by the Planck satellite in the absence
of non-galactic foregrounds. We also discuss the sensitivity of future
experiments to tensor modes in the presence of a magnetic signal generated by
weak lensing, and give lossless methods for analysing the electric polarization
field in the case that the magnetic component is negligible.Comment: 27 pages, 8 figures. New appendix on weak signal detection and
revised plots using a better statistic. Other changes to match version
accepted by PRD. Sample source code now available at
http://cosmologist.info/pola
Current cosmological constraints from a 10 parameter CMB analysis
We compute the constraints on a ``standard'' 10 parameter cold dark matter
(CDM) model from the most recent CMB and data and other observations, exploring
30 million discrete models and two continuous parameters. Our parameters are
the densities of CDM, baryons, neutrinos, vacuum energy and curvature, the
reionization optical depth, and the normalization and tilt for both scalar and
tensor fluctuations.
Our strongest constraints are on spatial curvature, -0.24 < Omega_k < 0.38,
and CDM density, h^2 Omega_cdm <0.3, both at 95%. Including SN 1a constraints
gives a positive cosmological constant at high significance.
We explore the robustness of our results to various assumptions. We find that
three different data subsets give qualitatively consistent constraints. Some of
the technical issues that have the largest impact are the inclusion of
calibration errors, closed models, gravity waves, reionization, nucleosynthesis
constraints and 10-dimensional likelihood interpolation.Comment: Replaced to match published ApJ version. More details added. 13 ApJ
pages. CMB movies and color figs at
http://www.hep.upenn.edu/~max/10par_frames.html or from [email protected]
Lensed CMB power spectra from all-sky correlation functions
Weak lensing of the CMB changes the unlensed temperature anisotropy and
polarization power spectra. Accounting for the lensing effect will be crucial
to obtain accurate parameter constraints from sensitive CMB observations.
Methods for computing the lensed power spectra using a low-order perturbative
expansion are not good enough for percent-level accuracy. Non-perturbative
flat-sky methods are more accurate, but curvature effects change the spectra at
the 0.3-1% level. We describe a new, accurate and fast, full-sky
correlation-function method for computing the lensing effect on CMB power
spectra to better than 0.1% at l<2500 (within the approximation that the
lensing potential is linear and Gaussian). We also discuss the effect of
non-linear evolution of the gravitational potential on the lensed power
spectra. Our fast numerical code is publicly available.Comment: 16 pages, 4 figures. Changes to match PRD version including new
section on non-linear corrections. CAMB code available at http://camb.info
The covariant perturbative approach to cosmic microwave background anisotropies
The Ehlers-Ellis 1+3 formulation of covariant hydrodynamics, when
supplemented with covariant radiative transport theory, gives an exact,
physically transparent description of the physics of the cosmic microwave
background radiation (CMB). Linearisation around a Friedmann-Robertson-Walker
(FRW) universe provides a very direct and seamless route through to the linear,
gauge-invariant perturbation equations for scalar, vector and tensor modes in
an almost-FRW model. In this contribution we review covariant radiative
transport theory and its application to the perturbative method for calculating
and understanding the anisotropy of the CMB. Particular emphasis is placed on
the inclusion of polarization in a fully covariant manner. With this inclusion,
the covariant perturbative approach offers a complete description of linearised
CMB physics in an almost-FRW universe.Comment: To appear in proceedings of SARS99 meeting in honour of G.F.R.Elli
All-sky convolution for polarimetry experiments
We discuss all-sky convolution of the instrument beam with the sky signal in
polarimetry experiments, such as the Planck mission which will map the
temperature anisotropy and polarization of the cosmic microwave background
(CMB). To account properly for stray light (from e.g. the galaxy, sun, and
planets) in the far side-lobes of such an experiment, it is necessary to
perform the beam convolution over the full sky. We discuss this process in
multipole space for an arbitrary beam response, fully including the effects of
beam asymmetry and cross-polarization. The form of the convolution in multipole
space is such that the Wandelt-Gorski fast technique for all-sky convolution of
scalar signals (e.g. temperature) can be applied with little modification. We
further show that for the special case of a pure co-polarized, axisymmetric
beam the effect of the convolution can be described by spin-weighted window
functions. In the limits of a small angle beam and large Legendre multipoles,
the spin-weight 2 window function for the linear polarization reduces to the
usual scalar window function used in previous analyses of beam effects in CMB
polarimetry experiments. While we focus on the example of polarimetry
experiments in the context of CMB studies, we emphasise that the formalism we
develop is applicable to anisotropic filtering of arbitrary tensor fields on
the sphere.Comment: 8 pages, 1 figure; Minor changes to match version accepted by Phys.
Rev.
Maize yield and rainfall on different spatial and temporal scales in Southern Brazil
This study aimed to establish relationships between maize yield and rainfall on different temporal and spatial scales, in order to provide a basis for crop monitoring and modelling. A 16-year series of maize yield and daily rainfall from 11 municipalities and micro-regions of Rio Grande do Sul State was used. Correlation and regression analyses were used to determine associations between crop yield and rainfall for the entire crop cycle, from tasseling to 30 days after, and from 5 days before tasseling to 40 days after. Close relationships between maize yield and rainfall were found, particularly during the reproductive period (45-day period comprising the flowering and grain filling). Relationships were closer on a regional scale than at smaller scales. Implications of the crop-rainfall relationships for crop modelling are discussed.TĂtulo em portuguĂȘs: Rendimento de milho e chuva em diferentes escalas espaço-temporais no Sul do Brasil
Stress effects in structure formation
Residual velocity dispersion in cold dark matter induces stresses which lead
to effects that are absent in the idealized dust model. A previous Newtonian
analysis showed how this approach can provide a theoretical foundation for the
phenomenological adhesion model. We develop a relativistic kinetic theory
generalization which also incorporates the anisotropic velocity dispersion that
will typically be present. In addition to density perturbations, we consider
the rotational and shape distortion properties of clustering. These quantities
together characterize the linear development of density inhomogeneity, and we
find exact solutions for their evolution. As expected, the corrections are
small and arise only in the decaying modes, but their effect is interesting.
One of the modes for density perturbations decays less rapidly than the
standard decaying mode. The new rotational mode generates precession of the
axis of rotation. The new shape modes produce additional distortion that
remains frozen in during the subsequent (linear) evolution, despite the rapid
decay of the terms that caused it.Comment: significantly improved discussion of kinetic theory of CDM velocity
dispersion; to appear Phys. Rev.
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