# Ishak-Boushaki, Mustapha

Permanent URI for this collectionhttps://hdl.handle.net/10735.1/2489

Mustapha Ishak-Boushaki is an Associate Professor in the Department of Physics. He also serves on the faculty of the UT Dallas Cosmology, Relativity, and Astrophysics Group. Dr. Ishak-Boushaki's research interests include:

- Gravitational Lensing and applications to cosmology.
- The Acceleration of the expansion of the Universe: Cosmological Constant, Dark Energy.
- Constraining cosmological parameters and cosmological models using probes such as gravitational lensing, the cosmic microwave background (CMB), and supernova searches.
- General Relativity and Cosmological Exact Solutions to Einstein's Equations.
- Higher dimensional cosmological models.
- Projects at the intersection of modern cosmology and General Relativity.
- Junction conditions for matching space-times and constructing wormholes and spacetime thin-shells.
- Computer Algebra (symbolic computing) and application to cosmology and general relativity.

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# Browsing Ishak-Boushaki, Mustapha by Subject "Cosmology"

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Item Effect of Inhomogeneities on High Precision Measurements of Cosmological Distances(American Physical Society, 2014-12-30) Peel, Austin; Troxel, Michael A.; Ishak-Boushaki, Mustapha; Peel, Austin; Troxel, Michael A.; Ishak-Boushaki, MustaphaWe study effects of inhomogeneities on distance measures in an exact relativistic Swiss-cheese model of the Universe, focusing on the distance modulus. The model has ΛCDM background dynamics, and the "holes" are nonsymmetric structures described by the Szekeres metric. The Szekeres exact solution of Einstein's equations, which is inhomogeneous and anisotropic, allows us to capture potentially relevant effects on light propagation due to nontrivial evolution of structures in an exact framework. Light beams traversing a single Szekeres structure in different ways can experience either magnification or demagnification, depending on the particular path. Consistent with expectations, we find a shift in the distance modulus μ to distant sources due to demagnification when the light beam travels primarily through the void regions of our model. Conversely, beams are magnified when they propagate mainly through the overdense regions of the structures, and we explore a small additional effect due to time evolution of the structures. We then study the probability distributions of Δμ = μ_{ΛCDM} – μ_{SC} for sources at different redshifts in various Swiss-cheese constructions, where the light beams travel through a large number of randomly oriented Szekeres holes with random impact parameters. We find for Δμ the dispersions 0.004 ≤ σ_{Δμ} ≤ 0.008 mag for sources with redshifts 1.0 ≤ ȥ ≤ 1.5, which are smaller than the intrinsic dispersion of, for example, magnitudes of type Ia supernovae. The shapes of the distributions we obtain for our Swiss-cheese constructions are peculiar in the sense that they are not consistently skewed toward the demagnification side, as they are in analyses of lensing in cosmological simulations. Depending on the source redshift, the distributions for our models can be skewed to either the demagnification or the magnification side, reflecting a limitation of these constructions. This could be the result of requiring the continuity of Einstein's equations throughout the overall spacetime patchwork, which imposes the condition that compensating overdense shells must accompany the underdense void regions in the holes. The possibility to explore other uses of these constructions that could circumvent this limitation and lead to different statistics remains open.Item Self-Calibrating the Gravitational Shear-Intrinsic Ellipticity-Intrinsic Ellipticity Cross-Correlation(2012-06-01) Troxel, Michael A.; Ishak-Boushaki, Mustapha; Troxel, Michael A.; Ishak-Boushaki, MustaphaWe extend the 3-point intrinsic alignment self-calibration technique to the gravitational shear-intrinsic ellipticity-intrinsic ellipticity (GII) bispectrum. While significantly decreased from using cross-correlations instead of autocorrelation in a single photo-z bin, the GII contamination persists in adjacent photo-z bins and must be accounted for and removed from the lensing signal. The proposed technique will allow the measurement and removal of the GII intrinsic alignment contamination from the cross-correlation weak lensing signal. We relate the GII and galaxy density-intrinsic ellipticity-intrinsic ellipticity (gII) bispectra through use of the galaxy bias, and develop the estimator necessary to isolate the gII bispectrum from