2000 (IPP)

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New cosmological model of quintessence and dark matter
(2000-01-14) Sahni, Varun; Wang, Limin
We propose a new class of quintessence models in which late times oscillations of a scalar ﬁeld give rise to an eﬀective equation of state which can be negative and hence drive the observed acceleration of the universe. Our ansatz provides a uniﬁed picture of quintessence and a new form of dark matter we call Frustrated Cold Dark Matter (FCDM). FCDM inhibits gravitational clustering on small scales and could provide a natural resolution to the core density problem for disc galaxy halos. Since the quintessence ﬁeld rolls towards a small value, constraints on slow-roll quintessence models are safely circumvented in our model.
Searching for gravitational waves from rotating neutron stars
(2000-01-15) Dhurandhar, Sanjeev
Rotating neutron stars are one of the important sources of gravitational waves (GW) for the ground based as well as space based detectors. Since the waves are emitted continuously, the source is termed as a continuous gravitational wave (CGW) source. The expected weakness of the signal requires long integration times ( year). The data analysis problem involves tracking the phase coherently over such large integration times, which makes it the most computationally intensive problem among all GW sources envisaged. In this article, the general problem of data analysis is discussed, and more so, in the context of searching for CGW sources orbiting another companion object. The problem is important because there are several pulsars, which could be deemed to be CGW sources orbiting another companion star. Differential geometric techniques for data analysis are described and used to obtain computational costs. These results are applied to known systems to assess whether such systems are detectable with current (or near future) computing resources.
Spherically symmetric empty space and its dual in general relativity
(2000-01-22) Dadhich, Naresh
Thermally driven outflows from pair-plasma pressure-mediated shock surfaces around schwarzschild black holes
(2000-01-23) Das, Tapas K.
Introducing a spherical, steady, self-supported pair-plasma pressure-mediated shock surface around a Schwarzschild black hole as the effective physical atmosphere that may be responsible for the generation of astrophysical mass outflows from relativistic quasi- spherical accretion, we calculate the mass outflow rate Rm Ç by simultaneously solving the set of equations governing transonic polytropic accretion and isothermal winds. Rm Ç is computed in terms of only three inflow parameters, which, we believe, has been done for the first time in our work. We then study the dependence of Rm Ç on various inflow as well as shock parameters, and establish the fact that the outflow rate is essentially controlled by the post- shock proton temperature.
Tree structure of the percolation universe
(2000-01-27) Colombi, S.; Pogosyan, D.; Souradeep, Tarun
We present a numerical study of topological descriptors of initially Gaussian and scale-free density perturbations evolving via gravitational instability in an expanding universe. We carefully evaluate and avoid numerical contamination in making accurate measurements on simulated ﬁelds on a grid in a ﬁnite box. Independent of extent of non linearity, the measured Euler number of the excursion set at the percolation threshold, δc, is positive and nearly equal to the number of isolated components, suggesting that these structures are trees. Our study of critical point counts reconciles the clumpy appearance of the density ﬁeld at δc with measured ﬁlamentary local curvature. In the Gaussian limit, we measure |δc| > σ in contrast to widely held belief that |δc| ∼ σ, where σ2 is the variance of the density ﬁeld.
Observing high-redshift Supernovae in lensed galaxies
(2000-01-28) Saini, Tarun Deep; Raychaudhury, Somak; Shchekinov, Yuri A.
Supernovae in distant galaxies that are grav- tationally lensed by foreground galaxy clusters make ex- cellent cosmological candles for measuring quantities like the density of the Universe in its various components and the Hubble constant. Distant supernovae will be more eas- ly detectable since foreground cluster lenses would mag- nify such supernovae by up to 3–4 magnitudes. We show that in the case of the lens cluster Abell 2218, the de- tectability of high-redshift supernovae is signiﬁcantly en- hanced due to the lensing eﬀects of the cluster. Since ensed supernovae will remain point images even when their host galaxies are stretched into arcs, the signal-to- noise ratio for their observation will be further enhanced, typically by an order of magnitude. We recommend moni- toring well-modelled clusters with several known arclets for the detection of cosmologically useful SNe around z = 1 and beyond.
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
Study of the angular size - redshift relation for models in which decays as the energy density
(2000-02-18) Vishwakarma, R. G.
By modifying the Chen and Wu ansatz, we have investigate some Friedmann models in which Λ varies as ρ. In order to test the con sistency of the models with observations, we study the angular size - red shift relation for 256 ultracompact radio sources selected by Jackson an Dodgson. The angular sizes of these sources were determined by using ver long-baseline interferometry in order to avoid any evolutionary eﬀects. Th models ﬁt the data very well and require an accelerating universe with a po itive cosmological constant. Open, ﬂat and closed models are almost equall probable, though the open model provides a comparatively better ﬁt to th data. The models are found to have intermediate density and imply the ex istence of dark matter, though not as much as in the canonical Einstein-d Sitter model.
