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Author: search.filters.author.Souradeep, TarunSubject: search.filters.subject.Cosmology

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Now showing 1 - 8 of 8
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    Generation of seed perturbations from quantum cosmology
    (2014-11-26) Souradeep, Tarun
    The origin of seed perturbations in the Universe is studied within the framework of a specific mini super space model. It is shown that the ‘creation’ of the Universe as a result of a quantum transition from a flat empty space time would lead to a flat FLRW (Friedmann Lemaitre Robertson- Walker) Universe with weak inhomogeneous perturbations at large wavelengths. The power spectrum of these perturbations is found to be scale invariant at horizon crossing (i.e., the Harrison- Zeldovich spectrum). It is also recognised that the seed perturbations generated in our model would be generically of the isocurvature kind.
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    Current status of observational cosmology
    (2011-07-06) Ostriker, Jeremiah P.; Souradeep, Tarun
    Observational cosmology has indeed made very rapid progress in recent years. The ability to quantify the universe has largely improved due to observational constraints coming from structure formation. The transition to precision cosmology has been spear- headed by measurements of the anisotropy in the cosmic microwave background (CMB) over the past decade. Observations of the large scale structure in the distribution of galax- ies, high red-shift supernova, have provided the required complementary information. We review the current status of cosmological parameter estimates from joint analysis of CMB anisotropy and large scale structure (LSS) data. We also sound a note of caution on overstating the successes achieved thus far.
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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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    Cosmology with CMB anisotropy
    (2006-07-12) Souradeep, Tarun
    Measurements of CMB anisotropy and, more recently, polarization have played a very important role allowing precise determination of various parameters of the ‘standard’ cosmological model. The expectation of the paradigm of inflation and the generic prediction of the simplest real- ization of inflationary scenario in the early universe have also been established – ‘acausally’ correlated initial perturbations in a flat, statistically isotropic universe, adiabatic nature of primordial density perturbations. Direct evidence for gravitational instability mechanism for structure formation from primordial perturbations has been established. In the next decade, future experiments promise to strengthen these deductions and uncover the remaining crucial signature of inflation – the primordial gravitational wave background.
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    Cosmological quests in the CMB sky
    (2006-06-21) Souradeep, Tarun
    Observational Cosmology has indeed made very rapid progress in recent years. The ability to quantify the universe has largely improved due to observational constraints coming from structure formation Measure- ments of CMB anisotropy and, more recently, polarization have played a very important role. Besides precise determination of various parameters of the ‘standard’ cosmological model, observations have also established some important basic tenets that underlie models of cosmology and struc- ture formation in the universe – ‘acausally’ correlated initial perturbations in a flat, statistically isotropic universe, adiabatic nature of primordial density perturbations. These are consistent with the expectation of the paradigm of inflation and the generic prediction of the simplest realization of inflationary scenario in the early universe. Further, gravitational instability is the established mechanism for structure formation from these initial perturbations. In the next decade, future experiments promise to strengthen these deductions and uncover the remaining crucial signature of inflation – the primordial gravitational wave background.
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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.
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    Beyond the standard cosmological model with CMB
    (2011-06-02) Souradeep, Tarun
    Measurements of CMB anisotropy and, more recently, polarization have played a very important role in cosmology. Besides precise determination of various parameters of the ‘standard’ cosmological model, observations have also established some important basic tenets that underlie models of cosmology and structure formation in the universe – ‘acausally’ correlated, adiabatic, primordial perturbations in a flat, statistically isotropic universe. These are consistent with the expectation of the paradigm of inflation and the generic prediction of the simplest realization of inflationary scenario in the early universe. Further, gravitational instability is the established mechanism for structure formation from these initial perturbations. Primordial perturbations observed as the CMB anisotropy and polarization is the most compelling evidence for new, possibly fundamental, physics in the early universe. The community is now looking beyond the parameter estimation of the ‘standard’ model, for subtle, characteristic signatures of early universe physics.