2006 (IPP)

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Analysis of trapped surfaces in higher dimensional dust collapse
(2006-04-12) Patil, K. D.
In the present work we analyze the dynamics of apparent horizon in higher dimensional (HD) dust collapse. For this study we have taken in to consideration the two different types of dust models. We propose the concept of ‘trapped range’ of initial data in the different higher dimensional spacetimes. We show that ‘trapped range’ of initial data increases with the increase in dimensions of the spacetimes.
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.
Astrophysical magnetic fields and nonlinear dynamo theory
(2006-01-10) Brandenburg, Axel; Subramanian, Kandaswamy
The current understanding of astrophysical magnetic ﬁelds is reviewed, focusing on their generation and maintenance by turbulence. In the astrophysical context this generation is usually explained by a self-excited dynamo, which involves ﬂows that can amplify a weak ‘seed’ magnetic ﬁeld exponentially fast. Particular emphasis is placed on the nonlinear saturation of the dynamo. Analytic and numerical results are discussed both for small scale dynamos, which are completely isotropic, and for large scale dynamos, where some form of parity breaking is crucial. Central to the discussion of large scale dynamos is the so-called alpha effect which explains the generation of a mean ﬁeld if the turbulence lacks mirror symmetry, i.e. if the ﬂow has kinetic helicity. Large scale dynamos produce small scale helical ﬁelds as a waste product that quench the large scale dynamo and hence the alpha effect. With this in mind, the microscopic theory of the alpha effect is revisited in full detail and recent results for the loss of helical magnetic ﬁelds are reviewed.
Avoidance of future singularities in loop quantum cosmology
(2006-05-21) Sami, M.; Singh, Parampreet; Tsujikawa, Shinji
We consider the fate of future singularities in the eﬀective dynamics of loop quantum cosmology. Non-perturbative quantum geometric eﬀects which lead to ρ2 modiﬁcation of the Friedmann equation at high energies result in generic resolution of singularities whenever energy density ρ diverges at future singularities of Friedmann dynamics. Such quantum eﬀects lead to the avoidance of a Big Rip, which is followed by a recollapsing universe stable against perturbations. Resolution of sudden singularity, the case when pressure diverges but energy density approaches a ﬁnite value depends on the ratio of the latter to a critical energy density of the order of Planck. If the value of this ratio is greater than unity, the universe escapes the sudden future singularity and becomes oscillatory.
Bulk motion of ultrarelativistic conical blazar jets
(2006-03-27) Krishna, Gopal; Wiita, Paul J.; Dhurde, Samir
Allowing for the conical shape of ultrarelativistic blazar jets with opening angles of a few degrees on parsec-scales we show that their bulk Lorentz factors and viewing angles can be much larger than the values usually inferred by combining their ﬂux variability and proper motion measurements. This is in accord with our earlier ﬁnding that such ultrarelativistic (Lorentz factor, Γ > 30) conical jets can reconcile the relatively slow apparent motions of VLBI knots in TeV blazars with the extremely fast ﬂows implied by their rapid γ-ray variability. This jet geometry also implies that deprojected jet opening angles will typically be signiﬁcantly underestimated from VLBI measurements. In addition, de-projected jet lengths will be considerably overestimated if high Lorentz factors and signiﬁcant opening angles are not taken into account.
Casimir effect confronts cosmological constant
(2006-04-02) Mahajan, Gaurang; Sarkar, Sudipta; Padmanabhan, T.
It has been speculated that the zero-point energy of the vacuum, regularized due to the existence of a suitable ultraviolet cut-oﬀ scale, could be the source of the non-vanishing cosmological constant that is driving the present acceleration of the universe. We show that the presence of such a cut-off can signiﬁcantly alter the results for the Casimir force between parallel conducting plates and even lead to repulsive Casimir force when the plate separation is smaller than the cut-off scale length. Using the current experimental data we rule out the possibility that the observed osmological constant arises from the zero-point energy which is made ﬁnite by a suitable cut-off. Any such cut-oﬀ which is consistent with the observed Casimir effect will lead to an energy density which is about 1012 times larger than the observed one, if gravity couples to these modes. The implications are discussed.
