2011 (IPP)

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5D radiating black holes in einstein-yang-mills-gauss-bonnet gravity
(2011-08-25) Ghosh, S. G.
We derive nonstatic spherically symmetric solutions of a null uid, in ve dimension (5D), to Einstein-Yang-Mills (EYM) equations with the coupling of Gauss-Bonnet (GB) combination of quadratic curvature terms, namely, 5D-EYMGB radiating black hole solution. It is shown that, in the limit, we can recover known radiating black hole solutions. The spherically symmetric known 5D static black hole solutions are also retrieved. The e ect of the GB term and Yang-Mills (YM) gauge charge on the structure and location of horizons, of the 5D radiating black hole, is also discussed.
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.
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.
Cross-correlation search for a hot spot of gravitational waves
(2011-05-31) Dhurandhar, Sanjeev; Tagoshi, H.; Okada, Yuta; et al.
The cross-correlation search has been previously applied to map the gravitational wave (GW) stochastic background in the sky and also to target GW from rotating neutron stars/pulsars. Here we investigate how the cross-correlation method can be used to target a small region in the sky spanning at most a few pixels, where a pixel in the sky is determined by the diﬀraction limit which depends on the (i) baseline joining a pair of detectors and (ii) detector bandwidth. Here as one of the promising targets, we consider the Virgo cluster - a ”hot spot” spanning few pixels - which could contain, as estimates suggest ~ 10¹¹ neutron stars, of which a small fraction would continuously emit GW in the bandwidth of the detectors. For the detector baselines, we consider advanced detector pairs among LCGT, LIGO, Virgo, ET etc. Our results show that suﬃcient signal to noise can be accumulated with integration times of the order of a year. The results improve for the multibaseline search. This analysis could as well be applied to other likely hot spots in the sky and other possible pairs of detectors.
Early universe with CMB polarization
(2011-04-16) Souradeep, Tarun
The Universe is the grandest conceivable scale on which the human mind can strive to understand nature. The amazing aspect of cosmology, the branch of science that attempts to understand the origin and evolution of the Universe, is that it is largely comprehensible by applying the same basic laws of physics that we use for other branches of physics. The observed cosmic microwave background (CMB) is understood by applying the basic laws of radiative processes and transfer, masterfully covered in the classic text by S. Chandrasekhar, in the cosmological context. In addition to the now widely acclaimed temperature anisotropy, there is also linear polarization information imprinted on the observed Cosmic Microwave background. CMB polarization already has addressed, and promises to do a lot more, to unravel the deepest fundamental queries about physics operating close to the origin of the Universe.
Entropy changes in the clustering of galaxies in an expanding universe
(2011-05-26) Iqbal, Naseer; Khan, Mohammad Shafi; Masood, Tabasum
In the present work the approach-thermody-namics and statistical mechanics of gravitating systems is applied to study the entropy changein gravitational clustering of galaxies in an ex-panding universe. We derive analytically the expressions for gravitational entropy in terms of temperature T and average density n of the par-ticles (galaxies) in the given phase space cell. It is found that during the initial stage of cluster-ing of galaxies, the entropy decreases and finally seems to be increasing when the system attains virial equilibrium. The entropy changes are studied for different range of measuring correlation parameter b. We attempt to provide a clearer account of this phenomena. The entropy esults for a system consisting of extended mass (non-point mass) particles show a imilar behaviour with that of point mass particles clustering gravitationally in an expanding uni-verse.
Evolution of shocks and turbulence in the formation of galaxy clusters embedded in megaparsec-scale filaments
(2011-04-26) Iapichino, L.; Miniati, F.; et al.; Paul, Surajit
Massive structures like cluster of galaxies, embedded in cosmic ﬁlaments, release enormous amount of energy through their interactions. These events are associated with production of Mpc-scale shocks and injection of considerable amount of turbulence, aﬀecting the non-thermal energy budget of the ICM. In order to study this thoroughly, we performed a set of cosmological simulations using the hydrodynamical code Enzo.We studied the formation of clusters undergoing major mergers, the propagation of merger shocks and their interaction with the ﬁlamentary cosmic web. This interaction is shown to pro- duce peripheral structures remarkably similar to giant radio relics observed, for example, in Abell 3376 and Abell 3667. We ﬁnd a relatively long timescale (about 4 Gyr) for turbulence decay in the centre of major merging clusters. This timescale is substantially longer than typically assumed in the turbulent re-acceleration models, invoked for explaining the statistics of observed radio halos.