observations. We find that the GII self-calibration technique performs at a level comparable to that of the gravitational shear-gravitational shear-intrinsic ellipticity correlation (GGI) self-calibration technique, with measurement error introduced through the gII estimator generally negligible when compared to minimum survey error. The accuracy of the relationship between the GII and gII bispectra typically allows the GII self-calibration to reduce the GII contamination by a factor of 10 or more for all adjacent photo-z bin combinations at ℓ > 300. For larger scales, we find that the GII contamination can be reduced by a factor of 3-5 or more. The GII self-calibration technique is complementary to the existing GGI self-calibration technique, which together will allow the total intrinsic alignment cross-correlation signal in 3-point weak lensing to be measured and removed.Item Self-Calibration for Three-Point Intrinsic Alignment Autocorrelations in Weak Lensing Surveys(2012-06-08) Troxel, Michael A. .; Ishak-Boushaki, Mustapha; 0000 0001 2874 3832 (Ishak-Boushaki, M); Troxel, Michael A. .; Ishak-Boushaki, MustaphaThe weak lensing signal (cosmic shear) has been shown to be strongly contaminated by the various types of galaxy intrinsic alignment (IA) correlations, which poses a barrier to precision weak lensing measurements. The redshift dependence of the IA signal has been used at the two-point level to reduce this contamination by only measuring cross-correlations between large redshift bins, which significantly reduces the galaxy intrinsic ellipticity-intrinsic ellipticity (II) correlation. A self-calibration technique based on the redshift dependencies of the IA correlations has also been proposed as a means to remove the two-point IA contamination from the lensing signal. We explore here the redshift dependencies of the IA and lensing bispectra in order to propose a self-calibration of the IA autocorrelations at the three-point level (i.e. GGI, GII and III), which can be well understood without the assumption of any particular IA model. We find that future weak lensing surveys will be able to measure the distinctive IA redshift dependence over ranges of |Δ_ȥ^P| ≤ 0.2. Using conservative estimates of photo-ȥ accuracy, we describe the three-point self-calibration technique for the total IA signal, which can be accomplished through lensing tomography of photo-ȥ bin size ∼0.01. We find that the three-point self-calibration can function at the accuracy of the two-point technique with modest constraints in redshift separation. This allows the three-point IA autocorrelation self-calibration technique proposed here to significantly reduce the contamination of the IA contamination to the weak lensing bispectrum.Item Self-Calibration Technique for Three-Point Intrinsic Alignment Correlations in Weak Lensing Surveys(2011-11-15) Troxel, Michael A.; Ishak-Boushaki, Mustapha; 0000 0001 2874 3832 (Ishak-Boushaki, M); Troxel, Michael A.; Ishak-Boushaki, MustaphaThe intrinsic alignment (IA) of galaxies has been shown to be a significant barrier to precision cosmic shear measurements. Recently, Zhang proposed a self-calibration technique for the power spectrum to calculate the induced gravitational shear-galaxy intrinsic ellipticity correlation (GI) in weak lensing surveys with photo-z measurements, which is expected to reduce the IA contamination by at least a factor of 10 for currently proposed surveys. We confirm this using an independent analysis and propose an expansion to the self-calibration technique for the bispectrum in order to calculate the dominant IA gravitational shear-gravitational shear-intrinsic ellipticity correlation (GGI) contamination. We first establish an estimator to extract the galaxy density-density-intrinsic ellipticity (ggI) correlation from the galaxy ellipticity-density-density measurement for a photo-z galaxy sample. We then develop a relation between the GGI and ggI bispectra, which allows for the estimation and removal of the GGI correlation from the cosmic shear signal. We explore the performance of these two methods, compare to other possible sources of error, and show that the GGI self-calibration technique can potentially reduce the IA contamination by up to a factor of 5-10 for all but a few bin choices, thus reducing the contamination to the per cent level. The self-calibration is less accurate for adjacent bins, but still allows for a factor of 3 reduction in the IA contamination. The self-calibration thus promises to be an efficient technique to isolate both the two-point and three-point intrinsic alignment signals from weak lensing measurements. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.