2000 (IPP)

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Absorption of Electro-magnetic Waves in a Magnetized Medium
(2000-07-04) Ganguly, Avijit K.; Konar, Sushan
In continuation to our earlier work, in which the structure of the vacuum polarisation tensor in a medium was analysed in presence of a background electro-magnetic ﬁeld, we discuss the absorptive part of the vacuum polarization tensor. Using the real time formalism of ﬁnite temperature ﬁeld theory we calculate the absorptive part of 1-loop vacuum polarisation tensor in the weak ﬁeld limit (eB < m2 ). Estimates of the absorption probability are also made for diﬀerent physical conditions of the background medium.
Adaptive filtering techniques for gravitational wave interferometric data : Removing long-term sinusoidal disturbances and oscillatory transients
(2000-02-12) Chassande-Mottin, E.; Dhurandhar, Sanjeev
We propose an adaptive denoising scheme for poorly modeled non-Gaussian features in the gravitational wave interferometric data. Preliminary tests on real data show encouraging results
Adaptive filtering techniques for gravitational wave interferometric data : Removing long-term sinusoidal disturbances and oscillatory transients
(2000-04-04) Chassande-Mottin, E.; Dhurandhar, Sanjeev
It is known by the experience gained from the gravitational wave detector proto-types that the interferometric output sig- nal will be corrupted by a signiﬁcant amount of non-Gaussian noise, large part of it being essentially composed of long-term sinusoids with slowly varying envelope (such as violin res- onances in the suspensions, or main power harmonics) and short-term ringdown noise (which may emanate from servo control systems, electronics in a non-linear state, etc.). Since non-Gaussian noise components make the detection and esti- mation of the gravitational wave signature more diﬃcult, a de- noising algorithm based on adaptive ﬁltering techniques (LMS methods) is proposed to separate and extract them from the stationary and Gaussian background noise. The strength of the method is that it does not require any precise model on the observed data : the signals are distinguished on the basis of their autocorrelation time. We believe that the robustness and simplicity of this method make it useful for data prepa- ration and for the understanding of the ﬁrst interferometric data. We present the detailed structure of the algorithm and its application to both simulated data and real data from the LIGO 40meter proto-type.
Case for a Positive Cosmological Lambda-term
(2000-04-25) Sahni, Varun; Starobinsky, A. A.
Recent observations of Type 1a supernovae indicating an accelerat- ing universe have once more drawn attention to the possible existence, at the present epoch, of a small positive Λ-term (cosmological constant). In this paper we review both observational and theoretical aspects of a small cosmological Λ-term. We discuss the current observational situation focusing on cosmolog- ical tests of Λ including the age of the universe, high redshift supernovae, gravitational lensing, galaxy clustering and the cosmic microwave background. We also review the theoretical debate surrounding Λ: the generation of Λ in models with spontaneous symmetry breaking and through quantum vacuum polarization eﬀects – mechanisms which are known to give rise to a large value of Λ hence leading to the ‘cosmological constant problem’. More recent at- tempts to generate a small cosmological constant at the present epoch using either ﬁeld theoretic techniques, or by modeling a dynamical Λ-term by scalar ﬁelds are also extensively discussed. Anthropic arguments favouring a small Λ-term are brieﬂy reviewed. A comprehensive bibliography of recent work on Λ is provided.
Collapsing shells of radiation in higher dimensional spacetime and cosmic censorship conjecture
(2001-06-26) Ghosh, S. G.; Saraykar, R. V.; Beesham, A.
Gravitational collapse of radiation shells in a non self-similar higher dimensional spherically symmetric spacetime is studied. Strong curvature naked singularities form for a highly inhomogeneous collapse, violating the cosmic censorship conjecture. As a special case, self similar models can be constructed.
Constraints on omegaB, omega M, and h from MAXIMA and BOOMERANG
(2000-10-24) Padmanabhan, T.; Sethi, Shiv K.
