arXiv subjects

Bjoern Malte Schaefer

Publications and source records attributed to Bjoern Malte Schaefer.

At least 55 records · Page 3Linked to original sources

arXiv2012

Investigating clustering dark energy with 3d weak cosmic shear

As observational evidence increasingly consolidates the case for a cosmological constant being the source of the Universe's accelerated expansion, the question whether, and if so, how well, future experiments could detect deviations from this standard scenario is raised with urgency. Assuming a dark energy component different from a cosmological constant, the observable effects in general include gravitational clustering described by the fluid's (rest-frame) speed of sound. We employ 3d weak cosmic shear, a proposed method to take advantage of the full three-dimensional information inherent to the cosmic shear field, to explore the capability of future surveys to detect dark energy clustering and the signature of an enhanced amplitude of the matter power spectrum on large scales. For this purpose, we present adequate numerical methods facilitating 3d weak cosmic shear calculations. We find that the possible constraints heavily depend on the dark energy equation of state w. If w is not very close to -1, constraining the squared sound speed within an order of magnitude seems possible with a combination of Euclid and Planck data.

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arXiv2012

Parameter likelihood of intrinsic ellipticity correlations

Subject of this paper are the statistical properties of ellipticity alignments between galaxies evoked by their coupled angular momenta. Starting from physical angular momentum models, we bridge the gap towards ellipticity correlations, ellipticity spectra and derived quantities such as aperture moments, comparing the intrinsic signals with those generated by gravitational lensing, with the projected galaxy sample of EUCLID in mind. We investigate the dependence of intrinsic ellipticity correlations on cosmological parameters and show that intrinsic ellipticity correlations give rise to non-Gaussian likelihoods as a result of nonlinear functional dependencies. Comparing intrinsic ellipticity spectra to weak lensing spectra we quantify the magnitude of their contaminating effect on the estimation of cosmological parameters and find that biases on dark energy parameters are very small in an angular-momentum based model in contrast to the linear alignment model commonly used. Finally, we quantify whether intrinsic ellipticities can be measured in the presence of the much stronger weak lensing induced ellipticity correlations, if prior knowledge on a cosmological model is assumed.

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arXiv2012

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

In the current era of high-precision CMB experiments, the imprint of gravitational lensing on the CMB temperature is exploited as a source of valuable information. Especially the reconstruction of the lensing potential power spectrum is of great interest. The reconstruction from the optimal quadratic estimator of the lensing potential, though, is biased. As long as the intrinsic CMB fluctuations are Gaussian this bias is well understood and controlled. In the presence of non-Gaussian primordial curvature perturbations, however, the CMB also acquires a non-Gaussian structure mimicking the lensing signal. Concentrating on primordial non-Gaussianity of local type, we address the resulting bias by extracting the lensing potential power spectrum from large samples of simulated lensed CMB temperature maps comprising different values of f_NL. We find that the bias is considerably larger than previous analytical calculations suggested. For current values of f_NL and a sensitivity like that of the Planck mission, however, the bias is completely negligible on all but the largest angular scales.

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arXiv2012

Evolution of intrinsic ellipticity correlations due to peculiar motion

Topic of this paper is the time-evolution of intrinsic correlations of galaxy ellipticities due to peculiar motion. In our model, the galaxy ellipticities are determined from the angular momentum of their host haloes, which can be computed from the fluctuations statistics of a Gaussian random field. Subsequent peculiar motion distorts the ellipticity field and causes changes in the ellipticity correlations. Using analogies between this problem of shifted ellipticity tensors and the displacements of polarisation tensors in gravitational lensing of the cosmic microwave background we compute E-mode and B-mode spectra of the time-evolved ellipticity field, where the displacements are modelled with first and second order Lagrangian perturbation theory. For EUCLID, ellipticity correlations are decreased on large multipoles l >1000, amounting to up to 10% in the E-mode spectrum C_E^epsilon(l) and up to 60% in the B-mode spectrum C_B^epsilon(l) at l~3000 due to the dispersing effect of peculiar motion. E/B-mode conversion in analogy to CMB-lensing is present but small. We conclude that distortions of the ellipticity field due to peculiar motion is not affecting the prediction of ellipticity models on the scales relevant for lensing in the case of EUCLID's galaxy distribution, but should affect larger scales for surveys at lower redshifts.

