2005 (IPP)

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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.
Solutions of Certain Types of Linear and Nonlinear Diffusion-Reaction Equations in One Dimension
(2005-01-01) Kaushal, R. S.
With a view to having further insight into the mathematical content of the non-Hermitian Hamiltonian associaterd with the diﬀusion-reaction (D-R) equation in one dimension, we investigate (a) the solitary wave solutions of certain types of its nonlinear versions, and (b) the problem of real eigenvalue spectrum associated with its linear version or with this class of non-Hermitian Hamiltonians. For the case (a) we use the standard techniques to handle the quadratic and cubic nonlinearities in the D-R equation whereas for the case (b) a newly proposed method, based on an extended complex phase space, is employed. For a particular class of solutions, an Ermakov system of equations is also found for the linear case. Further, corresponding to the ’classical’ version of the above one-dimensional complex Hamiltonian, an equivalent integrable system of two, two-dimensional real Hamiltonians is suggested.
Holographic Gravity and the Surface term in the Einstein-Hilbert Action
(2005-01-01) Padmanabhan, T.
Certain peculiar features of Einstein-Hilbert (EH) action provide clues towards a holographic approach to gravity which is independent of the detailed microstructure of spacetime. These features of the EH action include: (a) the existence of second derivatives of dynamical variables; (b) a non trivial relation between the surface term and the bulk term; (c) the fact that surface term is non analytic in the coupling constant, when gravity is treated as a spin-2 perturbation around flat spacetime and (d) the form of the variation of the surface term under infinitesimal coordinate transformations. The surface term can be derived directly from very general considerations and using (d) one can obtain Einstein's equations just from the surface term of the action. Further one can relate the bulk term to the surface term and derive the full EH action based on purely thermodynamic considerations. The features (a), (b) and (c) above emerge in a natural fashion in this approach. It is shown that action Agrav splits into two terms -8+(3E in a natural manner in any stationary spacetime with horizon, where E is essentially an integral over ADM energy density and 8 arises from the integral of the surface gravity over the horizon. This analysis shows that the true degrees of freedom of gravity reside in the surface term of the action, making gravity intrinsically holographic. It also provides a close connection between gravity and gauge theories, a
Estimating Cosmological Parameters from CMBR : Systematic effects
(2005-01-01) Sinha, Rita; Souradeep, Tarun
The fluctuations in the cosmic microwave background radiation depend on the cosmological parameters. We carry out Markov Chain Monte Carlo simulations to constrain the cosmological parameter space with the current CMB data for possible systematic effects! such as beam noncircularity in CMB measurements and incomplete sky coverage.
Primordial Magnetic Fields in the Post-recombination Era and Early Reionization
(2005-01-01) Sethi, Shiv K.; Subramanian, Kandaswamy
We explore the ways in which primordial magnetic ﬁelds inﬂuence the thermal and ion- ization history of the post-recombination universe. After recombination the universe becomes mostly neutral resulting also in a sharp drop in the radiative viscosity. Pri- mordial magnetic ﬁelds can then dissipate their energy into the intergalactic medium (IGM) via ambipolar diﬀusion and, for small enough scales, by generating decaying MHD turbulence. These processes can signiﬁcantly modify the thermal and ionization history of the post-recombination universe. We show that the dissipation eﬀects of magnetic ﬁelds which redshifts to a present value B0 = 3 × 10−9 Gauss smoothed on the magnetic Jeans scale and below, can give rise to Thomson scattering optical depths τ > ∼ 0.1, although not in the range of redshifts needed to explain the recent WMAP polarization observations.We also study the possibility that primordial ﬁelds could induce the formation of subgalactic structures for z > ∼ 15. We show that early structure formation induced by nano-Gauss magnetic ﬁelds is potentially capable of producing the early re-ionization implied by the WMAP data. Future CMB observations will be very useful to probe the modiﬁed ionization histories produ
Coupled dark energy: Towards a general description of the dynamics
(2005-01-01) Gumjudpai, Burin; Sami, M.; Naskar, Tapan; et al.
