IUCAA Preprints

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    Characterizing Foreground for redshifted 21-cm radiation: 150 MHz GMRT observations
    (2013-09-05) Ghosh, Abhik; Prasad, Jayanti; Bharadwaj, Somnath; Ali, Saiyad; Chengalur, Jayaram N.
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    Cosmological constant - The weight of the vacuum
    (2011-07-06) Padmanabhan, T.
    Recent cosmological observations suggest the existence of a positive cosmological constant Λ with the magnitude Λ(G~/c3) ≈ 10−123. This review discusses several aspects of the cosmological constant both from the cosmological (sections 1–6) and field theoretical (sections 7–11) perspectives. After a brief introduction to the key issues related to cosmological constant and a historical overview, a summary of the kinematics and dynamics of the standard Friedmann model of the universe is provided. The observational evidence for cosmological constant, especially from the supernova results, and the constraints from the age of the universe, structure for- mation, Cosmic Microwave Background Radiation (CMBR) anisotropies and a few others are described in detail, followed by a discussion of the theoretical models (quintessence, tachyonic scalar field, ...) from different perspectives. The latter part of the review (sections 7–11) concentrates on more conceptual and fundamental aspects of the cosmological constant like some alternative interpretations of the cosmological constant, relaxation mechanisms to reduce the cosmological constant to the currently observed value, the geometrical structure of the de Sitter space- time, thermodynamics of the de Sitter universe and the role of string theory in the cosmological constant problem.
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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.