2003 (IPP)

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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.
A faster implementation of the hierarchical search algorithm for detection of gravitational waves from inspiraling compact binaries
(2011-06-05) Sengupta, Anand; Dhurandhar, Sanjeev; Lazzarini, Albert
The ﬁrst scientiﬁc runs of kilometer scale laser interferometric detectors like LIGO are underway. Data from these detectors will be used to look for signatures of gravitational waves (GW) from astrophysical objects like inspiraling neutron star/blackhole binaries using matched ﬁltering. The computational resources required for online ﬂat-search implementation of the matched ﬁltering are large if searches are carried out for small total mass. Flat search is implemented by constructing a single discrete grid of densely populated template waveforms spanning the dynamical parameters - masses, spins - which are correlated with the interferometer data. The correlations over the kinematical parameters can be maximized apriori without constructing a template bank over them. Mohanty and Dhurandhar (1996) showed that a signiﬁcant reduction in computational resources can be accomplished by using a hierarchy of such template banks where candidate events triggered by a sparsely populated grid is followed up by the regular, dense ﬂat search grid. The estimated speed up in this method was a factor ∼ 25 over the ﬂat search. In this paper we report an improved implementation of the hierarchical search, wherein we extend the domain of hierarchy to an extra dimension - namely the time of arrival of the signal in the bandwidth of the interferometer. This is accomplished by lowering the Nyquist sampling rate of the signal in the trigger stage. We show that this leads to further improvement in the eﬃciency of data analysis and speeds up the online computation by a factor of ∼ 65 − 70 over the ﬂat search. We also take into account and discuss issues related to template placement, trigger thresholds and other peculiar problems that do not arise in earlier implementation schemes of the hierarchical search. We present simulation results for 2PN waveforms embedded in the noise expected for initial LIGO detectors.
Effects of curvature and interactions on the dynamics of the deconfinement phase transition
(2011-07-05) Chandra, Deepak; Goyal, Ashok
We study the dynamics of ﬁrst-order coﬁnement-deconﬁnement phase transition through nucleation of hadronic bubbles in an expanding quark gluon plasma in the context of heavy ion collisions for interacting quark and hadron gas and by incorporating the eﬀects of curvature energy. We ﬁnd that the interactions reduce the delay in the phase transition whereas the curvature energy has a mixed behavior. In contrast to the case of early Universe phase transition, here lower values of surface tension increase the supercooling and slow down the hadronization process. Higher values of bag pres- sure tend to speed up the transition. Another interesting feature is the start of the hadronization process as soon as the QGP is created.
On tripolar magnetic reconnection and coronal heating
(2011-07-05) Pandey, Kumud; Narain, Udit; Lohani, N. K.
Using recent data for the photosphere-chromosphere region of the solar atmosphere the magnetic reconnection in tripolar geometry has been investi- gated through the procedure of Sturrock (1999). Particular attention has been given to the width of the reconnecting region, wave number of the rapidly growing tearing mode, island length scales, frequency of MHD ﬂuctuations, tearing mode growth rate, energy dissipation rate and minimum magnetic ﬁeld strength required to heat chromospheric plasma to coronal temperatures. It is found that small length scales are formed in the upper chromosphere. The maximum growth rate of tearing mode instability coincides with the peak in the energy dissipation rate both of which occur in the upper chromo- sphere at the same height. It is realized that the distribution of magnetic ﬁeld with height is essential for a better understanding of the coronal heating problem.
Can dark energy be decaying?
(2011-07-05) Ujjaini, Alam; Sahni, Varun; Starobinsky, A. A.
We explore the fate of the universe given the possibility that the density associated with ‘dark energy’ may decay slowly with time. Decaying dark energy is modeled by a homogeneous scalar ﬁeld which couples minimally to gravity and whose potential has at least one local quadratic maximum. Dark energy decays as the scalar ﬁeld rolls down its potential, consequently the current acceleration epoch is a transient. We examine two models of decaying dark energy. In the ﬁrst, the dark energy potential is modeled by an analytical form which is generic close to the potential maximum. The second potential is the cosine, which can become negative as the ﬁeld evolves, ensuring that a spatially ﬂat universe collapses in the future. We examine the feasibility of both models using observations of high redshift type Ia supernovae. A maximum likelihood analysis is used to ﬁnd allowed regions in the {m, φ0} plane (m is the tachyon mass modulus and φ0 the initial scalar ﬁeld value; m ∼ H0 and φ0 ∼ MP by order of magnitude). For the ﬁrst model, the time for the potential to drop to half its maximum value is larger than ∼ 8 Gyrs. In the case of the cosine potential, the time left until the universe collapses is always greater than ∼ 18 Gyrs (both estimates are presented for Ω0m = 0.3, m/H0 ∼ 1, H0 ≃ 70 km/sec/Mpc, and at the 95.4% conﬁdence level).
