IUCAA Preprints

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Author: search.filters.author.Souradeep, TarunSubject: search.filters.subject.Cosmic microwave background

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Now showing 1 - 10 of 13
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    Dipole leakage and low CMB multipoles
    (IUCAA, 2015-02) Das, Santanu; Souradeep, Tarun
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    Window function for non-circular beam CMB anisotropy experiment
    (2001-07-05) Souradeep, Tarun; Ratra, Bharat
    We develop computationally rapid methods to compute the window function for a cosmic microwave background anisotropy experiment with a non-circular beam which scans over large angles on the sky. To concretely illustrate these methods we compute the window function for the Python V experiment which scans over large angles on the sky with an elliptical Gaussian beam.
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    Statistical isotropy of the cosmic microwave background
    (2011-07-05) Souradeep, Tarun; Hajian, Amir
    The breakdown of statistical homogeneity and isotropy of cosmic perturbations is a generic feature of ultra large scale structure of the cosmos, in particular, of non trivial cosmic topol- ogy. The statistical isotropy (SI) of the Cosmic Microwave Background temperature fluctuations (CMB anisotropy) is sensitive to this breakdown on the largest scales comparable to, and even be- yond the cosmic horizon. We propose a set of measures, κℓ (ℓ = 1, 2, 3, . . .) which for non-zero values indicate and quantify statistical isotropy violations in a CMB map. We numerically compute the predicted κℓ spectra for CMB anisotropy in flat torus universe models. Characteristic signature of different models in the κℓ spectrum are noted.
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    Measuring statistical isotropy of the CMB anisotropy
    (2003-07-05) Ajian, Amir; Souradeep, Tarun
    The statistical expectation values of the temperature fluctuations of cosmic microwave background (CMB) are assumed to be preserved under rotations of the sky. This assumption of statistical isotropy (SI) of the CMB anisotropy should be observationally verified since detection of violation of SI could have profound implications for cosmology. We propose a set of measures, κℓ (ℓ = 1, 2, 3, . . .) for detecting violation of statistical isotropy in an observed CMB anisotropy sky map indicated by non zero κℓ. We define an estimator for the κℓ spectrum and ana- lytically compute its cosmic bias and cosmic variance. The results match those obtained by measuring κℓ using simulated sky maps. Non-zero (bias corrected) κℓ larger than the SI cosmic variance will imply violation of SI. The SI mea- sure proposed in this paper is an appropriate statistics to investigate preliminary indication of SI violation in the recently released WMAP data.
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    Statistical Isotropy of the WMAP Data: A bipolar power spectrum analysis
    (2011-07-06) Hajian, Amir; Souradeep, Tarun; Neil, Cornish
    ABSTRACT The statistical expectation values of the temperature fluctuations of the cosmic microwave background (CMB) are assumed to be preserved under rotations of the sky. We use the bipolar power s pectrum (BiPS) proposed in Hajian & Souradeep to investigate the statistical isotropy (SI) of the CMB anisotropy maps recently measured by the Wilkinson Microwave Anisotropy Probe (WMAP). The method can isolate and probe specific regions of choice in multipole space using appropriate window functions. The BiPS is estimated for full sky CMB anisotropy maps based on the first year WMAP data using a range of window functions. The BiPS spectra computed for both full sky maps for all our window functions are consistent with zero, roughly within 2 σ. The null BiPS results may be interpreted as an absence of strong violation of statistical isotropy in the first-year WMAP data on angular scales larger than that corresponding to l ∼ 60. However, pending a careful direct comparison, our results do not necessarily conflict with the specific SI related anomalies reported using other statistical tests.
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    Estimating Cosmological Parameters from CMBR : Systematic effects
    (2005-01-01) Sinha, Rita; Souradeep, Tarun
    The fluctuations in the cosmic microwave background radiation depend on the cosmological parameters. We carry out Markov Chain Monte Carlo simulations to constrain the cosmological parameter space with the current CMB data for possible systematic effects! such as beam noncircularity in CMB measurements and incomplete sky coverage.