In dark energy models of scalar-ﬁeld coupled to a barotropic perfect ﬂuid, the existence of cosmological scaling solutions restricts the Lagrangian of the ﬁeld ϕ to p = Xg(Xeλϕ), where X = −g ν ∂ ϕ∂νϕ/2, λ is a constant and g is an arbitrary function. We derive general evolution equations in an autonomous form for this Lagrangian and investigate the stability of ﬁxed points for several diﬀerent dark energy models–(i) ordinary (phantom) ﬁeld, (ii) dilatonic ghost condensate, and (iii) (phantom) tachyon. We ﬁnd the existence of scalar-ﬁeld dominant ﬁxed points (Ωϕ = 1) with an accelerated expansion in all models irrespective of the presence of the coupling Q between dark energy and dark matter. These ﬁxed points are always classically stable for a phantom ﬁeld, implying that the universe is eventually dominated by the energy density of a scalar ﬁeld if phantom is responsible for dark energy. When the equation of state wϕ for the ﬁeld ϕ is larger than −1, we ﬁnd that scaling solutions are stable if the scalar-ﬁeld dominant solution is unstable, and vice versa. Therefore in this case the ﬁnal attractor is either a scaling solution with constant Ωϕ satisfying 0 < Ωϕ < 1 or a scalar-ﬁeld dominant solution with Ωϕ = 1
Fate of (phantom) dark energy universe with string curvature corrections
(2005-01-01) Sami, M.; Tsujikawa, Shinji; Tretjakov, P. V.; et al.
We study the evolution of (phantom) dark energy universe by taking into account the higher-order string corrections to Einstein-Hilbert action with ﬁxed dilaton and modulus ﬁelds. While the presence of a cosmological constant gives stable de-Sitter ﬁxed points in the cases of heterotic and bosonic strings, no stable de-Sitter solutions exist when a phantom ﬂuid is present. We ﬁnd that the universe can exhibit a Big Crunch singularity with a ﬁnite time for type II string, whereas it reaches a Big Rip singularity for heterotic and bosonic strings. Thus the fate of dark energy universe crucially depends upon the type of string theory under consideration.
Cosmological cosmic rays and the observed Li6 plateau in metal poor halo stars
(2005-01-01) Rollinde, Emmanuel; Elisabeth, Vangioni-Flam; Olive, Keith A
Very recent observations of the 6Li isotope in halo stars reveal a 6Li plateau about 1000 times above the predicted BBN abundance. We calculate the evolution of 6Li versus redshift generated from an initial burst of cosmological cosmic rays (CCRs) up to the formation of the Galaxy. We show that the pregalactic production of the 6Li isotope can account for the 6Li plateau observed in metal poor halo stars without additional overproduction of 7Li. The derived relation between the amplitude of the CCR energy spectra and the redshift of the initial CCR production puts constraints on the physics and history of the objects, such as pop III stars, responsible for these early cosmic rays. Consequently, we consider the evolution of 6Li in the Galaxy. Since 6Li is also produced in Galactic cosmic ray nucleosynthesis, we argue that halo stars with metalIicities between [Fe/H) = -2 and -1, must be somewhat depleted in 6Li.