Relativistic anisotropic charged fluid spheres with varying cosmological constant
(2011-07-05) Ray, Saibal; Bhadra, Sumana; Sengupta, Anand
Static spherically symmetric anisotropic source has been studied for the Einstein- Maxwell ﬁeld equations assuming the erstwhile cosmological constant Λ to be a space-variable scalar, viz., Λ = Λ(r). Two cases are examined out of which one reduces to isotropic sphere. The solutions thus obtained are shown to be electromagnetic in origin as a particular case. It is also shown that the generally used pure charge condition, viz., ρ+pr = 0 is not always required for constructing electromagnetic mass models.
Variational monte carlo calculations of 5/\He hypernucleus
(2011-07-05) Usmani, A . A.; Murtaza, S.
We perform a realistic study of L 5 He hypernucleus using variational Monte Carlo technique. The Hamil- tonian for 4 He nuclear core of the hypernucleus is written using Argonne v18 NN potential and Urbana model-IX NNN potential, where N stands for nucleon. For the strange sector, we use phenomenological LN potential having central, spin, and exchange components and LNN potential which includes spin-dependent dispersive force and two-pion exchange force. Using this Hamiltonian and a fully correlated variational wave function, we reproduce the experimental L binding energy. Without three-body LNN potential in the Hamil- tonian and its corresponding correlation in the wave function, the hypernucleus is overbound by 0.56~4! MeV, which is about a quarter of the previous reported values of 2–3 MeV due to the use of central forces. We present the detailed energy breakdown of the hypernucleus and also show the effect of LNN correlation on it. The one-body density proﬁles for nucleon and L in the hypernucleus and in its nuclear core have been critically examined. The nuclear core polarization due to presence of L is precisely determined.
Quasi normal modes in Schawarzschild-DeSitter spacetime: A simple derivation of the level spacing of the frequencies
(2011-07-05) Choudhury, T. Roy; Padmanabhan, T.
It is known that the imaginary parts of the quasi normal mode (QNM) frequencies for the Schwarzschild black hole are evenly spaced with a spacing that depends only on the surface gravity. On the other hand, for massless minimally coupled scalar ﬁelds, there exist no QNMs in the pure DeSitter spacetime. It is not clear what the structure of the QNMs would be for the Schwarzschild- DeSitter (SDS) spacetime, which is characterized by two diﬀerent surface gravities. We provide a simple derivation of the imaginary parts of the QNM frequencies for the SDS spacetime by calcu- lating the scattering amplitude in the ﬁrst Born approximation and determining its poles. We ﬁnd that, for the usual set of boundary conditions in which the incident wave is scattered oﬀ the black hole horizon, the imaginary parts of the QNM frequencies have a equally spaced structure with the level spacing depending on the surface gravity of the black hole. Several conceptual issues related to the QNM are discussed in the light of this result and comparison with previous work is presented.
Morphology of the supercluster-void network in /\ CDM cosmology
(2011-07-05) Shandarin, S.F.; Sheth, Jatush V.; Sahni, Varun
We report here the ﬁrst systematic study of the supercluster-void network in the ΛCDM concordance cosmology in which voids and superclusters are treated on an equal footing. Superclusters are deﬁned as individual members of an over-dense excur- sion set and voids are deﬁned as individual members of a complementary under-dense excursion set at the same density threshold. We determine the geometric, topological and morphological properties of the cosmic web at a large set of density levels by computing Minkowski functionals for every supercluster and void using SURFGEN (Sheth et al. 2003). The properties of the largest (percolating) supercluster and the complementary void are found to be very diﬀerent from properties of individual su- perclusters and voids. Individual superclusters totally occupy no more than about 5% of the total volume and contain no more than 20% of mass if the largest supercluster is excluded. Likewise, individual voids totally occupy no more than 14% of volume and contain no more than 4% of mass if the largest void is excluded. Although super- clusters are more massive and voids are more voluminous the diﬀerence in maximum volumes is not greater than by an order of magnitude. The genus value of individual superclusters can be ∼ 5 while the genus of individual voids can reach ∼ 40, implying signiﬁcant amount of substructure in superclusters and especially in voids. One of our main results is that large voids, as deﬁned through the density ﬁeld (read dark matter distribution) can be distinctly non-spherical.
Origin and interpretation of kilohertz QPOs from strange stars in X-ray Binary system: Theoretical hydrodynamical description
(2011-07-05) Mukhopadhyay, B.; Ray, Subharthi; Dey, Jishnu; et al.
