Philipp M. Merkel

Born-corrections to weak lensing of the cosmic microwave background temperature and polarization anisotropies

Many weak lensing calculations make use of the Born approximation where the light ray is approximated by a straight path. We examine the effect of Born-corrections for lensing of the cosmic microwave background in an analytical approach by taking perturbative corrections to the geodesic into account. The resulting extra power in the lensing potential spectrum is comparable to the power generated by nonlinear structure formation and affects especially the polarisation spectra, leading to relative changes of the order of one per cent for the

Intrinsic ellipticity correlations of galaxies: models, likelihoods and interplay with weak lensing

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The cross-correlation between 3D cosmic shear and the integrated Sachs–Wolfe effect

-mode spectrum and up to 10 per cent on all scales to the

A theoretical estimate of intrinsic ellipticity bispectra induced by angular momenta alignments

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Gravitational lensing of the cosmic microwave background by nonlinear structures

-mode spectrum. In contrast, there is only little change of spectra involving the CMB temperature. Additionally, the corrections excite one more degree of freedom resulting in a deflection component which can not be described as a gradient of the lensing potential as it is related to image rotation in lens-lens coupling. We estimate the magnitude of this effect on the CMB-spectra and find it to be negligible.

Angular spectra of the intrinsic galaxy ellipticity field, their observability and their impact on lensing in tomographic surveys

Subject of this paper are the statistical properties of elli pticity alignments between galaxies evoked by their coupled angular momenta. Starting from phys ical angular momentum models, we bridge the gap towards ellipticity correlations, el lipticity spectra and derived quantities such as aperture moments, comparing the intrinsic signals w ith those generated by gravitational lensing, with the projected galaxy sample of EUCLI D in mind. We investigate the dependence of intrinsic ellipticity correlations on cosmo logical parameters and show that intrinsic ellipticity correlations give rise to non-Gaussia n likelihoods as a result of nonlinear functional dependencies. Comparing intrinsic ellipticit y spectra to weak lensing spectra we quantify the magnitude of their contaminating e ffect on the estimation of cosmological parameters and find that biases on dark energy parameters are very s mall in an angular-momentum based model in contrast to the linear alignment model common ly used. Finally, we quantify whether intrinsic ellipticities can be measured in the pres ence of the much stronger weak lensing induced ellipticity correlations, if prior knowledge o n a cosmological model is assumed.

Parameter constraints from weak-lensing tomography of galaxy shapes and cosmic microwave background fluctuations

We present the first calculation of the cross-correlation between three-dimensional cosmic shear and the integrated Sachs-Wolfe (iSW) effect. Both signals are combined in a single formalism, which permits the computation of the full covariance matrix. In order to avoid the uncertainties presented by the non-linear evolution of the matter power spectrum and intrinsic alignments of galaxies, our analysis is restricted to large scales, i.e. multipoles below l=1000. We demonstrate in a Fisher analysis that this reduction compared to other studies of three-dimensional weak lensing extending to smaller scales is compensated by the information that is gained if the additional iSW signal and in particular its cross-correlation with lensing data are considered. Given the observational standards of upcoming weak lensing surveys like Euclid, marginal errors on cosmological parameters decrease by ten per cent compared to a cosmic shear experiment if both types of information are combined without a CMB prior. Once the constraining power of CMB data is added, the improvement becomes marginal.

The interplay of CMB temperature lensing power reconstruction with primordial non-Gaussianity of local type

Intrinsically aligned galaxy shapes are one of the most important systematics in cosmic shear measurements. So far theoretical studies of intrinsic alignments almost exclusively focus on their statistics at the two-point level. Results from numerical simulations, however, suggest that third-order measures might be even stronger affected. We therefore investigate the (angular) bispectrum of intrinsic alignments. In our fully analytical study we describe intrinsic galaxy ellipticities by a physical alignment model, which makes use of tidal torque theory. We derive expressions for the various combinations of intrinsic and gravitationally induced ellipticities, i.e. III-, GII- and GGI-alignments, and compare our results to the shear bispectrum, the GGG-term. The latter is computed using hyper-extended perturbation theory. Considering equilateral and squeezed configurations we find that for a Euclid-like survey intrinsic alignments (III-alignments) start to dominate on angular scales smaller than 20 arcmin and 13 arcmin, respectively. This sensitivity to the configuration-space geometry may allow to exploit the cosmological information contained in both the intrinsic and gravitationally induced ellipticity field. On smallest scales (l ~ 3000) III-alignments exceed the lensing signal by at least one order of magnitude. The amplitude of the GGI-alignments is the weakest. It stays below that of the shear field on all angular scales irrespective of the wave-vector configuration.

Contributions to the non-linear integrated Sachs–Wolfe effect: Birkinshaw–Gull effect and gravitational self-energy density

Weak gravitational lensing changes the angular power spectra of the cosmic microwave background (CMB) temperature and polarization in a characteristic way containing valuable information for cosmological parameter estimation and weak lensing reconstructions. So far, analytical expressions for the lensed CMB power spectra assume the probability density function (PDF) of the lensing excursion angle to be Gaussian. However, coherent light deflection by nonlinear structures at low redshifts causes deviations from a pure Gaussian PDF. Working in the flat-sky limit we develop a method for computing the lensed CMB power spectra which takes these non-Gaussian features into account. Our method does not assume any specific PDF but uses instead an expansion of the characteristic function of the lensing excursion angle into its moments. Measuring these in the CMB lensing deflection field obtained from the Millennium Simulation we show that the change in the lensed power spectra is only at the 0.1% - 0.4% level on very small scales (below 4 arcmin) and demonstrate that the assumption of a Gaussian lensing excursion angle PDF is well applicable.

Imitating intrinsic alignments: a bias to the CMB lensing-galaxy shape cross-correlation power spectrum induced by the large-scale structure bispectrum

Subject of this paper are intrinsic ellipticity correlations between galaxies, their statistical properties, their observability with future surveys and their interference with weak gravitational lensing measurements. Using an angular momentum-based, quadratic intrinsic alignment model we derive correlation functions of the ellipticity components and project them to yield the four non-zero angular ellipticity spectra