Computational study for neutral and cationic pericondensed polycyclic aromatic hydrocarbons
(2006-02-10) Pathak, Amit; Rastogi, Shantanu
Quantum chemical calculations using density functional theory are presented for small to medium sized pericondensed PAHs includ- ing some being reported for the first time. Bond lengths and charge distribution have been computed for these PAHs in both neutral and cationic forms. Upon ionization, significant change in fractional charge on atoms is present particularly for the outer carbon atoms. The charge on the internal carbon atoms tends towards zero in cations. Vibrational frequencies and infrared intensities have been calculated for the optimized structures of PAH neutrals and cations. The drastic intensity alterations occurring upon ionization are discussed and related to specific changes occurring in the charge distribution. The C-H stretch intensity depends on the partial charge on peripheral hydrogen atoms and reduces in cations as hydrogen atoms become more positive. Pericondensed PAHs show better matching with the observed interstellar infrared bands. The co-added model spectra show profiles similar to the observed astrophysical bands.
Confronting braneworld cosmology with supernova data and baryon oscillations
(2006-03-18) Alam, Ujjaini; Sahni, Varun
Braneworld cosmology has several attractive and distinctive features. For instance the eﬀective equation of state in braneworld models can be both quintessence-like (w0 ≥ −1) as well as phantom like (w0 ≤ −1). Models with w0 ≥ −1 (w0 ≤ −1) are referred to as Brane 2 (Brane 1) and correspond to complementary embeddings of the brane in the bulk. (The equation of state in Brane 1 can successfully cross the ‘phantom divide’ at w = −1.) In this paper we compare the predictions of braneworld models to two recently released supernova data sets: the ‘Gold’ data (Riess et al., 2004) and the data from the Supernova Legacy Survey (SNLS) (Astier et al., 2005). We also incorporate the recent discovery of the baryon acoustic peak in the Sloan Digital Sky Survey (Eisenstein et al., 2005) into our analysis. Our main results are that braneworld models satisfy both sets of SNe data. Brane 1 (with w0 ≤ −1) shows very good agreement with data for values of the matter density bounded from below: Ω0m > ∼ 0.25 (Gold) and Ω0m > ∼ 0.2 (SNLS). On the other hand Brane 2 (with w0 ≥ −1) shows excellent agreement with data for values of the matter density which are bounded from above: Ω0m < ∼ 0.45 (Gold) and Ω0m < ∼ 0.35 (SNLS). The DGP model is excluded at 3σ by SNLS and at 1σ by the Gold dataset. Braneworld models with future ‘quiescent’ singularities (at which the Hubble parameter and the matter density remain ﬁnite but higher derivatives of the expansion factor diverge) are excluded by both datasets.
Cosmic mimicry: Is LCDM a braneworld in disguise?
(2006-01-01) Sahni, Varun; Shtanov, Yuri; Viznyuk, Alexander
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 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.
Critical properties and stability of stationary solutions in multi-transonic pseudo-schwarzschild accretion
(2006-09-13) Chaudhury, Soumini; Ray, Arnab K.; Das, Tapas K.
For inviscid, rotational accretion ows, both isothermal and polytropic, a simple dynamical systems analysis of the critical points has given a very accurate mathematical scheme to understand the nature of these points, for any pseudo-potential by which the ow may be driven on to a Schwarzschild black hole. This allows for a complete classi cation of the critical points for a wide range of ow parameters, and shows that the only possible critical points for this kind of ow are saddle points and centre-type points. A restrictive upper bound on the angular momentum of critical solutions has been established. A time-dependent perturbative study reveals that the form of the perturbation equation, for both isothermal and polytropic ows, is invariant under the choice of any particular pseudo-potential. Under generically true outer boundary conditions, the inviscid ow has been shown to be stable under an adiabatic and radially propagating perturbation. The perturbation equation has also served the dual purpose of enabling an understanding of the acoustic geometry for inviscid and rotationalows.
Dark energy
(2006-01-10) Sahni, Varun
The cosmological constant problem as well as the case for dark energy are briefly reviewed and some theoretical models of dark energy are discussed in detail. These include: the cosmological constant, quintessence, the Chaplygin gas and Braneworld models. I also discuss model independent measures of dark energy and conclude by mentioning some properties of the Statefinder diagnostic which can successfully differentiate between different families of dark energy models.
Dark energy cosmology from higher-order, string-inspired gravity, and its reconstruction
(2006-04-10) Nojiri, Shin'ichi; Odintsov, Sergi D.; Sami, M.