We analyse the BOOMERANG and MAXIMA results in the context of models with ΩTotal = 1 and ns = 1. We attempt to constrain three other parameters—h, ΩB, and Ωm—from these observations. We show that: (a) the value of ΩBh2 is too high to be compatible with primordial nucleosynthesis observations at 95% conﬁdence level (b) universe with age greater than 12Gyr is ruled out at 95% conﬁdence level and (c) the value of Ωmh is too high to be compatible with the shape of the power spectrum of gravitational clustering. In eﬀect, our analysis shows that models with ΩTotal = 1 and n = 1 are ruled out by BOOMERANG and MAXIMA observations.
Cosmic microwave background temperature at a redshift of 2.33771
(2000-12-18) Srianand, R.; Petitjean, Patrick; Ledoux, C.
The Cosmic Microwave Background radiation is a fundamental prediction of Hot Big Bang cosmology. The temperature of its black-body spectrum has been measured at the present time, TCMBR,0 = 2.726±0.010 K, and is predicted to have been higher in the past. At earlier time, the temperature can be measured, in principle, using the excitation of atomic ﬁne structure levels by the radiation ﬁeld. All previous measurements however give only upper limits as they assume that no other signiﬁcant source of excitation is present. Here we report the detection of absorption from the ﬁrst and second ﬁne-structure levels of neutral carbon atoms in an isolated remote cloud at a redshift of 2.33771. In addition, the unusual detection of molecular hydrogen in several rotational levels and the presence of ionized carbon in its excited ﬁne structure level make the absorption system unique to constrain, directly from observation, the diﬀerent excitation processes at play. It is shown for the ﬁrst time that the cosmic radiation was warmer in the past. We ﬁnd 6.0 < TCMBR < 14 K at z = 2.33771 when 9.1 K is expected in the Hot Big Bang cosmology.
Counterexample to Claimed COBE Constraints on Compact Toroidal Universe Models
(2000-03-12) Roukema, B. F.
It has been suggested that if the Universe satisﬁes a ﬂat, multiply connected, perturbed Friedmann- Lemaˆıtre model, then cosmic microwave back- ground data from the COBE satellite implies that the minimum size of the injectivity diameter (shortest closed spatial geodesic) must be larger than about two ﬁfths of the horizon diameter. To show that this claim is misleading, a simple T2×R universe model of injectivity diameter a quarter of this size, i.e. a tenth of the horizon diameter, is shown to be consistent with COBE four year ob- servational maps of the cosmic microwave back- ground. This is done using the identiﬁed circles principle.
Data-analysis strategy for detecting gravitational-wave signals from inspiraling compact binaries with a network of laser-interferometric detectors
(2000-04-24) Pai, A.; Dhurandhar, Sanjeev; Bose, Sukanta
A data-analysis strategy based on the maximum-likelihood method (MLM) is presented for the detection of gravitational waves from inspiraling compact binaries with a network of laser- nterferometric detectors having arbitrary orientations and arbitrary locations around the globe. For simplicity, we restrict ourselves to the Newtonian inspiral waveform. However, the formalism we develop here is also applicable to a waveform with post-Newtonian (PN) corrections. The Newtonian waveform depends on eight parameters: the distance r to the binary, the phase δc of the waveform at the time of ﬁnal coalescence, the polarization-ellipse angle ψ, the angle of inclination ǫ of the binary orbit to the line of sight, the source-direction angles {θ, φ}, the time of ﬁnal coalescence tc at the ﬁducial detector, and the chirp time ξ. All these parameters are relevant for a chirp search with multiple detectors, unlike the case of a single detector. The primary construct on which the MLM s based is the network likelihood ratio (LR). We obtain this ratio here. For the Newtonian inspiral waveform, the LR is a function of the eight signal-parameters. In the MLM-based detection strategy, the LR must be maximized over all of these parameters. Here, we show that it is possible to maxi- mize it analytically with respect to four of the eight parameters, namely, {r, δc, ψ, ǫ}. Maximization over the time of arrival is handled most eﬃciently by using the Fast-Fourier-Transform algorithm, as in the case of a single detector. This