We model and interpret the Kilohertz QPOs from the hydrodynamical description of accretion disk around a rapidly rotating compact strange star. The higher QPO frequency is described by the viscous eﬀects of accretion disk leading to shocks, while the lower one is taken to be the Keplerian motion of the accreting matter. Comparing our results with the observations for two of the fastest rotating compact stellar candidates namely, 4U 1636−53 and KS 1731−260, we ﬁnd that they match to a very good approximation, thus interpreting them as strange stars.
Matching of the continuous gravitational wave in an all sky search
(2011-07-05) Sahay, S. K.
We investigate the matching of continuous gravitational wave (CGW) signals in an all sky search with reference to Earth based laser interferometric detectors. We consider the source location as the parameters of the signal manifold and templates corresponding to different source locations. It has been found that the matching of signals from locations in the sky that differ in their co-latitude and longitude by π radians decreases with source frequency. We have also made an analysis with the other parameters affecting the symmetries. We observe that it may not be relevant to ake care of the symmetries in the sky locations for the search of CGWfrom the output of LIGO-I, GEO600 and TAMA detectors.
Traversable wormholes with arbitrarily small energy condition violations
(2011-07-05) Matt, Visser; Kar, Sayan; Dadhich, Naresh
Traversable wormholes necessarily require violations of the averaged null energy condition; this being the deﬁnition of “exotic matter”. However, the theorems which guarantee the energy condi- tion violation are remarkably silent when it comes to making quantitative statements regarding the “total amount” of energy condition violating matter in the spacetime. We develop a suitable mea- sure for quantifying this notion, and demonstrate the existence of spacetime geometries containing traversable wormholes that are supported by arbitrarily small quantities of “exotic matter”.
Is the present expansion of the universe really accelerating?
(2011-07-05) Vishwakarma, R. G.
The current observations are usually explained by an accelerating ex- pansion of the present universe. However, with the present quality of the supernovae Ia data, the allowed parameter space is wide enough to accommodate the decelerating models as well. This is shown by considering a particular example of the dark energy equation-of-state wφ ≡ pφ/ρφ = −1/3, which is equivalent to modifying the geometrical curvature index k of the standard cosmology by shifting it to (k − α) where α is a constant. The resulting decelerating model is consistent with the recent CMB observations made by WMAP, as well as, with the high redshift supernovae Ia data including SN 1997ﬀ at z = 1.755. It is also consistent with the newly discovered supernovae SN 2002dc at z = 0.475 and SN 2002dd at z = 0.95 which have a general tendency to improve the ﬁt.
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.
Can brane cosmology with a vanishing /\ explain the observations?
(2011-07-05) Vishwakarma, R. G.; Singh, Parampreet
A plethora of models of the universe have been proposed in recent years claiming that the present universe is accelerating, being driven by some hypothetical source with negative pressure collectively known as dark energy which though do not appear to resemble any known form of matter tested in the laboratory. These models are motivated by the high redshift supernovae Ia observations. Though low density models, without dark energy, also appear to ﬁt the SN Ia data reasonably well, however, they are ruled out by the CMB observations. In this paper, we present a warped brane model with an additional surface term of brane curvature scalar in the action. This results in shifting the dynamical curvature of the model from its geometrical counterpart, which creates profound consequences. Even for Λ = 0, the low energy decelerating model successfully explains the observed locations of the peaks in the angular power spectrum of CMB. This model also ﬁts the high redshift supernovae Ia observations, taken together with the recently observed SN 1997ﬀ at z ≈ 1.7, very well. Additionally, it also ﬁts the data on the angular size and redshift of the compact radio sources very well.
VLT-UVEs survey for molecular hydrogen in high-redshift damped lyman-alfa system
(2011-07-05) Ledoux, C.; Petitjean, Patrick; Srianand, R.