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arXiv2011

Weak lensing tomography with orthogonal polynomials

The topic of this article is weak cosmic shear tomography where the line of sight-weighting is carried out with a set of specifically constructed orthogonal polynomials, dubbed TaRDiS (Tomography with orthogonAl Radial Distance polynomIal Systems). We investigate the properties of these polynomials and employ weak convergence spectra, which have been obtained by weighting with these polynomials, for the estimation of cosmological parameters. We quantify their power in constraining parameters in a Fisher-matrix technique and demonstrate how each polynomial projects out statistically independent information, and how the combination of multiple polynomials lifts degeneracies. The assumption of a reference cosmology is needed for the construction of the polynomials, and as a last point we investigate how errors in the construction with a wrong cosmological model propagate to misestimates in cosmological parameters. TaRDiS performs on a similar level as traditional tomographic methods and some key features of tomography are made easier to understand.

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arXiv2011

A weak lensing view on primordial non-Gaussianities

We investigate the signature of primordial non-Gaussianities in the weak lensing bispectrum, in particular the signals generated by local, orthogonal and equilateral non-Gaussianities. The questions we address include the signal-to-noise ratio generated in the Euclid weak lensing survey (we find the 1sigma-errors for fNL are 200, 575 and 1628 for local, orthogonal and equilateral non-Gaussianities, respectively), misestimations of fNL if one chooses the wrong non-Gaussianity model (misestimations by up to a factor of +/-3 in fNL are possible, depending on the choice of the model), the probability of noticing such a mistake (improbably large values for the chi^2-functional occur from fNL 200 on), degeneracies of the primordial bispectrum with other cosmological parameters (only the matter density Omega_m plays a significant role), and the subtraction of the much larger, structure-formation generated bispectrum. If a prior on a standard wCDM-parameter set is available from Euclid and Planck, the structure formation bispectrum can be predicted accurately enough for subtraction, and any residual structure formation bispectrum would influence the estimation of fNL to a minor degree. Configuration-space integrations which appear in the evaluation of chi^2-functionals and related quantities can be carried out very efficiently with Monte-Carlo techniques, which reduce the complexity by a factor of O(10^4) while delivering sub-percent accuracies. Weak lensing probes smaller scales than the CMB and hence provide an additional constraint on non-Gaussianities, even though they are not as sensitive to primordial non-Gaussianities as the CMB.

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arXiv2011

On the validity of the Born approximation for weak cosmic flexions

Weak lensing calculations are often made under the assumption of the Born approximation, where the ray path is approximated as a straight radial line. In addition, lens-lens couplings where there are several deflections along the light ray are often neglected. We examine the effect of dropping the Born approximation and taking lens-lens couplings into account, for weak lensing effects up to second order (cosmic flexion), by making a perturbative expansion in the light path. We present a diagrammatic representation of the resulting corrections to the lensing effects. The flexion signal, which measures the derivative of the density field, acquires correction terms proportional to the squared gravitational shear; we also find that by dropping the Born approximation, two further degrees of freedom of the lensing distortion can be excited (the twist components), in addition to the four standard flexion components. We derive angular power spectra of the flexion and twist, with and without the Born-approximation and lens-lens couplings and confirm that the Born approximation is an excellent approximation for weak cosmic flexions, except at very small scales.

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arXiv2011

Galactic angular momenta and angular momentum couplings in the large-scale structure

In this paper, we revisit the aquisition of angular momentum of galaxies by tidal shearing and compute the angular momentum variance sigma_L^2 as well as the angular momentum correlation function C_L(r) from a peak-restricted Gaussian random process. This stochastic process describing the initial conditions treats both the tidal shear as well as the inertia as dynamical fields and explicitly accounts for the discreteness of the inertia field. We describe the way in which the correlations in angular momentum result from an interplay of long-ranged correlations in the tidal shear, and short ranged correlations in the inertia field and which reflects the correlation between the eigensystems of these two symmetric tensors. We propose a new form of the angular momentum correlation function which is able to distinguish between parallel and antiparallel alignment of angular momentum vectors, and comment on implications of intrinsic alignments for weak lensing measurements. We confirm the scaling L/M propto. M^{2/3} and find the angular momentum distribution of Milky Way-sized haloes to be correlated on scales of ~1 Mpc/h. The correlation function can be well fitted by an empirical relation of the form C_L(r) propto. exp(-[r/r_0]^beta).

