Browsing by Author "Souradeep, Tarun"

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AIGO: a southern hemisphere detector for the worldwide array of ground based interferometric gravitational wave detectors
(2009-10-01) Dhurandhar, Sanjeev; Souradeep, Tarun; Coward, D
This paper describes the proposed AlGO detector for the worldwide array of interferometric gravitational wave detectors. The first part of the paper summarises the benefits that AlGO provides to the worldwide array of detectors. The second part gives a technical description of the detector, which will follow closely the Advanced LIGO design. Possible technical variations in the design are discussed.
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.
Assumptions of the primordial spectrum and cosmological parameter estimation
(2009-01-01) Shafieloo, Arman; Souradeep, Tarun
The observables of the perturbed universe, CMB anisotropy and large structures, depend on a set of cosmological parameters, as well as, the assumed nature of primordial perturbations. In particular, the shape of the primordial power spectrum (PPS) is, at best, a well motivated assumption. It is known that the assumed functional form of the PPS in cosmological parameter estimation can aﬀect the best ﬁt parameters and their relative conﬁdence limits. In this letter, we demonstrate that a speciﬁc assumed form actually drives the best ﬁt parameters into distinct basins of likelihood in the space of cosmological parameters where the likelihood resists improvement via modiﬁcations to the PPS. The regions where considerably better likelihoods are obtained allowing free form PPS lie outside these basins. In the absence of a preferred model of inﬂation, this raises a concern that current cosmological parameters estimates are strongly prejudiced by the assumed form of PPS. Our results strongly motivate approaches toward simultaneous estimation of the cosmological parameters and the shape of the primordial spectrum from upcoming cosmological data. It is equally important for theorists to keep an open mind towards early universe scenarios that produce features in the PPS.
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.
Binned cosmic microwave background anisotropy power spectra : Peak location
(2001-02-15) Podariu, Silviu; Souradeep, Tarun; Gott, J. Richard; et al.
We use weighted mean and median statistics techniques to combine individual cosmic microwave background (CMB) anisotropy detections and determine binned, multipole- space, CMB anisotropy power spectra. The resultant power spectra are peaked. The derived weighted-mean CMB anisotropy power spectrum is not a good representation of the individual measurements in a number of multipole-space bins, if the CMB anisotropy is Gaussian and correlations between individual measurements are small. This could mean that some observational error bars are underestimated, possibly as a consequence of undetected systematic eﬀects. Discarding the most discrepant 5% of the measure- ments alleviates but does not completely resolve this problem. The median-statistics power spectrum of this culled data set is not as constraining as the weighted-mean power spectrum. Nevertheless it indicates that there is more power at multipoles ℓ ∼ 150 − 250 than is expected in an open cold dark matter (CDM) model, and it is more consistent with a ﬂat CDM model. Unlike the weighted-mean power spectrum, the median-statistics power spectrum at ℓ ∼ 400 − 500 does not exclude a second peak in the ﬂat CDM model.
Bipolar Harmonic encoding of CMB correlation patterns
(2009-12-01) Joshi, Nidhi; Jhingan, S.; Souradeep, Tarun; et al.
Deviations from statistical isotropy can be modeled in various ways, for instance, anisotropic cosmological models (Bianchi models), compact topologies and presence of primordial magnetic ﬁeld. Signature of anisotropy manifests itself in CMB correlation patterns. Here we explore the symmetries of the correlation function and its implications on the observable measures constructed within the Bipolar harmonic formalism for these variety of models. Diﬀerent quantiﬁers within the Bipolar harmonic representation are used to distinguish between plausible models of breakdown of statistical isotropy and as a spectroscopic tool for discriminating between distinct cosmic topology.
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.
CMB anisotropy power spectrum using linear combinations of WMAP maps
(2007-06-25) Saha, Rajib; Souradeep, Tarun; Jain, Pankaj; et al.