Evolution of the CMB power spectrum across WMAP data releases: A nonparametric analysis
(2011-07-04) Aghamousa, Amir; Arjunwadkar, Mihir; Souradeep, Tarun
Exploring the dust content of SDSS DR7 damped lyman alpha systems at 2.156 Zab <5.2
(2011-09-02) Khare, Pushpa; et al.; Berk, D. V.; York, D. G.
We have studied a sample of 1084 intervening absorption systems with 2.156 zab 65.2, having log(NHI) > 20.0 in the spectra of QSOs in Sloan Digital Sky Survey (SDSS) data release 7 (DR7), with the aim of understanding the nature and abundance of the dust and the chemical abundances in the DLA absorbers. Composite spectra were constructed for the full sample and several subsamples, chosen on the basis of absorber and QSO properties. Average extinction curves were obtained for the samples by comparing their geometric mean composite spectra with those of two samples of QSOs, matching in zem and i magnitude with the DLA sample, one sample without any absorbers along their lines of sight and the other without any DLAs along their lines of sight irrespective of the presence of other absorption systems. We also derived relative extinction curves of several pairs of subsamples. While the average reddening in the DLA sample is small, we find definite evidence for the presence of dust in subsamples based on absorber properties, in particular the strength of metal absorption lines. DLAs along lines of sight to QSOs which are not colour selected are found to be more dusty compared to those along the lines of sight to the more numerous colour selected QSOs. From these studies and from the strengths of absorption lines in the composite spectra, we conclude that 6 10% of the DLAs in SDSS DR7 cause significant reddening, have stronger absorption lines and have higher abundances as compared to the rest of the sample. The rest of the sample shows little reddening.While due to the dominant color selection method used to target QSOs in the SDSS DR7, this fraction of 10% likely represents a lower limit for the global fraction of dusty DLAs at high-z, it is also possible that the dust grain sizes at high redshifts are larger, giving rise to a flat extinction curve over the observed range of wavelengths.
FLAGCAL: A flagging and calibration package for radio interferometric data
(2011-11-28) Prasad, Jayanti; Chengalur, Jayaram
We describe a flagging and calibration pipeline intended for making quick look images from GMRT data. The package identifies and flags cor-rupted visibilities, computes calibration solutions and interpolates these onto the target source. These flagged calibrated visibilities can be directly imaged using any standard imaging package. The pipeline is written in “C” with the most compute intensive algorithms being parallelized using OpenMP.
Generalizing the cosmic energy equation
(2011-01-01) Shtanov, Yuri; Sahni, Varun
We generalize the cosmic energy equation to the case when massive particles in- teract via a modified gravitational potential of the form .(a, r), which is allowed to explicitly depend on the cosmological time through the expansion factor a(t). Using the nonrelativistic approximation for particle dynamics, we derive the equation for the cosmological expansion which has the form of the Friedmann equation with a renormalized gravitational constant. The generalized Layzer–Irvine cosmic energy equation and the associated cosmic virial theorem are applied to some recently pro- posed modifications of the Newtonian gravitational interaction between dark-matter particles. We also draw attention to the possibility that the cosmic energy equation may be used to probe the expansion history of the universe thereby throwing light on the nature of dark matter and dark energy.
Gravitational wave astronomy- astronomy of the 21st century
(2011-02-07) Dhurandhar, Sanjeev
An enigmatic prediction of Einstein’s eneral theory of relativity are gravitational waves. With the observed decay in the orbit of the Hulse-Taylor binary pulsar agreeing within a fraction of a percent with the theoretically computed decay from Einstein’s theory the existence of gravitational waves was ﬁrmly established. Currently there is a worldwide eﬀort to detect gravitational waves with inteferomet-ric gravitational wave observatories or detectors and several such detectors have been built or being built. The initial detectors have reached their design sensitivities and now the eﬀort is on to construct advanced detectors which are expected to detect gravitational waves from astrophysical sources. The era of GravitationalWave Astronomy has arrived. This article describes the worldwide eﬀort which includes the eﬀort on the Indian front - the IndIGO project -, the principle underlying interferometric detectors both on ground and in space, the principal noise sources that plague such detectors, the astrophysical sources of gravitational waves that one expects to detect by these detectors and some glimpse of the data analysis methods involved in extracting the very weak gravitational wave signals from detector noise.
Green’s function formalism of holography with arbitrary mass, spin, and dimensionality
(2011-09-23) Batiz, Zoltan; Chauhan, Bhag C.