not only allows us to scan the parameter space continu- ously over these ﬁve parameters but also cuts down substantially on the computational costs. The analytical maximization over the four parameters yields the optimal statistic on which the decision must be based. The value of the statistic also depends on the nature of the noises in the detectors. Here, we model these noises to be mainly Gaussian, stationary, and uncorrelated for every pair of detectors. Instances of non-Gaussianity, as are present in detector outputs, can be accommodated n our formalism by implementing vetoing techniques similar to those applied for single detectors. Our formalism not only allows us to express the likelihood ratio for the network in a very simple and compact form, but also is at the basis of giving an elegant geometric interpretation to the de- tection problem. Maximization of the LR over the remaining three parameters is handled as follows. Owing to the arbitrary locations of the detectors in a network, the time of arrival of a signal at any detector will, in general, be diﬀerent from those at the others and, consequently, will result in signal time-delays. For a given network, these time delays are determined by the source-direction angles {θ, φ}. Therefore, to maximize the LR over the parameters {θ, φ} one needs to scan over the possible time-delays allowed by a network. We opt for obtaining a bank of templates for the chirp time and the time-delays. This means that we construct a bank of templates over ξ, θ, and φ. We ﬁrst discuss “idealized” networks with all the detectors having a common noise curve for simplicity. Such an exercise nevertheless yields useful estimates about computational costs, and also tests the formalism developed here. We then consider realistic cases of networks comprising of the LIGO and VIRGO detectors: These include two-detector networks, which pair up the two LIGOs or VIRGO with one of the LIGOs, and the three-detector network that includes VIRGO and both the LIGOs. For these networks we present the computational speed requirements, network sensitivities, and source-direction resolutions.
Density dependent strong coupling constant of QCD derived from compact star data
(2000-05-16) Ray, Subharthi; Dey, Jishnu; Dey, Mira
The present work is an endeavour to connect the properties of tiny nearly massless objects with those of some of the most massive ones, the compact stars. Since 1996 there is major inﬂux of X-ray and γ ray data from binary stars, one or both of which are compact objects that are diﬃcult to explain as neutron stars since they contain a mass M in too small a radius R . The suggestion has been put forward that these are strange quark stars (SS) explainable in a simple model with chiral symmetry restoration (CSR) for the quarks and the M, R and other properties like QPOs (quasi periodic oscillations) in their X-ray power spectrum. It would be nice if this astrophysical data could shed some light on fundamental properties of quarks obeying QCD. One can relate the strong coupling constant of QCD, αs to the quark mass through the Dyson-Schwinger gap equation using the real time formalism of Dolan and Jackiw. This enables us to obtain the density dependence of αs from the simple CSR referred to above. This way fundamental physics, diﬃcult to extract from other models like for example lattice QCD, can be constrained from present - day compact star data and may be put back to modelling the dense quark phase of early universe.
Detection of gravitational waves from inspiraling compact binaries using a network of interferometric detectors
(2000-06-28) Bose, Sukanta; Pai, A.; Dhurandhar, Sanjeev
We formulate the data analysis problem for the detection of the New- tonian waveform from an inspiraling compact-binary by a network of arbi- trarily oriented and arbitrarily distributed laser interferometric gravitational wave detectors. We obtain for the ﬁrst time the relation between the opti- mal statistic and the magnitude of the network correlation vector, which is constructed from the matched network-ﬁlter. This generalizes the calculation reported in an earlier work (gr-qc/9906064), where the detectors are taken to be coincident.
Fundamental plane distances to early-type field galaxies in the South Equatorial Strip
(2000-07-25) Muller, K. R.; Wegner, G.; Freudling, W.
