R. Benton Metcalf

Gravitational magnification of the cosmic microwave background

Some aspects of gravitational lensing by large-scale structure are investigated. We show that lensing causes the damping tail of the cosmic microwave background (CMB) power spectrum to fall less rapidly with decreasing angular scale than previously expected. This is because of a transfer of power from larger to smaller angular scales, which produces a fractional change in power spectrum that increases rapidly beyond l~2000. We also find that lensing produces a nonzero mean magnification of structures on surfaces of constant redshift if weighted by area on the sky. This is a result of the fact that light rays that are evenly distributed on the sky oversample overdense regions. However, this mean magnification has a negligible affect on the CMB power spectrum. A new expression for the lensed power spectrum is derived, and it is found that future precision observations of the high-l tail of the power spectrum will need to take lensing into account when determining cosmological parameters.

Recovering the non-linear density field from the galaxy distribution with a Poisson–lognormal filter

We present a general expression for a lognormal filter given an arbitrary non-linear galaxy bias. We derive this filter as the maximum a posteriori solution assuming a lognormal prior distribution for the matter field with a given mean field and modelling the observed galaxy distribution by a Poissonian process. We have performed a three-dimensional implementation of this filter with a very efficient Newton-Krylov inversion scheme. Furthermore, we have tested it with a dark matter N-body simulation assuming a unit galaxy bias relation and compared the results with previous density field estimators like the inverse weighting scheme and Wiener filtering. Our results show good agreement with the underlying dark matter field for overdensities even above δ ∼ 1000 which exceeds by one order of magnitude the regime in which the lognormal is expected to be valid. The reason is that for our filter the lognormal assumption enters as a prior distribution function, but the maximum a posteriori solution is also conditioned on the data. We find that the lognormal filter is superior to the previous filtering schemes in terms of higher correlation coefficients and smaller Euclidean distances to the underlying matter field. We also show how it is able to recover the positive tail of the matter overdensity field distribution for a unit bias relation down to scales of about ≳2 Mpc h ―1 .

Cosmic cartography of the large-scale structure with sloan digital sky survey data release 6

We present the largest Wiener reconstruction of the cosmic density field made to date. The reconstruction is based on the Sloan Digital Sky Survey (SDSS) data release 6 covering the northern Galactic cap. We use a novel supersampling algorithm to suppress aliasing effects and a Krylov-space inversion method to enable high performance with high resolution. These techniques are implemented in the argo computer code. We reconstruct the field over a 500 Mpc cube with Mpc grid resolution while accounting for both the angular and the radial selection functions of the SDSS, and the shot noise giving an effective resolution of the order of similar to 10 Mpc. In addition, we correct for the redshift distortions in the linear and non-linear regimes in an approximate way. We show that the commonly used method of inverse weighting the galaxies by the corresponding selection function heads to excess noise in regions where the density of the observed galaxies is small. It is more accurate and conservative to adopt a Bayesian framework in which we model the galaxy selection/detection process to be Poisson binomial. This results in heavier smoothing in regions of reduced sampling density. Our results show a complex cosmic web structure with huge void regions indicating that the recovered matter distribution is highly non-Gaussian. Filamentary structures are clearly visible on scales of up to similar to 20 Mpc. We also calculate the statistical distribution of density after smoothing the reconstruction with Gaussian kernels of different radii r(S) and find good agreement with a lognormal distribution for 10 Mpc less than or similar to r(S) less than or similar to 30 Mpc.

A Fundamental Test of the Nature of Dark Matter

Dark matter may consist of weakly interacting elementary particles or of macroscopic compact objects. We show that the statistics of the gravitational lensing of high-redshift supernovae strongly discriminate between these two classes of dark matter candidates. We develop a method of calculating the magnification distribution of supernovae, which can be interpreted in terms of the properties of the lensing objects. With simulated data, we show that 50 well-measured Type Ia supernovae (Δm ~ 0.2 mag) at redshifts ~1 can clearly distinguish macroscopic from microscopic dark matter if Ω0 0.2 and all dark matter is in one form or the other.

Gravitational lensing of high‐redshift Type Ia supernovae: a probe of medium‐scale structure

The dispersion in the peak luminosities of high-redshift Type Ia supernovae will change with redshift because of gravitational lensing. This lensing is investigated with an emphasis on the prospects of measuring it and separating it from other possible sources of redshift-dependent dispersion. Measuring the lensing-induced dispersion would directly constrain the power spectrum of density fluctuations on smaller length-scales than are easily probed in any other way. The skew of the magnification distribution is related to the bispectrum of density fluctuations. Using cold dark matter models it is found that the amount and quality of data needed are attainable in a few years. A parametrization of the signal as a power law of the angular size distance to the supernova is motivated by these models. This information can be used in detecting lensing, detecting other systematic changes in supernovae and calculating the uncertainties in cosmological parameter estimates.