Physics of CMBR Anisotropies
(2005-01-01) Subramanian, Kandaswamy
The observed structures in the universe are thought to have arisen from gravitational instability acting on small ﬂuctuations generated in the early universe. These spatial ﬂuctuations are imprinted on the CMBR as angular anisotropies. The physics which connects initial ﬂuctuations in the early universe to the observed anisotropies is fairly well understood, since for most part it involves linear perturbation theory. This makes CMBR anisotropies one of the cleanest probes of the initial ﬂuctuations, various cosmological parameters governing their evolution and also the geometry of the universe. We review here in a fairly pedagogical manner the physics of the CMBR anisotropies and explain the role they play in probing cosmological parameters, especially in the light of the latest observations from the WMAP satellite
Exploring the Geometry, Topology and Morphology of Large Scale Structure using Minkowski Functionals
(2005-02-01) Sheth, Jatush V.; Sahni, Varun
Modern redshift surveys such as the 2 degree ﬁeld Galaxy Redshift Survey (2dFGRS) and the Sloan Digital Sky Survey (SDSS) reveal the fully 3 dimensional distribution of a million or so galaxies over a large cosmological volume. Visually galaxies appear to be distributed along sheet-like and/or ﬁlamentary superclusters. The CfA Great Wall, Southern Great Wall and the recently discovered SDSS Great Wall are very spectacular superclusters. Clearly theoretical predictions for galaxy clustering must be tested against these rich datasets. This can be achieved by means of the Minkowski Functionals (MFs). A MF-based approach provides an excellent description of superclusters and voids and allows one to quantify the properties of the cosmic web. In this review we give a summary of the progress made in this direction. After reviewing the status of observations and of numerical simulations, we comment upon the nature of bias which serves as a link between theoretical predictions and observations. We also summarise the methods developed for eﬃcient numerical estimation of MFs for cosmological datasets and list several important results obtained using these methods. Speciﬁcally, we stress the discriminatory power of MFs and of the derived morphological statistics, the Shapeﬁnders. Shapeﬁnders are an excellent tool with which to study the shapes and sizes of superclusters and voids. We also discuss some of the important eﬀects of scale-dependent bias which are brought out by a MF-based study of the mock catalogues of galaxies. Such eﬀects, we note, should be accounted for before comparing theoretical models with observations.
Statistical isotropy of CMB anisotropy from WMAP
(2005-02-01) Souradeep, Tarun; Hajian, Amir
The statistical expectation values of the temperature ﬂuctuations of cosmic microwave background (CMB) are assumed to be preserved under rotations of the sky. We investigate the statistical isotropy of the CMB anisotropy maps recently measured by the Wilkinson Microwave Anisotropy Probe (WMAP) using bipolar spherical harmonic power spectrum proposed in Hajian & Souradeep 2003. The Bipolar Power Spectrum (BiPS) is estimated for the full sky CMB anisotropy maps of the ﬁrst year WMAP data. The method allows us to isolate regions in multipole space and study each region independently. This search shows no evidence for violation of statistical isotropy in the ﬁrst-year WMAP data on angular scales larger than that corresponding to l ≈ 60 .
Long-term optical photometric monitoring of the quasar SDSS J153259.96 - 003944.1
(2005-02-01) Stalin, C. S.; Srianand, R.
We report optical Cousins R and I band monitoring observations of the high redshift (z = 4.67) QSO SDSS J153259.96−003944.1 that does not show detectable emission lines in its optical spectrum. We show this object varies with a maximum amplitude of ∼0.4 mag during a year and three months of monitoring. Combined with two other epochs of photometric data available in the literature, we show the object has gradually faded by ∼0.9 mag during the period June 1998 − April 2001. A linear least squares ﬁt to all available observations gives a slope of ∼0.35 mag/yr which translates to ∼1.9 mag/yr in the rest frame of the quasar. Such a variability is higher than that typically seen in QSOs but consistent with that of BL Lacs, suggesting that the optical continuum is Doppler boosted. Alternatively, within photometric errors, the observed lightcurve is also consistent with the object going through a microlensing event. Photoionization model calculations show the mass of the Broad Line Region to be few tens of M⊙ similar to that of low luminosity Seyfert galaxies, but ∼2 orders of magnitude less than that of luminous quasars. Further frequent photometric/spectroscopic monitoring is needed to support or refute the diﬀerent alternatives discussed here on the nature of SDSS J153259.96−003944.1
Understanding Our Universe : Current Status and Open Issues
(2005-03-01) Padmanabhan, T.
Last couple of decades have been the golden age for cosmology. High quality data conﬁrmed the broad paradigm of standard cosmology but have thrusted upon us a preposterous composition for the universe which deﬁes any simple explanation, thereby posing probably the greatest challenge theoretical physics has ever faced. Several aspects of these developments are critically reviewed, concentrating on conceptual issues and open questions.
Did the Universe loiter at high redshifts?