In this paper we investigate the cosmological effects of modiﬁed gravity with string curvature corrections added to the Einstein-Hilbert action in the presence of a dynamically evolving scalar ﬁeld coupled to Riemann invariants. The scenario exhibits several features of cosmological interest for the late universe. We show that higher-order stringy corrections can lead to a class of dark energy models consistent with recent observations. The models can give rise to quintessence without recourse to a scalar ﬁeld potential. The detailed treatment of the reconstruction program for general scalar-Gauss-Bonnet gravity is presented for any given cosmology. The explicit examples of reconstructed scalar potentials are given for an accelerated (quintessence, cosmological constant, or phantom) universe. Finally, the relation with modiﬁed F G gravity is established at the classical level and is extended to include third order terms on the curvature.
Detecting cold gas at intermediate redshifts: GMRT survey using MgII systems
(2006-11-28) Gupta, Neeraj; Srianand, R.; Petitjean, Patrick; et al.
Intervening H i 21-cm absorption systems at z ≥ 1.0 are very rare and only 4 conﬁrmed detections have been reported in the literature. Despite their scarcity, they provide interesting and unique insights into the physical conditions in the interstellar medium of high-z galaxies. Moreover, they can provide independent constraints on the variation of fundamental constants. We report 3 new detections based on our ongoing Giant Metrewave Radio Telescope (GMRT) survey for 21-cm absorbers at 1.10 ≤ zabs ≤ 1.45 from candidate damped Lyman-α systems. The 21-cm lines are narrow for the zabs= 1.3710 system towards SDSS J0108−0037 and zabs= 1.1726 system toward SDSS J2358−1020. Based on line full-width at half maximum, the kinetic temperatures are ≤ 5200 K and ≤ 800 K, respectively. The 21-cm absorption proﬁle of the third system, zabs=1.1908 system towards SDSS J0804+3012, is shallow, broad and complex, extending up to 100 km s−ᶥ . The centroids of the 21-cm lines are found to be shifted with respect to the corresponding centroids of the metal lines derived from SDSS spectra. This may mean that the 21-cm absorption is not associated with the strongest metal line component.
Detecting gravitational waves from inspiraling binaries with a network of detectors: Coherent versus coincident strategies
(2006-08-22) Mukhopadhyay, Himan; Sago, Norichica; Tagoshi, H.; et al.
We compare two strategies of multi-detector detection of compact binary inspiral signals, namely, the coincidence and the coherent. For simplicity we consider here two identical detectors having the same power spectral density of noise, that of initial LIGO, located in the same place and having the same orientation. We consider the cases of independent noise as well as that of correlated noise. The coincident strategy involves separately making two candidate event lists, one for each detector, and from these choosing those pairs of events from the two lists which lie within a suitable parameter window, which then are called as coincidence detections. The coherent strategy on the other hand involves combining the data phase coherently, so as to obtain a single network statistic which is then compared with a single threshold. Here we attempt to shed light on the question as to which strategy is better. We compare the performances of the two methods by plotting the receiver operating characteristics (ROC) for the two strategies. Several of the results are obtained analytically in order to gain insight. Further we perform numerical simulations in order to determine certain parameters in the analytic formulae and thus obtain the ﬁnal complete results. We consider here several cases from the relatively simple to the astrophysically more relevant in order to establish our results. The bottom line is that the coherent strategy although more computationally expensive in general than the coincidence strategy, is superior to the coincidence strategy - onsiderably less false dismissal probability for the same false alarm probability in the viable false alarm regime.
Dynamics of dark energy
(2006-06-16) Copeland, E. J.; Sami, M.; Tsujikawa, Shinji
In this paper we review in detail a number of approaches that have been adopted to try and explain the remarkable observation of our accelerating Universe. In particular we discuss the arguments for and recent progress made towards understanding the nature of dark energy. We review the observational evidence for the current accelerated expansion of the universe and present a number of dark energy models in addition to the conventional cosmological constant, paying particular attention to scalar ﬁeld models such as quintessence, K-essence, tachyon, phantom and dilatonic models. The importance of cosmological scaling solutions is emphasized when studying the dynamical system of scalar ﬁelds including coupled dark energy. We study the evolution of cosmological perturbations allowing us to confront them with the observation of the Cosmic Microwave Background and Large Scale Structure and demonstrate how it is possible in principle to reconstruct the equation of state of dark energy by also using Supernovae Ia observational data. We also discuss in detail the nature of tracking solutions in cosmology, particle physics and braneworld models of dark energy, the nature of possible future singularities, the eﬀect of higher order curvature terms to avoid a Big Rip singularity, and approaches to modifying gravity which leads to a late-time accelerated expansion without recourse to a new form of dark energy.