Cosmology with a SKA HI intensity mapping survey

HI intensity mapping (IM) is a novel technique capable of mapping the large-scale structure of the Universe in three dimensions and delivering exquisite constraints on cosmology, by using HI as a biased tracer of the dark matter density field. This is achieved by measuring the intensity of the redshifted 21cm line over the sky in a range of redshifts without the requirement to resolve individual galaxies. In this chapter, we investigate the potential of SKA1 to deliver HI intensity maps over a broad range of frequencies and a substantial fraction of the sky. By pinning down the baryon acoustic oscillation and redshift space distortion features in the matter power spectrum -- thus determining the expansion and growth history of the Universe -- these surveys can provide powerful tests of dark energy models and modifications to General Relativity. They can also be used to probe physics on extremely large scales, where precise measurements of spatial curvature and primordial non-Gaussianity can be used to test inflation; on small scales, by measuring the sum of neutrino masses; and at high redshifts where non-standard evolution models can be probed. We discuss the impact of foregrounds as well as various instrumental and survey design parameters on the achievable constraints. In particular we analyse the feasibility of using the SKA1 autocorrelations to probe the large-scale signal.

Mass and concentration estimates from weak and strong gravitational lensing: a systematic study

We study how well halo properties of galaxy clusters, like mass and concentration, are recovered using lensing data. In order to generate a large sample of systems at dierent redshifts we use the code MOKA. We measure halo mass and concentration using weak lensing data alone (WL), best fitting with a NFW profile the reduced tangential shear profile, or combining it with strong lensing data, by using the information about the size of the Einstein ring (WL+SL). For dierent redshifts, we measure the mass and the concentration bias finding that these are mainly caused by the random orientation of the halo ellipsoid with respect to the line-of-sight. Since our simulations account for the presence of a bright central galaxy, we perform mass and concentration measurements using a generalized NFW profile which allows for a free inner slope. This reduces both the mass and the concentration biases but introduces an additional free parameter. We discuss how the mass function and the concentration mass relation change when using WL and WL+SL estimates. Finally, we investigate how selection eects impact the measured concentration-mass relation showing that strong lens clusters may have a concentration 20 30% higher than the average, at fixed mass, considering also the particular case of strong lensing selected samples of relaxed clusters.

glamer – I. A code for gravitational lensing simulations with adaptive mesh refinement

A computer code is described for the simulation of gravitational lensing data. The code incorporates adaptive mesh refinement in choosing which rays to shoot based on the requirements of the source size, location and surface brightness distribution or to find critical curves/caustics. A variety of source surface brightness models are implemented to represent galaxies and quasar emission regions. The lensing mass can be represented by point masses (stars), smoothed simulation particles, analytic halo models, pixelized mass maps or any combination of these. The deflection and beam distortions (convergence and shear) are calculated by modified tree algorithm when halos, point masses or particles are used and by FFT when mass maps are used. The combination of these methods allow for a very large dynamical range to be represented in a single simulation. Individual images of galaxies can be represented in a simulation that covers many square degrees. For an individual strongly lensed quasar, source sizes from the size of the quasars host galaxy (~ 100 kpc) down to microlensing scales (~ 10^-4 pc) can be probed in a self consistent simulation. Descriptions of various tests of the codes accuracy are given.

glamer – II. Multiple-plane gravitational lensing

We present an extension to multiple planes of the gravitational lensing code {\small GLAMER}. The method entails projecting the mass in the observed light-cone onto a discrete number of lens planes and inverse ray-shooting from the image to the source plane. The mass on each plane can be represented as halos, simulation particles, a projected mass map extracted form a numerical simulation or any combination of these. The image finding is done in a source oriented fashion, where only regions of interest are iteratively refined on an initially coarse image plane grid. The calculations are performed in parallel on shared memory machines. The code is able to handle different types of analytic halos (NFW, NSIE, power-law, etc.), haloes extracted from numerical simulations and clusters constructed from semi-analytic models ({\small MOKA}). Likewise, there are several different options for modeling the source(s) which can be distributed throughout the light-cone. The distribution of matter in the light-cone can be either taken from a pre-existing N-body numerical simulations, from halo catalogs, or are generated from an analytic mass function. We present several tests of the code and demonstrate some of its applications such as generating mock images of galaxy and galaxy cluster lenses.

Characterizing dark interactions with the halo mass accretion history and structural properties

We study the halo mass accretion history (MAH) and its correlation with the internal structural properties in coupled dark energy (cDE) cosmologies. To accurately predict all the non-linear eects caused by dark interactions, we use the COupled Dark Energy Cosmological Simulations (CoDECS). We measure the halo concentration at z = 0 and the number of substructures above a mass resolution threshold for each halo. Tracing the halo merging history trees back in time, following the mass of the main halo, we develope a MAH model that accurately reproduces the halo growth in term of M200 in the cold dark matter (CDM Universe; we then compare the MAH