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    Blind estimation of the power spectrum of CMB anisotropy from WMAP
    (2005-06-01) Saha, Rajib; Jain, Pankaj; Souradeep, Tarun
    Accurate measurements of angular power spectrum of Cosmic Microwave Background (CMB) radiation has lead to marked improvement in the estimates of different cosmological parameters. This has required removal of foreground contamination as well as detector noise bias with reliability and precision. We present the estimation of CMB angular power spectrum from the multi-frequency observations ofWMAP using a novel model-independent method. The primary product of WMAP are the observations of CMB in 10 independent difference assemblies (DA) that have uncorrelated noise. Our method utilizes maximum information available within WMAP data by linearly combining all the DA maps to remove foregrounds and estimating the power spectrum from cross power spectra of clean maps with independent noise. We compute 24 cross power spectra which are the basis of the final power spectrum. The binned average power matches with WMAP team’s published power spectrum closely. A small systematic difference at large multipoles is accounted for by the correction for the expected residual power from unresolved point sources. The correction is small and significantly tempered. Previous estimates have depended on foreground templates built using extraneous observational input. This is the first demonstration that the CMB angular spectrum can be reliably estimated with precision from a self contained analysis of the WMAP data.
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    Non-circular beam correction to the CMB power spectrum
    (2006-08-24) Souradeep, Tarun; Mitra, Sanjit; Sengupta, Anand; et al.
    In the era of high precision CMB measurements, systematic effects are beginning to limit the ability to extract subtler cosmological information. The non-circularity of the experimental beam has become progressively important as CMB experiments strive to attain higher angular resolution and sensitivity. The effect of non-circular beam on the power spectrum is important at multipoles larger than the beam-width. For recent experiments with high angular resolution, optimal methods of power spectrum estimation are computationally prohibitive and sub-optimal approaches, such as the Pseudo-Cl method, are used. We provide an analytic framework for correcting the power spectrum for the effect of beam non-circularity and non-uniform sky coverage (including incomplete/masked sky maps). The approach is perturbative in the distortion of the beam from non-circularity allowing for rapid computations when the beam is mildly non-circular. When non-circular beam effect is important, we advocate that it is computationally advantageous to employ ‘soft’ azimuthally apodized masks whose spherical harmonic transform die down fast with m.
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    Measuring statistical isotropy of CMB anisotropy
    (2006-07-25) Souradeep, Tarun; Hajian, Amir; Basak, Soumen
    The statistical expectation values of the temperature fluctuations and polarization of cosmic microwave background (CMB) are assumed to be preserved under rotations of the sky. We investigate the statistical isotropy (SI) of the CMB maps recently measured by the Wilkinson Microwave Anisotropy Probe (WMAP) using the bipolar spherical harmonic formalism proposed in Hajian & Souradeep 2003 for CMB temperature anisotropy and extended to CMB polarization in Basak, Hajian & Souradeep 2006. The Bipolar Power Spectrum (BiPS) had been measured for the full sky CMB anisotropy maps of the first year WMAP data and now for the recently released three years of WMAP data. We also introduce and measure directional sensitive reduced Bipolar coefficients on the three year WMAP ILC map. Consistent with our published results from first year WMAP data we have no evidence for violation of statistical isotropy on large angular scales. Preliminary analysis of the recently released firstWMAP polarization maps, however, indicate significant viola- tion of SI even when the foreground contaminated regions are masked out. Further work is required to confirm a possible cosmic origin and rule out the (more likely) origin in observational artifact such as foreground residuals at high galactic latitude.
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    Angular power spectrum of CMB anisotropy from WMAP
    (2011-08-09) Souradeep, Tarun; Saha, Rajib; Jain, Pankaj
    The remarkable improvement in the estimates of different cosmological parameters in recent years has been largely spearheaded by accurate measurements of the angular power spectrum of Cosmic Microwave Background (CMB) radiation. This has required removal of foreground contamination as well as detector noise bias with reliability and precision. Recently, a novel model independent method for the estimation of CMB angular power spectrum from multi-frequency observations has been proposed and implemented on the first year WMAP (WMAP-1) data by Saha et al. 2006. We review the results from WMAP-1 and also present the new angular power spectrum based on three years of the WMAP data (WMAP-3). Previous estimates have depended on foreground templates built using extraneous observational input to remove foreground contamination. This is the first demonstration that the CMB angular spectrum can be reliably estimated with precision from a self contained analysis of the WMAP data. The primary product of WMAP are the observations of CMB in 10 independent difference assemblies (DA) distributed over 5 frequency bands that have uncorrelated noise. Our method utilizes maximum information available within WMAP data by linearly combining DA maps from different frequencies to remove foregrounds and estimating the power spectrum from the 24 cross power spectra of clean maps that have independent noise. An important merit of the method is that the expected residual power from unresolved point sources is significantly tempered to a constant offset at large multipoles (in contrast to the ∼ L² contribution expected from a Poisson distribution) leading to a small correction at large multipoles. Hence, the power spectrum estimates are less susceptible to uncertainties in the model of point sources.