109 research outputs found
Cosmic Microwave Background-Weak Lensing Correlation: Analytical and Numerical Study of Nonlinearity and Implications for Dark Energy
Evolution of density fluctuations yields secondary anisotropies in the cosmic microwave background ( CMB), which are correlated with the same density fluctuations that can be measured by weak lensing (WL) surveys. We study the CMB-WL correlation induced by the integrated Sachs-Wolfe (ISW) effect and its nonlinear extension, the Rees-Sciama (RS) effect, using analytical models as well as N-body simulations. We show that an analytical model based on the time derivative of matter power spectrum agrees with simulations. All-sky cosmic-variance-limited CMB and WL surveys allow us to measure the correlation from the nonlinear RS effect with high significance (50 sigma) for l(max) = 10(4) whereas forthcoming missions such as Planck and LSST are expected to yield 4 l p 10 1.5 sigma detections, on the assumption of that the point-source contributions are negligible. We find that the CMB-WL correlation has a characteristic scale which is sensitive to the nature of dark energy.Alfred P. Sloan FellowshipAstronom
Can Non-Gaussian Cosmological Models Explain the WMAP's High Optical Depth for Reionization?
The first-year Wilkinson Microwave Anisotropy Probe data suggest a high
optical depth for Thomson scattering of 0.17 +/- 0.04, implying that the
universe was reionized at an early epoch, z ~ 20. Such early reionization is
likely to be caused by UV photons from first stars, but it appears that the
observed high optical depth can be reconciled within the standard structure
formation model only if star-formation in the early universe was extremely
efficient. With normal star-formation efficiencies, cosmological models with
non-Gaussian density fluctuations may circumvent this conflict as high density
peaks collapse at an earlier epoch than in models with Gaussian fluctuations.
We study cosmic reionization in non-Gaussian models and explore to what extent,
within available constraints, non-Gaussianities affect the reionization
history. For mild non-Gaussian fluctuations at redshifts of 30 to 50, the
increase in optical depth remains at a level of a few percent and appears
unlikely to aid significantly in explaining the measured high optical depth. On
the other hand, within available observational constraints, increasing the
non-Gaussian nature of density fluctuations can easily reproduce the optical
depth and may remain viable in underlying models of non-Gaussianity with a
scale-dependence.Comment: 5 pages, 2 figure
A complex rupture image of the 2011 off the Pacific coast of Tohoku Earthquake revealed by the MeSO-net
Simulations of Early Structure Formation: Primordial Gas Clouds
(abridged) We use large cosmological simulations to study the origin of
primordial star-forming clouds in a Lambda CDM universe, by following the
formation of dark matter halos and the cooling of gas within them. To model the
physics of chemically pristine gas, we employ a non-equilibrium treatment of
the chemistry of 9 species and include cooling by molecular hydrogen. We
explore the hierarchical growth of bound structures forming at redshifts z = 25
- 30 with total masses in the range 10^5 - 10^6 Msun. The complex interplay
between the gravitational formation of dark halos and the thermodynamic and
chemical evolution of the gas clouds compromises analytic estimates of the
critical H2 fraction. Dynamical heating from mass accretion and mergers opposes
relatively inefficient cooling by molecular hydrogen, delaying the production
of star-forming clouds in rapidly growing halos. We also investigate the impact
of photo-dissociating ultra-violet (UV) radiation on the formation of
primordial gas clouds. We consider two extreme cases by first including a
uniform radiation field in the optically thin limit and secondly by accounting
for the maximum effect of gas self-shielding in virialized regions. In both the
cases we consider, the overall impact can be described by computing an
equilibrium H2 abundance for the radiation flux and defining an effective
shielding factor.
Based on our numerical results, we develop a semi-analytic model of the
formation of the first stars, and demonstrate how it can be coupled with large
N-body simulations to predict the star formation rate in the early universe.Comment: Revised version accepted by ApJ. Description of semi-analytic models
extende
Simulations of Wide-Field Weak Lensing Surveys I: Basic Statistics and Non-Gaussian Effects
We study the lensing convergence power spectrum and its covariance for a
standard LCDM cosmology. We run 400 cosmological N-body simulations and use the
outputs to perform a total of 1000 independent ray-tracing simulations. We
compare the simulation results with analytic model predictions. The
semi-analytic model based on Smith et al.(2003) fitting formula underestimates
the convergence power by ~30% at arc-minute angular scales. For the convergence
power spectrum covariance, the halo model reproduces the simulation results
remarkably well over a wide range of angular scales and source redshifts. The
dominant contribution at small angular scales comes from the sample variance
due to the number fluctuations of halos in a finite survey volume. The
signal-to-noise ratio for the convergence power spectrum is degraded by the
non-Gaussian covariances by up to a factor 5 for a weak lensing survey to z_s
~1. The probability distribution of the convergence power spectrum estimators,
among the realizations, is well approximated by a chi-square distribution with
broadened variance given by the non-Gaussian covariance, but has a larger
positive tail. The skewness and kurtosis have non-negligible values especially
for a shallow survey. We argue that a prior knowledge on the full distribution
may be needed to obtain an unbiased estimate on the ensemble averaged band
power at each angular scale from a finite volume survey.Comment: 11 pages, 11 figures. Accepted for publication in the Astrophysical
Journal. Corrected typo in the equation of survey window function below
Equation (18). The results unchange
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