In this work we present a mathematical description of how one can produce and read a thin hologram. We use different kinds of waves, such as scalar, vector (electromagnetic field, Maxwell-Proca fields, acoustic waves, etc.). For reading of the hologram, we use the Green’s function formalism. With the help of computer simulations, we investigate the aberrations created by this procedure for the simplest case: 2d-scalar wave case.
Infrared emission from the composite grains: Effects of inclusions and porosities on the 10 and 18 µm features
(2011-01-11) Vaidya, D.B.; Gupta, Ranjan
In this paper we study the effects of inclusions and porosities on the emission properties of silicate grains and compare the model curves with the observed infrared emission from circumstellar dust. Methods. We calculate the absorption efficiency of the composite grain, made up of a host silicate oblate spheroid and inclusions of ice/graphite/or voids, in the spectral region 5.0-25.0 m. The absorption efficiencies of the composite spheroidal oblate grains for three axial ratios are computed using the discrete dipole approximation (DDA). We study the absorption as a function of the volume fraction of the inclusions and porosity. In particular, we study the variation in the 10 m and 18 m emission features with the volume fraction of the inclusions and porosities. We then calculate the infrared fluxes for these composite grains at several dust temperatures (T=200-350K) and compare the model curves with the average observed IRAS-LRS curve, obtained for circumstellar dust shells around oxygen rich M-type stars. The model curves are also compared with two other individual stars. Results. The results on the composite grains show variation in the absorption efficiencies with the variation in the inclusions and porosities. In particular, it is found that the wavelength of peak absorption at 10 m, shifts towards longer wavelengths with variation in the volume fraction of the inclusions of graphite. The spheroidal composite grains with axial ratio ∼ 1.33; volume fraction of f=0.1 and dust temperature between 210-340K, fit the observed infra-red emission from circumstellar dust reasonably well in the wavelength range 5-25 m. The model flux ratio, R=Flux(18 )/Flux(10 ), compares well with the observed ratio for the circumstellar dust.Conclusions. The results on the composite grains clearly indicate that the silicate feature at 10 µm shifts with the volume fraction of graphite inclusions. The feature does not shift with the porosity. Both the features do not show any broadening with the inclusions or porosity. The absorption efficiencies of the composite grains calculated using DDA and Effective Medium Approximation (EMA)do not agree. The composite grain models presented in this study need to be compared with the observed IR emission from the circumstellar dust around a few more stars.
Numerical simulation of time delay interferometry for LISA with one arm dysfunctional
(2011-02-25) Dhurandhar, Sanjeev; Ni, W.-T.; Wang, G.
In order to attain the requisite sensitivity for LISA, laser frequency noise must be suppressed below the secondary noises such as the optical path noise, acceleration noise etc. In a previous paper (Dhurandhar et al., Class. Quantum Grav., 27, 135013, 2010), we have found a large family of second generation analytic solutions of time delay interferometry with one arm dysfunctional and also estimated the laser noise due to residual time-delay semi-analytically from orbit perturbations due to Earth. Since other planets and solar-system bodies also perturb the orbits of LISA spacecraft and affect the time delay interferometry (TDI), we simulate the time delay numerically in this paper for all solutions with n ≤ 3. To conform to the actual LISA planning, we have worked out a set of 3-year optimized mission orbits of LISA spacecraft starting at June 21, 2021 using CGC2.7 ephemeris framework. We then use this numerical solution to calculate the residual optical path differences in the second generation solutions of our previous paper, and compare with the semi-analytic error estimate. The accuracy of this calculation is better than 1 cm (or 30 ps). The maximum path length difference, for all configuration calculated, is below 1 m (3 ns). This is well below the limit under which the laser frequency noise is required to be suppressed.
Odd-parity bipolar spherical harmonics
(2011-09-13) Book, Laura G.; Kamionkowski, Marc; Souradeep, Tarun
Bipolar spherical harmonics (BiPoSHs) provide a general formalism for quantifying departures in the cosmic microwave background (CMB) from statistical isotropy (SI) and from Gaussianity. However, prior work has focused only on BiPoSHs with even parity. Here we show that there is another set of BiPoSHs with odd parity, and we explore their cosmological applications. We describe systematic artifacts in a CMB map that could be sought by measurement of these odd-parity BiPoSH modes. These BiPoSH modes may also be produced cosmologically through lensing by gravitational waves (GWs), among other sources. We derive expressions for the BiPoSH modes induced by the weak lensing of both scalar and tensor perturbations. Our estimate of the expected signal-to-noise with which we could measure the weak lensing of GWs from the correlation of two BiPoSH modes is respectable, indicating that such a measurement may be merited. We also investigate the possibility to detect parity-breaking physics, such as chiral GWs, by cross-correlating opposite parity BiPoSH modes with multipole moments of the CMB polarization. We nd that the expected signal-to-noise of such a detection is modest, with errors dominated by cosmic variance.