We have searched for molecular hydrogen in damped Lyman-α (DLA) and sub- DLA systems at high redshift (zabs > 1.8) using UVES at the VLT down to a detection limit of typically N(H2) = 2×1014 cm−2. Out of the 33 systems in our sample, 8 have ﬁrm and 2 have tentative detections of associated H2 absorption lines. Considering that 3 detections were already known from past searches, molecular hydrogen is detected in 13 to 20 percent of the newly-surveyed systems.We report new detections of molecular hydrogen at zabs = 2.087 and 2.595 toward, respectively, Q1444+014 and Q0405−443, and also reanalyse the system at zabs = 3.025 toward Q0347−383. In all of the systems, we measure metallicities relative to Solar, [X/H] (with either X=Zn, or S, or Si), and depletion factors of Fe, [X/Fe], supposedly onto dust grains, and compare the characteristics of our sample with those of the global population of DLA systems (60 systems in total). We ﬁnd that there is a correlation between metallicity and depletion factor in both our sample and also the global population of DLA systems. Although H2 molecules are detected in systems with [Zn/Fe] as small as 0.3, the DLA and sub-DLA systems where H2 is detected are usually amongst those having the highest metallicities and the largest depletion factors. In particular, H2 is detected in the ﬁve systems having the largest depletion factors. Moreover, the individual components where H2 is detected have depletion factors systematically larger than other components in the proﬁles. In two diﬀerent systems, one of the H2- detected components even has [Zn/Fe] > 1.4. These are the largest depletion factors ever seen in DLA systems. All this clearly demonstrates the presence of dust in a large fraction of the DLA systems. The mean H2 molecular fraction, f = 2N(H2)/[2N(H2) + N(Hi)], is generally small in DLA systems (typically log f < −1) and similar to what is observed in the Magellanic Clouds. There is no correlation between the observed amount of H2 and the Hi column density. In fact, two systems where H2 is detected have logN(Hi) < 20.3 and, therefore, are sub-DLA systems. From 58 to 75 percent of the DLA systems have log f < −6. This can be explained if the formation rate of H2 onto dust grains is reduced in those systems, probably because the gas is warm (T > 1000 K) and/or the ionizing ﬂux is enhanced relative to what is observed in our Galaxy.
Tolman - Bayin type static charged fluid spheres in general relativity
(2011-07-05) Ray, Saibal; Das, Basanti
In a static spherically symmetric Einstein-Maxwell spacetime the class of astrophysical solu- tion found out by Ray and Das (2002) and Pant and Sah (1979) are revisited here in connection to the phenomenological relationship between the gravitational and electromagnetic ﬁelds. It is qualitatively shown that the charged relativistic stars of Tolman (1939) and Bayin (1978) type are of purely electromagnetic origin. The existence of this type of astrophysical solutions is a probable extension of Lorentz’s conjecture that electron-like extended charged particle possesses only ‘electromagnetic mass’ and no ‘material mass’.
Energy density in general relativity : a possible role of cosmological constant
(2011-07-05) Ray, Saibal; Bhadra, Sumana
We consider a static spherically symmetric charged anisotropic ﬂuid source of radius ∼ 10−16 cm by introducing a variable Λ dependent on the radial coordinate r under general relativity. From the solution sets a possible role of the cosmological constant is investigated which indicates the dependency of energy density on it.
Hyperdiffusion in non-linear, large and small-scale turbulent dynamos
(2011-07-05) Subramanian, Kandaswamy
The generation of large-scale magnetic ﬁelds is generically accompanied by the more rapid growth of small-scale ﬁelds. The growing Lorentz force due to these ﬁelds back reacts on the turbulence to saturate the mean-ﬁeld and small-scale dynamos. For the mean-ﬁeld dynamo, in a quasi-linear treatment of this saturation, it is generally thought that, while the alpha-eﬀect gets renormalised and suppressed by non-linear eﬀects, the turbulent diﬀusion is left unchanged. We show here that this is not true and the eﬀect of the Lorentz forces, is also to generate additional non-linear hyperdiﬀusion of the mean ﬁeld. A combination of such non-linear hyperdiﬀusion with diﬀusion at small scales, also arises in a similar treatment of small-scale dynamos, and is crucial to understand its saturation.
Theoretician's analysis of the supernova data and the limitations in determining the nature of dark energy
(2011-07-05) Padmanabhan, T.; Choudhury, T. Roy
Current cosmological observations show a strong signature of the existence of a dark en-ergy component with negative pressure. The most obvious candidate for this dark energy is the cosmological constant (with the equation of state wX = p/ρ = −1), which, however,raises several theoretical ifﬁculties. This has led to models for dark energy componentwhich evolves with time. We discuss certain questions related to the determination of the nature of dark energy component from observations of high redshift supernova. The main results of our analysis are: (i) Even if the precise value of wX is known from observations, it is not possible to determine the nature of the unknown dark energy source using only kinematical and geometrical measurements.We have given explicit examples to show that different types of sources can give rise to a given wX. (ii) Although the full data set of supernova observa- tions (which are currently available) strongly rule out models without dark energy, the high (z > 0.25) and low (z < 0.25) redshift data sets, individually, admit decelerating models with zero dark energy. Any possible evolution in the absolute magnitude of the supernovae, if detected, might allow the decelerating models to be consistent with the data. (iii) We have in- troduced two parameters, which can be obtained entirely from theory, to study the sensitivity of the luminosity distance on wX. Using these two parameters, we have argued that although one can determine the present value of wX accurately from the data, one cannot constrain the evolution of wX.