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arXiv2010

Parameter estimation biases due to contributions from the Rees-Sciama effect to the integrated Sachs-Wolfe spectrum

The subject of this paper is an investigation of the nonlinear contributions to the spectrum of the integrated Sachs-Wolfe (iSW) effect. We derive the corrections to the iSW-auto spectrum and the iSW-tracer cross-spectrum consistently to third order in perturbation theory and analyse the cumulative signal-to-noise ratio for a cross-correlation between the PLANCK and EUCLID data sets as a function of multipole order. We quantify the parameter sensitivity and the statistical error bounds on the cosmological parameters Omega_m, sigma_8, h, n_s and w from the linear iSW-effect and the systematical parameter estimation bias due to the nonlinear corrections in a Fisher-formalism, analysing the error budget in its dependence on multipole order. Our results include: (i) the spectrum of the nonlinear iSW-effect can be measured with 0.8σstatistical significance, (ii) nonlinear corrections dominate the spectrum starting from l=100, (iii) an anticorrelation of the CMB temperature with tracer density on high multipoles in the nonlinear regime, (iv) a much weaker dependence of the nonlinear effect on the dark energy model compared to the linear iSW-effect, (v) parameter estimation biases amount to less than 0.1 sigma and weaker than other systematics.

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arXiv2010

Gravitational lensing of the cosmic microwave background by nonlinear structures

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.

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arXiv2009

The Growth of Structure in Interacting Dark Energy Models

If dark energy interacts with dark matter, there is a change in the background evolution of the universe, since the dark matter density no longer evolves as a^{-3}. In addition, the non-gravitational interaction affects the growth of structure. In principle, these changes allow us to detect and constrain an interaction in the dark sector. Here we investigate the growth factor and the weak lensing signal for a new class of interacting dark energy models. In these models, the interaction generalises the simple cases where one dark fluid decays into the other. In order to calculate the effect on structure formation, we perform a careful analysis of the perturbed interaction and its effect on peculiar velocities. Assuming a normalization to today's values of dark matter density and overdensity, the signal of the interaction is an enhancement (suppression) of both the growth factor and the lensing power, when the energy transfer in the background is from dark matter to dark energy (dark energy to dark matter).

astro-ph.CO↗

arXiv2009

Twist and Turn: Weak Lensing Image Distortions to Second Order

We account for all the image distortions relevant to weak gravitational lensing to second order. Besides the familiar shear, convergence, rotation and flexions, we find a new image distortion with two distinct descriptions, the twist and the turn. Like rotation, this distortion is not activated gravitationally to first order, but will be activated by systematic effects. We examine the rotational properties of twist and turn, and their effect on images in real and shapelet space. We construct estimators for the new distortion, taking into account the centroid shift which it generates. We then use these estimators to make first constraints on twist using the STAGES HST survey; we find that the mean twist estimator is consistent with zero. We measure correlation functions for our twist estimator on the survey, again finding no evidence of systematic effects.

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arXiv2009

Constraints on early dark energy from CMB lensing and weak lensing tomography

Dark energy can be studied by its influence on the expansion of the Universe as well as on the growth history of the large-scale structure. In this paper, we follow the growth of the cosmic density field in early dark energy cosmologies by combining observations of the primary CMB temperature and polarisation power spectra at high redshift, of the CMB lensing deflection field at intermediate redshift and of weak cosmic shear at low redshifts for constraining the allowed amount of early dark energy. We present these forecasts using the Fisher-matrix formalism and consider the combination of Planck-data with the weak lensing survey of Euclid. We find that combining these data sets gives powerful constraints on early dark energy and is able to break degeneracies in the parameter set inherent to the various observational channels. The derived statistical 1-sigma-bound on the early dark energy density parameter is sigma(Omega_d^e)=0.0022 which suggests that early dark energy models can be well examined in our approach. In addition, we derive the dark energy figure of merit for the considered dark energy parameterisation and comment on the applicability of the growth index to early dark energy cosmologies.