Radial velocities and central velocity disper- sions are derived for 238 E/S0 galaxies from medium- resolution spectroscopy. New spectroscopic data have been obtained as part of a study of the Fundamental Plane distances and peculiar motions of early-type galaxies in three selected directions of the South Equatorial Strip, undertaken in order to investigate the reality of large- scale streaming motion; results of this study have been reported in M¨uller et al. (1998). The new APM South Equatorial Strip Catalog (−17◦.5 < δ < +2◦.5) was used to select the sample of ﬁeld galaxies in three directions: (1) 15h10 – 16h10; (2) 20h30 – 21h50; (3) 00h10 – 01h30. The spectra obtained have a median S/N per ˚ A of 23, an in- strumental resolution (FWHM) of ∼ 4 ˚ A, and the spectro- graph resolution (dispersion) is ∼ 100 km s−1. The Fourier cross-correlation method was used to derive the radial ve- locities and velocity dispersions. The velocity dispersions have been corrected for the size of the aperture and for the galaxy eﬀective radius. Comparisons of the derived radial velocities with data from the literature show that our values are accurate to 40 km s−1. A comparison with results from Jørgensen et al. (1995) shows that the derived central velocity dispersion have an rms scatter of 0.036 in log σ. There is no oﬀset relative to the velocity dispersions of Davies et al. (1987).
Global monopoles and scalar fields as the electrogravity dual of Schwarzschild spacetime
(2000-08-15) Dadhich, Naresh; Banerjee, Narayan
We prove that both global monopole and minimally coupled static zero mass scalar ﬁeld are electrogravity dual of the Schwarzschild solution or ﬂat space and they share the same equation of state, T0 0 − Ti i = 0. This property was however known for the global monopole spacetime while it is for the ﬁrst time being established for the scalar ﬁeld. In particular, it turns out that the Xanthopoulos - Zannias scalar ﬁeld solution is dual to ﬂat space.
H2 molecules and the nature of damped Lyman-alpha systems
(2000-12-14) Petitjean, Patrick; Srianand, R.; Ledoux, C.
We report results from our mini-survey for molecular hydrogen in eight high-redshift damped Lyman- α (DLA) systems using the ESO Ultra-violet and Visible Spectrograph on the VLT. In addition, we investigate two systems using ESO public data. We include in the sam- ple the only system where H2 was previously detected and studied at high-spectral resolution. Altogether our sample consists of eleven absorbers with 1.85 < zabs < 3.4. We conﬁrm the presence of H2 in the zabs = 2.3377, metal-poor ([Si/H] = −1.20), system toward PKS 1232+082. The derived molecular fraction, f = 2N(H2)/(2N(H2)+N(H i)) = 4×10−4, is two orders of magnitude less than what has been claimed previ- ously from low-resolution data. The physical conditions within the cloud can be constrained directly from ob- servation. The kinetic temperature and particle density are in the ranges, respectively, 100 < T < 300 K and 30 < nH < 50 cm−3. In addition, UV pumping is of the same order of magnitude than in our Galaxy. The upper limits on the molecular fraction derived in nine of the systems are in the range 1.2×10−7−1.6×10−5. There is no evidence in this sample for any correlation be- tween H2 abundance and relative heavy element depletion into dust grains. This should be investigated using a larger sample however. The molecular abundance in a few DLA systems (and in particular in the two systems where H2 is detected) is consistent with what is seen in the Magel- lanic clouds. But most of the DLA measurements are well below these values. This is probably partly due to small amounts of dust and/or high UV ﬂux. We argue however that the lack of molecules is a direct consequence of high kinetic temperature (T > 3000 K) implying a low forma- tion rate of H2 onto dust grains. Therefore, most of the DLA systems arise in warm and diﬀuse neutral gas.
Higher Dimensional Radiation Collapse and Cosmic Censorship
(2000-11-12) Ghosh, S. G.; Saraykar, R. V.
We study the occurrence of naked singularities in the spherically symmetric collapse of radiation shells in a higher dimensional spacetime. The necessary conditions for the formation of a naked singularity or a black hole are obtained. The naked singularities are found to be strong in the Tipler’s sense and thus violating cosmic censorship conjecture.
Hypothesis of path integral duality. III: Applications to QED
(2000-11-14) Shankaranarayanan, S.; Padmanabhan, T.
