603 research outputs found
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
The radiative transfer for polarized radiation at second order in cosmological perturbations
This article investigates the full Boltzmann equation up to second order in
the cosmological perturbations. Describing the distribution of polarized
radiation by using a tensor valued distribution function, the second order
Boltzmann equation, including polarization, is derived without relying on the
Stokes parameters.Comment: 4 pages, no figure; replaced to match published versio
Nonlinear Effects in the Cosmic Microwave Background
Major advances in the observation and theory of cosmic microwave background
anisotropies have opened up a new era in cosmology. This has encouraged the
hope that the fundamental parameters of cosmology will be determined to high
accuracy in the near future. However, this optimism should not obscure the
ongoing need for theoretical developments that go beyond the highly successful
but simplified standard model. Such developments include improvements in
observational modelling (e.g. foregrounds, non-Gaussian features), extensions
and alternatives to the simplest inflationary paradigm (e.g. non-adiabatic
effects, defects), and investigation of nonlinear effects. In addition to well
known nonlinear effects such as the Rees-Sciama and Ostriker-Vishniac effects,
further nonlinear effects have recently been identified. These include a
Rees-Sciama-type tensor effect, time-delay effects of scalar and tensor
lensing, nonlinear Thomson scattering effects and a nonlinear shear effect.
Some of the nonlinear effects and their potential implications are discussed.Comment: Invited contribution to Relativistic Cosmology Symposium (celebrating
the 60th year of GFR Ellis); to appear Gen. Rel. Gra
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
Cosmic microwave background anisotropies in the CDM model: a covariant and gauge-invariant approach
We present a fully covariant and gauge-invariant calculation of the evolution
of anisotropies in the cosmic microwave background (CMB) radiation. We use the
physically appealing covariant approach to cosmological perturbations, which
ensures that all variables are gauge-invariant and have a clear physical
interpretation. We derive the complete set of frame-independent, linearised
equations describing the (Boltzmann) evolution of anisotropy and inhomogeneity
in an almost Friedmann-Robertson-Walker (FRW) cold dark matter (CDM) universe.
These equations include the contributions of scalar, vector and tensor modes in
a unified manner. Frame-independent equations for scalar and tensor
perturbations, which are valid for any value of the background curvature, are
obtained straightforwardly from the complete set of equations. We discuss the
scalar equations in detail, including the integral solution and relation with
the line of sight approach, analytic solutions in the early radiation dominated
era, and the numerical solution in the standard CDM model. Our results confirm
those obtained by other groups, who have worked carefully with non-covariant
methods in specific gauges, but are derived here in a completely transparent
fashion.Comment: Two new sections added to earlier version. To appear in Astrophys.
A covariant and gauge-invariant analysis of cosmic microwave background anisotropies from scalar perturbations
We present a new, fully covariant and manifestly gauge-invariant expression
for the temperature anisotropy in the cosmic microwave background radiation
resulting from scalar perturbations. We pay particular attention to gauge
issues such as the definition of the temperature perturbation and the placing
of the last scattering surface. In the instantaneous recombination
approximation, the expression may be integrated up to a Rees-Sciama term for
arbitrary matter descriptions in flat, open and closed universes. We discuss
the interpretation of our result in the baryon-dominated limit using numerical
solutions for conditions on the last scattering surface, and confirm that for
adiabatic perturbations the dominant contribution to the anisotropy on
intermediate scales (the location of the Doppler peaks) may be understood in
terms of the spatial inhomogeneity of the radiation temperature in the baryon
rest frame. Finally, we show how this term enters the usual Sachs-Wolfe type
calculations (it is rarely seen in such analyses) when subtle gauge effects at
the last scattering surface are treated correctly.Comment: 20 pages, 2 Postscript figure
CLOVER - A new instrument for measuring the B-mode polarization of the CMB
We describe the design and expected performance of Clover, a new instrument
designed to measure the B-mode polarization of the cosmic microwave background.