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arXiv2009

Implications of bias evolution on measurements of the integrated Sachs-Wolfe effect: errors and biases in parameter estimation

The subject of this paper is a quantification of the impact of uncertainties in bias and bias evolution on the interpretation of measurements of the integrated Sachs-Wolfe effect, in particular on the estimation of cosmological parameters. We carry out a Fisher-matrix analysis for quantifying the degeneracies between the parameters of a dark energy cosmology and bias evolution, for the combination of the PLANCK microwave sky survey with the EUCLID main galaxy sample, where bias evolution b(a)=b_0+(1-a)b_a is modelled with two parameters b_0 and b_a. Using a realistic bias model introduces a characteristic suppression of the iSW-spectrum on large angular scales, due to the altered distance-weighting functions. The errors in estimating cosmological parameters if the data with evolving bias is interpreted in the framework of cosmologies with constant bias is quantified in an extended Fisher-formalism. We find that the best-fit values of all parameters are shifted by an amount comparable to the statistical accuracy: The estimation bias in units of the statistical accuracy amounts to 1.19 for Omega_m, 0.27 for sigma_8, and 0.72 for w for bias evolution with b_a=1. Leaving b_a open as a free parameter deteriorates the statistical accuracy, in particular on Omega_m and w.

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arXiv2008

Review: galactic angular momenta and angular momentum correlations in the cosmological large-scale structure

I review the theory of angular momentum acquisition of galaxies by tidal torquing, the resulting angular momentum distribution, the angular momentum correlation function and discuss the implications of angular momentum alignments on weak lensing measurements: Starting from linear models for tidal torquing I summarise perturbative approaches and the results from n-body simulations of cosmic structure formation. Then I continue to discuss the validity of decompositions of the tidal shear and inertia fields, the effects of angular momentum biasing, the applicability of parameterised angular momentum correlation models and the consequences of angular momentum correlations for shape alignments. I compile the result of observations of shape alignments in recent galaxy surveys as well as in n-body simulations. Finally, I review the contamination of weak lensing surveys by spin-induced shape alignments and methods for suppressing this contamination.

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arXiv2008

Constraints on the decay of dark matter to dark energy from weak lensing bispectrum tomography

We consider a phenomenological model for a coupling between the dark matter and dark energy fluids and investigate the sensitivity of a weak lensing measurement for constraining the size of this coupling term. Physically, the functional form of the coupling term in our model describes the decay of dark matter into dark energy. We present forecasts for tomographic measurements of the weak shear bispectrum for the DUNE experiment in a Fisher-matrix formalism, where we describe the nonlinearities in structure formation by hyper-extended perturbation theory. Physically, CDM decay tends to increase the growth rate of density perturbations due to higher values for the CDM density at early times, and amplifies the lensing signal because of stronger fluctuations in the gravitational potential. We focus on degeneracies between the dark energy equation of state properties and the CDM decay constant relevant for structure formation and weak lensing. A typical lower bound on the CDM decay time ~7.7/H_0 = 75.3 Gyr/h$ which could be provided by DUNE would imply that it would not possible to produce the dark energy content of the universe by CDM decay within the age of the Universe for a constant equation of state parameter of w close to -1.

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arXiv2008

The integrated Sachs-Wolfe effect in cosmologies with coupled dark matter and dark energy

The subject of this paper is the derivation of the integrated Sachs-Wolfe (iSW) effect in cosmologies with coupled dark matter and dark energy fluids. These couplings influence the iSW-effect in three ways: The Hubble function assumes a different scaling, the structure growth rate shows a different time evolution, and in addition, the Poisson equation, which relates the density perturbations to fluctuations in the gravitational potential, is changed, due to the violation of the scaling rho ~ a^{-3} of the matter density rho with scale factor a. Exemplarily, I derive the iSW-spectra for a model in which dark matter decays into dark energy, investigate the influence of the dark matter decay rate and the dark energy equation of state on the iSW-signal, and discuss the analogies for gravitational lensing. Quite generally iSW-measurements should reach similar accuracy in determining the dark energy equation of state parameter and the coupling constant.

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arXiv2007

Spherical collapse in modified gravity with the Birkhoff-theorem

We study structure formation in a phenomenological model of modified gravity which interpolates between LambdaCDM and phenomenological DGP-gravity. Generalisation of spherical collapse by using the Birkhoff-theorem along with the modified growth equation shows that the overdensity for spherical collapse delta_c in these models is significantly lowered compared to LambdaCDM, leading to enhanced number densities of massive clusters and enhanced cluster merging rates. We find that delta_c(z) is well fitted by a function of the form delta_c(z) = a - b\exp(-cz). We examine the sensitivity of PLANCK's and SPT's Sunyaev-Zel'dovich survey to constrain the modified gravity parameterisation and find that these experiments can easily distinguish between models with a cosmological constant and modified gravity, if prior constraints from CMB temperature and polarisation anisotropies are included.

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