Electrical spinning particle in Einstein's unifield theory
(2006-08-10) Pandey, S. N.; Sinha, B. K.; Kumar, Raj
Previous work on exact solutions has been shown that sources need to be appended to the field equations of Einstein’s unified field theory in order to achieve physically meaningful results, such sources can be included in a variational formulation by Borchsenius and Moffat. The resulting field equations and conservation identities related to the theory that can be used to derive the equations of structure and motion of a pole-dipole particle according to an explicitly covariant approach by Dixon6 . In this present paper it is shown that, under certain conditions for the energy tensor of the spinning particle, the equations of structure and motion in an electromagnetic field turn out to be formally identical to those occurring in Einstein-Maxwell theory.
Evolving turbulence and magnetic fields in galaxy clusters
(2006-01-10) Subramanian, Kandaswamy; Shukurov, A.; Haugen, N. E. L.
We discuss, using simple analytical models and MHD simulations, the origin and parameters of turbulence and magnetic ﬁelds in galaxy clusters. Any pre-existing tangled magnetic ﬁeld must decay in a few hundred million years by generating gas motions even if the electric conductivity of the intracluster gas is high. We argue that tur- bulent motions can be maintained in the intracluster gas and its dynamo action can prevent such a decay and amplify a random seed magnetic ﬁeld by a net factor typically 10⁴ in 5Gyr. Three physically distinct regimes can be identiﬁed in the evolution of turbulence and magnetic ﬁeld in galaxy clusters. Firstly, the ﬂuctuation dynamo will produce microgauss-strong, random magnetic ﬁelds during the epoch of cluster formation and major mergers. At this stage pervasive turbulent ﬂows with r.m.s. velocity of about 300 kms−ᶥ can be maintained at scales 100–200 kpc. The magnetic ﬁeld is intermittent, has a smaller scale of 20–30 kpc and average strength of 2 G. Secondly, turbulence will decay after the end of the major merger epoch; we discuss the dynamics of the decaying turbulence and the behavior of magnetic ﬁeld in it. Magnetic ﬁeld and turbulent speed undergo a power-law decay, decreasing by a factor of two during this stage, whereas their scales increase by about the same factor. Thirdly, smaller-mass subclusters and cluster galaxies will produce turbulent wakes where magnetic ﬁelds will be generated as well. Although the wakes plausibly occupy only a small fraction of the cluster volume, we show that their area covering factor can be close to unity, and thus they can produce some of the signatures of turbulence along virtually all lines of sight. The latter could potentially allow one to reconcile the possibility of turbulence with ordered ﬁlamentary gas structures, as in the Perseus cluster. The turbulent speeds and magnetic ﬁelds in the wakes are estimated to be of order 300 kms−ᶥ and 2 G, respectively, whereas the turbulent scales are of order 200 kpc for wakes behind subclusters of a mass 3 × 10ᶥᶟM⊙ and about 10 kpc in the galactic wakes. Magnetic ﬁeld in the wakes is intermittent and has the scale of about 30 kpc and 1 kpc in the subcluster and galactic wakes, respectively. Random Faraday rotation measure is estimated to be typically 100–200 radm−², in agreement with observations. We predict detectable polarization of synchrotron emission from cluster radio halos at wavelengths 3–6 cm, if observed at suﬃciently high resolution.
Exact non-spherical radiating collapse
(2006-07-30) Ghosh, S. G.; Deshkar, D. W.
We study the junction conditions for non-spherical collapsing radiating star consisting of a shearing fluid undergoing radial heat flow with outgoing radiation. Radiation of the system is described by plane symmetric version of Vaidya solution. Junction conditions which match the collapse solutions to an exterior Vaidya metric show that, at the boundary, the pressure is proportional to the magnitude of the heat flow vector. Physical quantities, analogous to spherical symmetry related to the local conservation of momentum and surface red-shift, are also obtained. Finally, exact gravitational collapse solutions, for both shear and shear-free case, have been obtained by integrating a field equation.