(2005-04-01) Sahni, Varun
We show that loitering at high redshifts (z > ∼ 6) can easily arise in braneworld models of dark energy which, in addition to being spatially ﬂat, also accelerate at late times. Loitering is characterized by the fact that the Hubble parameter dips in value over a narrow redshift range which we shall refer to as the ‘loitering epoch’. During loitering, density perturbations are expected to grow rapidly. In addition, since the expansion of the universe slows down, its age near loitering dramatically increases. An early epoch of loitering is expected to boost the formation of high redshift gravitationally bound systems such as 109M⊙ black holes at z ∼ 6 and lower-mass black holes and/or Population III stars at z > 10, whose existence could be problematic within the LCDM scenario. Loitering models also help to reduce the redshift of reionization from its currently (high) value of zreion ≃ 17 in LCDM cosmology, thus alleviating a signiﬁcant source of tension between observations of the high-redshift universe and theoretical model building. Currently a loitering universe accelerates with an eﬀective equation of state w < −1 thus mimicking phantom dark energy. Unlike phantom, however, the late-time expansion of the universe in our model is singularity free, and a universe that loitered in the past will approach a LCDM model symptotically in the distant future.
CMB Anisotropy Due to Tangled magnetic fields in re-ionized models
(2005-04-01) Seshadri, T. R.; Subramanian, Kandaswamy
Primordial tangled cosmological Magnetic Fields source rotational velocity perturbations of the baryon ﬂuid, even in the post-recombination universe. These vortical modes inturn leave a characteristic imprint on the temperature anisotropy of the Cosmic Microwave Background (CMB), if the CMB photons can be re-scatterred after recombination. Observations from WMAP in- dicate that the Universe underwent a relatively early re-ionization (zri ∼ 15), which does indeed lead to a signiﬁcant optical depth for re-scattering of CMB photons after the re-ionization epoch. We compute the resulting additional temperature anisotropies, induced by primordial magnetic ﬁelds in the postrecombination universe. We show that in models with early re-ionization, a nearly scale-invariant spectrum of tangled magnetic ﬁelds which redshift to a present value of B0 ∼ 3 × 10−9 Gauss, produce vector mode perturbations which in turn induce additional temperature anisotropy of about 0.3 to 0.4 µK over very small angular scales, with l upto ∼ 10000 or so.98.62.En, 98.70.Vc, 98.80.Cq, 95.30.Qd
Cosmological Surprises from Braneworld models of Dark Energy
(2005-04-01) Sahni, Varun
Properties of Braneworld models of dark energy are reviewed. Braneworld models admit the following interesting possibilities: (i) The eﬀective equation of state can be w ≤ −1 as well as w ≥ −1. In the former case the expansion of the universe is well behaved at all times and the universe does not run into a future ‘Big Rip’ singularity which is usually encountered by Phantom models. (ii) For a class of Braneworld models the acceleration of the universe can be a transient phenomenon. In this case the current acceleration of the universe is sandwiched between two matter dominated epochs. Such a braneworld does not have a horizon in contrast to LCDM and most Quintessence models. (iii) For a speciﬁc set of parameter values the universe can either originate from, or end its existence in a Quiescent singularity, at which the density, pressure and Hubble parameter remain ﬁnite, while the deceleration parameter and all invariants of the Riemann tensor diverge to inﬁnity within a ﬁnite interval of cosmic time. (iv) Braneworld models of dark energy can loiter at high redshifts: 6 < ∼ z < ∼ 40. The Hubble parameter decreases during the loitering epoch relative to its value in LCDM. As a result the age of the universe at loitering dramatically increases and this is expected to boost the formation of high redshift gravitationally bound systems such as 109 M⊙ black holes at z ∼ 6 and lower-mass black holes and/or Population III stars at z > 10, whose existence could be problematic within the LCDM scenario. (v) Braneworld models with a time-like extra dimension bounce at early times thereby avoiding the initial ‘Big Bang singularity’. (vi) Both Inﬂation and Dark Energy can be successfully uniﬁed within a single scheme (Quintessential Inﬂation)
Scattering properties and composition of cometary dust
(2005-04-01) Gupta, Ranjan; Vaidya, D.B.; Dobbie, J.S.; et al.