In recent years the goal of estimating diﬀerent cosmological parameters precisely has set new challenges in the eﬀort to accurately measure the angular power spectrum of CMB. 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 solely from multi-frequency observations has been proposed and implemented on the ﬁrst yearWMAP data by Saha et al. 2006. All previous estimates of power spectrum of CMB are based upon foreground templates using data sets from diﬀerent experiments. However our methodology demonstrates that CMB angular spectrum can be reliably estimated with precision from a self contained analysis of the WMAP data. In this work we provide a detailed description of this method. We also study and identify the biases present in our power spectrum estimate. We apply our methodoly to extract the power spectrum from the WMAP 1 year and 3 year data.
CMB Polarization and Temperature Power Spectra Estimation using Linear Combination of WMAP 5-year Maps
(2009-03-01) Souradeep, Tarun; Jain, Pankaj; Saha, Rajib; et al.
We estimate CMB polarization and temperature power spectra using WMAP 5-year foreground contaminated maps. The power spectrum is estimated by using a model independent method, which does not utilize directly the diﬀuse foreground templates nor the detec tor noise model. The method essentially consists of two steps, (i) removal of diﬀuse foregrounds contamination by making linear combination of individual maps in harmonic space and (ii) cross-correlation of foreground cleaned maps to minimize detector noise bias. For temperature power spectrum we also estimate and subtract residual unre- solved point source contamination in the cross-power spectrum using the point source model provided by the WMAP science team. Our 1TT, TE and EE power spectra are in good agreement with the published results of the WMAP science team. The error bars on the polarization power spectra, however, turn out to be smaller in comparison to what is obtained by the WMAP science team. We perform detailed numerical simulations to test for bias in our procedure. We ﬁnd that the bias is small in all cases. A negative bias at low l in TT power spectrum has been pointed in an earlier publication. We ﬁnd that the bias corrected quadrupole power (l(l + 1)Cl/2π) is 532 µK2, approximately 2.5 times the estimate (213.4 µK2) made by the WMAP team.
CMB power spectrum estimation using non-circular beam
(2011-07-06) Mitra, Sanjit; Sengupta, Anand; Souradeep, Tarun
The measurements of the angular power spectrum of the Cosmic Microwave Background (CMB) anisotropy has proved crucial to the emergence of cosmology as a precision science in recent years. In this remarkable data rich period, the limitations to precision now arise from the the inability to account for ﬁner systematic eﬀects in data analysis. The non-circularity of the experimental beam has become progressively important as CMB experiments strive to attain higher angular resolution and sensitivity. We present an analytic framework for studying the leading order eﬀects of a non- circular beam on the CMB power spectrum estimation. We consider a non-circular beam of ﬁxed shape but variable orientation. We compute the bias in the pseudo-Cl power spectrum estimator and then construct an unbiased estimator using the bias matrix. The covariance matrix of the unbiased estimator is computed for smooth, non-circular beams. Quantitative results are shown for CMB maps made by a hypothetical experiment with a non-circular beam comparable to our ﬁts to the WMAP beam maps described in the appendix and uses a toy scan strategy. We ﬁnd that signiﬁcant eﬀects on CMB power spectrum can arise due to non-circular beam on multipoles comparable to, and beyond, the inverse average beam-width where the pseudo-Cl approach may be the method of choice due to computational limitations of analyzing the large datasets from current and near future CMB experiments.
CMB power spectrum estimation with non-circular beam and incomplete sky coverage
(2007-02-05) Mitra, Sanjit; Sengupta, Anand; Souradeep, Tarun; et al.
Over the last decade, measurements of the CMB anisotropy has spearheaded the remarkable transition of cosmology into a precision science. However, addressing the systematic eﬀects in the increasingly sensitive, high resolution, ‘full’ sky measurements from diﬀerent CMB experiments pose a stiﬀ challenge. The analysis techniques must not only be computationally fast to contend with the huge size of the data, but, the higher sensitivity also limits the simplifying assumptions which can then be invoked to achieve the desired speed without compromising the ﬁnal precision goals. While maximum likelihood is desirable, the enormous computational cost makes the suboptimal method of power spectrum estimation using Pseudo-Cl unavoidable for high resolution data. The debiasing of the Pseudo-Cl needs account for non-circular beams, together with non-uniform sky coverage. We provide an analytic framework for correcting the power spectrum for the eﬀect of beam noncircularity 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.