We use the modiﬁed propagator for quantum ﬁeld based on a “principle of path integral duality” proposed earlier in a paper by Padmanabhan to investigate several results in QED. This procedure modiﬁes the Feynman propagator by the introduction of a funda- mental length scale. We use this modiﬁed propagator for the Dirac particles to evaluate the ﬁrst order radiative corrections in QED. We ﬁnd that the extra factor of the modiﬁed propagator acts like a regulator at the Planck scales thereby removing the divergences that otherwise appear in the conventional radiative correction calculations of QED. We ﬁnd that:(i) all the three renormalization factors Z1, Z2, and Z3 pick up ﬁnite correc- tions and (ii) the modiﬁed propagator breaks the gauge invariance at a very small level of O(10−45). The implications of this result to generation of the primordial seed magnetic ﬁelds are discussed.
Inheriting geodesic ﬂows
(2000-12-12) Lortan, D. B.; Maharaj, S. D.; Dadhich, Naresh
We investigate the propagation equations for the expansion, vorticity and shear for per- fect ﬂuid space-times which are geodesic. It is assumed that space-time admits a conformal Killing vector which is inheriting so that ﬂuid ﬂow lines are mapped conformally. Simple constraints on the electric and magnetic parts of the Weyl tensor are found for conformal symmetry. For homothetic vectors the vorticity and shear are free; they vanish for nonhomothetic vectors. We prove a conjecture for conformal symmetries in the special case of inheriting geodesic ﬂows: there exist no proper con- formal Killing vectors ( ;ab =0) for perfect ﬂuids except for Robertson–Walker space-times. For a nonhomothetic vector ﬁeld the propagation of the quantityln(Rabuaub ) along the integral curves of the symmetry vector is homogeneous.
Issue of choosing nothing : What determines the low energy vacuum state of nature?
(2000-09-25) Padmanabhan, T.; Choudhury, T. Roy
Starting from an (unknown) quantum gravitational model, one can invoke a sequence of approx- imations to progressively arrive at quantum ﬁeld theory (QFT) in curved spacetime, QFT in ﬂat spacetime, nonrelativistic quantum mechanics and newtonian mechanics. The more exact theory can put restrictions on the range of possibilities allowed for the approximate theory which are not derivable from the latter – an example being the symmetry restrictions on the wave function for a pair of electrons. We argue that the choice of vacuum state at low energies could be such a ‘relic’ arising from combining the principles of quantum theory and general relativity, and demonstrate this result in a simple toy model. Our analysis suggests that the wave function of the universe, when it describes the large volume limit of the universe, dynamically selects a vacuum state for matter ﬁelds — which in turn deﬁnes the concept of particle in the low energy limit. The result also has the potential for providing a concrete quantum mechanical version of Mach’s principle.
Lens mapping algorithm for weak lensing
(2000-09-24) Saini, Tarun Deep; Raychaudhury, Somak
We develop an algorithm for the reconstruction of the two-dimensional mass distribution of a gravitational lens from the observable distortion of background galaxies. From the measured reduced shear γi/(1 − κ) the lens mapping is obtained, from which a mass distribution is derived. This is unlike other methods where the convergence κ is directly obtained. We show that this method works best for sub-critical lenses, but can be applied to a critical lens away from the critical lines. For ﬁnite ﬁelds the usual mass-sheet degeneracy is shown to exist in this method as well. We show that the algorithm reproduces the mass distribution within acceptable limits when applied to simulated noisy data.
Lifting cosmic degeneracy within a single quasar survey
(2000-12-30) Roukema, B. F.; Mamon, G. A.
In the almost Friedmann-Lemaˆıtre model of the Universe, the density parameter, Ωm, and the cosmo- logical constant, ΩΛ, measure curvature. Several linearly degenerate relations between these two parameters have recently been measured. Here, large scale structure cor- relations at ∼ 100 − 150 h−1 Mpc are found in the co- moving three-dimensional separations of redshift z ≈ 2 quasars. These function as a comoving standard rod of length LLSS ≈ 130 ± 10 h−1 Mpc. A local maximum in the correlation function at ≈ LLSS/2 also appears to be signiﬁcant. By combining separate radial and tangential standard ruler analyses, the lifting of the Ωm − ΩΛ linear degeneracy within a single data set is demonstrated for the ﬁrst time.