Composition of the Comet dust obtained by the dust impact analyzer on the Halley probes indicated that the comet dust is a mixture of silicate and carbonaceous material. The collected interplanetary dust particles (IDP’s) are ﬂuﬀy and composite, having grains of several diﬀerent types stuck together. Using Discrete Dipole Approximation (DDA) we study the scattering properties of composite grains. In particular, we study the angular distribution of the scattered intensity and linear polarization of composite grains. We assume that the composite grains are made up of a host silicate sphere/spheroid with the inclusions of graphite. Results of our calculations on the composite grains show that the angle of maximum polarization shifts, and the degree of polarization varies with the volume fraction of the inclusions. We use these results on the composite grains to interpret the observed scattering in cometary dust.
Analysis of the distribution of background star polarization in dark clouds
(2005-04-01) Sen, A.K.; Mukai, T.; Gupta, Ranjan; et al.
The polarization observed for stars background to dark clouds (Bok Globules) is often used as diagnostic to study the ongoing star formation processes in these clouds. Such polarization maps in the optical have been reported for eight nearby clouds CB3, CB25, CB39, CB52, CB54, CB58, CB62 and CB246 in one of our previous work (Sen et al 2000). With a view to understand the origin of this polarization, in the present work attempts are made to look for any possible relation between this observed polarization and other physical parameters in the cloud (like temperature, turbulence etc.). The observed polarization does not seem to be clearly related to the dust and gas temperatures (Td and Tg) in the cloud as expected from Davis-Greenstein grain alignment mechanism (Davis & Greenstein 1952). However, the average observed polarization (pav) appears to be related to the turbulence ∆V (measured by 12CO line width) by the mathematical relation pav = 2.95 exp(−0.24∆V ). The possible relation between the direction of polarization vector and other physical parameters are also discussed. For this analysis in addition to the data on above eight dark clouds, the data on CB4 (Kane et al. 1995) are also included for comparison.
Density structure around quasars from optical depth statistics
(2005-05-01) Rollinde, Emmanuel; Srianand, R.; Chand, Hum; et al.
We present a method for studying the proximity effect and the density structure around redshift z=2-3 quasars. It is based on the probability distribution of Lyman-α pixel optical depths and its evolution with redshift. We validate the method using mock spectra obtained from hydrodynamical simulations, and then apply it to a sample of 12 bright quasars at redshifts 2-3 observed with UVES at the VLT-UT2 Kueyen ESO telescope. These quasars do not show signatures of associated absorption and have a mean monochromatic luminosity of 5.4 × 1031 h−2 erg s−1 Hz−1 at the Lyman limit. The observed distribution of optical depth within 10 h−1Mpc from the QSO is statistically different from that measured in the general intergalacticmedium at the same redshift. Such a change will result from the combined effects of the increase in photoionisation rate above the mean UV-background due to the extra ionizing photons from the quasar radiation (proximity effect), and the higher density of the IGM if the quasars reside in overdense regions (as expected from biased galaxy formation). The ﬁrst factor decreases the optical depth whereas the second one increases the optical depth, but our measurement cannot distinguish a high background from a low overdensity. An overdensity of the order of a few is required if we use the amplitude of the UV-background inferred from the mean Lyman-α opacity. If no overdensity is present, then we require the UV-background to be higher, and consistent with the existing measurements based on standard analysis of the proximity effect.
Nanoflares as a possible coronal heating agent
(2005-05-01) Lalan, Prasad; Lohani, N. K.
We present the concept of coronal heating by nanoﬂares and the characteristics of active regions which result presumbly from a series of discrete events of various energies analogeus to a large number of more or less random heating agents. The ﬂare associated with small impulsive events of magnetic energy dissipation is from 5 x 1023 to 1025 ergs, called nanoﬂares and it is treated as heating agents. The analysis of these agents are represented by a power law distribution as a function of their energies with a negative slope of 2. We discuss the physical characteristics of nanoﬂare heating process in respect of power law distribution and formulate the coronal luminosity. We estimate the coronal radiation energy and generation rate by nanoﬂares.