The proposed instrument will comprise three independent telescopes operating at
90, 150 and 220 GHz and is planned to be sited at Dome C, Antarctica. Each
telescope will feed a focal plane array of 128 background-limited detectors and
will measure polarized signals over angular multipoles 20 < l < 1000. The
unique design of the telescope and careful control of systematics should enable
the B-mode signature of gravitational waves to be measured to a
lensing-confusion-limited tensor-to-scalar ratio r~0.005.Comment: 4 pages, 5 figures. To appear in the proceedings of the XXXVIXth
Rencontres de Moriond "Exploring the Universe
Recommended from our members
Influences of increasing temperature on Indian wheat: quantifying limits to predictability
As climate changes, temperatures will play an increasing role in determining crop yield. Both
climate model error and lack of constrained physiological thresholds limit the predictability of
yield. We used a perturbed-parameter climate model ensemble with two methods of
bias-correction as input to a regional-scale wheat simulation model over India to examine
future yields. This model configuration accounted for uncertainty in climate, planting date,
optimization, temperature-induced changes in development rate and reproduction. It also
accounts for lethal temperatures, which have been somewhat neglected to date. Using
uncertainty decomposition, we found that fractional uncertainty due to temperature-driven
processes in the crop model was on average larger than climate model uncertainty (0.56 versus
0.44), and that the crop model uncertainty is dominated by crop development. Simulations
with the raw compared to the bias-corrected climate data did not agree on the impact on future
wheat yield, nor its geographical distribution. However the method of bias-correction was not
an important source of uncertainty. We conclude that bias-correction of climate model data
and improved constraints on especially crop development are critical for robust impact
predictions
Comparing the effects of calibration and climate errors on a statistical crop model and a process-based crop model
Understanding the relationship between climate and crop productivity is a key component of projections of future food production, and hence assessments of food security. Climate models and crop yield datasets have errors, but the effects of these errors on regional scale crop models is not well categorized and understood. In this study we compare the effect of synthetic errors in temperature and precipitation observations on the hindcast skill of a process-based crop model and a statistical crop model. We find that errors in temperature data have a significantly stronger influence on both models than errors in precipitation. We also identify key differences in the responses of these models to different types of input data error. Statistical and process-based model responses differ depending on whether synthetic errors are overestimates or underestimates. We also investigate the impact of crop yield calibration data on model skill for both models, using datasets of yield at three different spatial scales. Whilst important for both models, the statistical model is more strongly influenced by crop yield scale than the process-based crop model. However, our results question the value of high resolution yield data for improving the skill of crop models; we find a focus on accuracy to be more likely to be valuable. For both crop models, and for all three spatial scales of yield calibration data, we found that model skill is greatest where growing area is above 10-15 %. Thus information on area harvested would appear to be a priority for data collection efforts. These results are important for three reasons. First, understanding how different crop models rely on different characteristics of temperature, precipitation and crop yield data allows us to match the model type to the available data. Second, we can prioritize where improvements in climate and crop yield data should be directed. Third, as better climate and crop yield data becomes available, we can predict how crop model skill should improve
An improved approach to measuring H_0 using X-ray and SZ observations of galaxy clusters
We present an improved method for predicting the Sunyaev-Zeldovich (SZ)
effect in galaxy clusters from spatially-resolved, spectroscopic X-ray data.
Using the deprojected electron density and temperature profiles measured within
a fraction of the virial radius, and assuming a Navarro, Frenk & White (1995)
mass model, we show how the pressure profile of the X-ray gas can be
extrapolated to large radii, allowing the Comptonization parameter profile for
the cluster to be predicted precisely. We apply our method to Chandra
observations of three X-ray luminous, dynamically relaxed clusters with
published SZ data: RX J1347.5-1145, Abell 1835 and Abell 478. Combining the
predicted and observed SZ signals, we determine improved estimates for the
Hubble constant from each cluster and obtain a weighted mean of
H_0=69\pm8km/s/Mpc for a cosmology with Omega_m=0.3 and Omega_Lambda=0.7. This
result is in good agreement with independent findings from the Hubble Key
Project and the combination of cosmic microwave background and galaxy cluster
data.Comment: 9 pages, 2 figures, 4 tables. Accepted for publication in MNRA
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