Cosmic microwave background bipolar power spectrum: Basic formalism and applications
(2005-01-01) Hajian, Amir; Souradeep, Tarun
We study the statistical isotropy (SI) of temperature ﬂuctuations of the CMB as dis- tinct from Gaussianity. We present a detailed formalism of the bipolar power spectrum BiPS) which was introduced as a fast method of measuring the statistical isotropy by Hajian & Souradeep 2003. The method exploits the existence of patterns in the real space correlations of the CMB temperature ﬁeld. We discuss the applications of BiPS in constraining the topology of the universe and other theoretical scenarios of SI violation. Unlike the traditional methods of search for cosmic topology, this method is computationally fast. We also show that BiPS is potentially a good tool to detect the eﬀect of observational artifacts in a CMB map such as non-circular beam, anisotropic noise, etc. Our method has been successfully applied to the Wilkinson Microwave Anisotropy Probe sky maps by Hajian et al. 2004, but no strong evidence of SI violation was found.
Cosmological parameter estimation using particle swarm optimization (PSO)
(2011-08-30) Prasad, Jayanti; Souradeep, Tarun
Obtaining the set of cosmological parameters consistent with observational data is an important exercise in current cosmological research. It involves ﬁnding the global maximum of the likelihood function in the multi-dimensional parameter space. Currently sampling based methods, which are in general stochastic in nature, like Markov-Chain Monte Carlo(MCMC), are being commonly used for parameter estimation. The beauty of stochastic methods is that the computational cost grows, at the most, linearly in place of exponentially (as in grid based approaches) with the dimensionality of the search space. MCMC methods sample the full joint probability distribution (posterior) from which one and two dimensional probability distributions, best ﬁt (average) values of parameters and then error bars can be computed. In the present work we demonstrate the application of another stochastic method, named Particle Swarm Optimization (PSO), that is widely used in the ﬁeld of engineering and artiﬁcial intelligence, for cosmological parameter estimation from WMAP seven years data. We ﬁnd that there is a good agreement between the values of the best ﬁt parameters obtained from PSO and publicly available code COSMOMC. However, there is a slight disagreement between error bars mainly due to the fact that errors are computed diﬀerently in PSO. Apart from presenting the results of our exercise, we also discuss the merits of PSO and explain its usefulness in more extensive search in higher dimensional parameter space.
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.
Cosmology
(2013-07-16) Sahni, Varun; Souradeep, Tarun
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.
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.
Density perturbations, gravity waves and the cosmic microwave background
(2014-07-30) Souradeep, Tarun; Sahni, Varun
Dipole leakage and low CMB multipoles
(IUCAA, 2015-02) Das, Santanu; Souradeep, Tarun
Double inflation and the low CMB multipoles
(2008-09) Souradeep, Tarun
We investigate inﬂationary scenarios driven by a class of potentials which are similar in form to those that arise in certain minimal supersymmetric extensions of the standard model. We show that these potentials allow a period of fast roll sandwiched between two stages of slow roll inﬂation. We ﬁnd that the modes which exit the Hubble radius during the period of fast roll have lower power when comparedto the amplitude of the nearly scale invariant spectrum associated with the modes that leave during the second stage of slow roll inﬂation. We set the scales such that the drop in the scalar power spectrum occurs at a length scale that corresponds to the Hubble radius today—a feature that seems necessary to explain the lower power observed in the quadrupole moment of the Cosmic Microwave Background (CMB) anisotropies. We perform a Markov Chain Monte Carlo analysis to determine the values of the model parameters that provide the best ﬁt to the recent WMAP 5-year data for the CMB angular power spectrum. We ﬁnd that an inﬂationary spectrum with a suppression of power at large scales that we obtain leads to a much better ﬁt (with just one extra parameter, χ2 eﬀ improves by 6.62) of the observed data when compared to the best ﬁt reference ΛCDM model with a featureless, power law, primordial spectrum.