Oscillation in the inflaton potential: Exact numerical analysis and comparison with the recent and forthcoming CMB datasets
(2011-06-15) Aich, Moumita; Hazra, Dhiraj Kumar; Sriramkumar, L.; et al.
Amongst the multitude of inﬂationary models currently available, models that lead to features in the primordial scalar spectrum are drawing increasing attention, since certain features have been found to provide a better ﬁt to the CMB data than the conventional, nearly scale invariant, primordial spectrum. In this work, we carry out an exact numerical analysis of two models that lead to oscillations over all scales in the scalar power spectrum. We consider the model described by a quadratic potential which is superposed by a sinusoidal modulation and the recently popular axion monodromy model. Since the oscillations continue even onto smaller scales, in addition to the WMAP data, we also compare the models with the small scale data from ACT. Though, both the models, broadly, result in oscillations in the spectrum, interestingly, we ﬁnd that, while the monodromy model leads to a considerably better ﬁt to the data in comparison to the standard power law spectrum, the quadratic potential superposed with a sinusoidal modulation does not improve the ﬁt to a similar extent. We also carry out forecasting of the parameters using simulated Planck data for both the models. We show that the Planck mock data performs better in constraining the model parameters as compared to the presently available CMB datasets.
Perturbations in dark energy models with evolving speed of sound
(2011-04-26) Ansari, Rizwan Ul Haq; Unnikrishnan, Sanil
The behavior of perturbation in scalar ﬁeld dark energy and its consequent effect on the cold dark matter (CDM) power spectrum is well understood to be governed by the equation of state (EOS)parameter and the eﬀective speed of sound (ESS) of dark energy. In this paper, we investigate whether dark energy models whose ESS are epoch dependent leaves any distinct imprints on the large scale CDM power spectrum. In particular, we compare the cases where the ESS is decreasing with time with those where it increases. The CDM power spectrum is found to be generically suppressed in these cases as compared to the ΛCDM model. The degree of suppression at different length scales can, in principle, reﬂect the evolving nature of the ESS of dark energy. However, we ﬁnd that the eﬀect on the CDM power spectrum in cases where the ESS of dark energy is evolving with constant EOS parameter is signiﬁcantly smaller as compared to the situation where ESS is constant whereas EOS parameter is evolving. Further, it is also shown that the eﬀect of diﬀerent evolution of ESS for a given evolution of EOS parameter of dark energy on the CDM power spectrum is signiﬁcant only at the intermediate scales (around k ∼ 0.01h/Mpc). At scales much smaller and larger than the Hubble radius, it is the evolution of EOS parameter of dark energy which governs the degree of suppression of CDM power spectrum with respect to the ΛCDM model.
Soft time lags in the X-ray emission of mrk 1040
(2011-01-30) Tripathi, Shruti; Misra, Ranjeev; Dewangan, Gulab Chand; et al.
Temporal analysis of X-ray binaries and Active Galactic Nuclei have shown that hard X-rays react to variation of soft ones after a time delay. The opposite trend, or soft lag, has only been seen in a few rare Quasi-periodic Oscillations in X-ray binaries and recently for the AGN, 1H 0707-495, on short timescales of ∼ 103 secs. Here, we report analysis of a XMM-Newton observation of Mrk 1040, which reveals that on the dominant variability timescale of ∼ 104 secs, the source seems to exhibit soft lags. If the lags are frequency independent, they could be due to reverberation eﬀects of a relativistically blurred reﬂection component responding to a varying continuum. Alternatively, they could be due to Comptonization delays in the case when high energy photons impinge back on the soft photon source. Both models can be veriﬁed and their parameters tightly constrained, because they will need to predict the photon spectrum, the r.m.s variability and time lag as a function of energy. A successful application of either model will provide unprecedented information on the radiative process, geometry and more importantly the size of the system, which in turn may provide stringent test of strong general relativistic effects.
'Standard' Cosmological model & beyond with CMB
(2011-04-16) Souradeep, Tarun
Observational Cosmology has indeed made very rapid progress in the past decade. The ability to quantify the universe has largely improved due to observational constraints coming from structure formation Measurements 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 structure 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. The signature of 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 estimation of parameters of a working 'standard’ model of cosmology for subtle, characteristic signatures from early universe physics.