Negative Energy Condition and Black Holes on the Brane
(2000-02-24) Dadhich, Naresh
We propose that for non-localizable energy distribution the rele- vant energy condition is determined by the gravitational ﬁeld energy which is negative for positive non-gravitational energy. That is neg- ativity of the non-localized energy is the ”positive” energy condition. This would have direct application and relevance for a black hole on the brane which would be sitting in a trace free stresses induced by the Weyl curvature of the bulk.
LRS Bianchi I cosmological universe models with varying cosmological term
(2000-02-25) Pradhan, Anirudh; Kumar, Ambarish
Einstein's eld equations with cosmological term varying with time are considered in the context of general homogeneous, anisotropic universes in a way which conserves the energy{momentum tensor of matter content. It is shown that the eld equations are solvable for any arbitrary cosmic scale functions. Some physical and geometrical features of the models are discussed.
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.
Physical conditions in broad and associated narrow absorption-line systems toward APM 08279+5255
(2000-03-21) Srianand, R.; Petitjean, Patrick
Results of a careful analysis of the absorption systems with zabs≃zem seen toward the bright, zem ∼3.91, gravitationally lensed quasar APM 08279+5255 are pre- sented. Two of the narrow-line systems, at zabs = 3.8931 and zabs = 3.9135, show absorptions from singly ionized species with weak or no N v and O vi absorptions at the same redshift. Absorption due to ﬁne structure transitions of C ii and Si ii (excitation energies cor- responding to, respectively, 156µm and 34µm) are de- tected at zabs = 3.8931. Excitation by IR radiation is favored as the column density ratios are consistent with the shape of APM 08279+5255 IR spectrum. The low- ionization state of the system favors a picture where the cloud is closer to the IR source than to the UV source, supporting the idea that the extension of the IR source is larger than ∼200 pc. The absence of ﬁne structure lines at zabs = 3.9135 suggests that the gas responsible for this system is farther away from the IR source. Abundances are ∼0.01 and 1 Z⊙ at zabs = 3.913 and 3.8931 and alu- minum could be over-abundant with respect to silicon and carbon by at least a factor of two and ﬁve. All this sug- gests that whereas the zabs= 3.8931 system is probably located within 200 pc from the QSO and ejected at a ve- locity larger than 1000 km s−1, the zabs= 3.9135 system is farther away and part of the host-galaxy. Several narrow-line systems have strong absorption lines due to C iv, O vi and N v and very low neutral hy- drogen optical depths. This probably implies metallicities Z ≥ Z⊙ although ﬁrm conclusion cannot be drawn as the exact value depends strongly on the shape of the ionizing spectrum. The C iv broad absorption has a complex structure with mini-BALs (width ≤ 1000 km s−1) and narrow compo-nents superposed on a continuous absorption of smaller optical depth. The continuous absorption is much stronger in O vi indicating that the corresponding gas-component is of higher ionization than the other components in the ﬂow and that absorption structures in the BAL-ﬂow are mainly due to density inhomogeneities. There is a ten- dency for mini-BALs to have diﬀerent covering factors for diﬀerent species. It is shown that a few of the absorbing clouds do not cover all the three QSO images, especially we conclude that the zabs = 3.712 system covers only im- age C. Finally we identify narrow components within the BAL- ﬂow with velocity separations within 5 km s−1 of the O vi, N v and Si iv doublet splittings suggesting that line driven radiative acceleration is an important process to explain the out-ﬂow.
Observational Approaches to the topology of the universe
(2000-03-25) Roukema, B. F.
Many diﬀerent and complementary strategies for translating the basic principle of multiple topological imaging into observational analysis are now available, both for three-dimensional and two-dimensional catalogues.
Using Gravitational Lensing to study damped Lyman - alpha clouds
(2000-03-29) Saini, Tarun Deep; Bharadwaj, Somnath; Sethi, Shiv K.
We investigate the possibility of detecting HI emission from gravitationally lensed HI clouds (akin to damped Lyman-α clouds) at high redshift by carrying out deep radio observations in the ﬁelds of known cluster lenses. Such observations will be possible with present radio telescopes only if the lens substantially magniﬁes the ﬂux of the HI emission. While at present this holds the only possibility of detecting the HI emission from such clouds, it has the disadvantage of being restricted to clouds that lie very close to the caustics of the lens. We ﬁnd that observations at a detection threshold of 50 Jy at 320 MHz (possible with the GMRT) have a greater than 20% probability of detecting an HI cloud in the ﬁeld of a cluster, provided the clouds have HI masses in the range 5 × 108 M⊙ ≤ MHI ≤ 2.5 × 1010 M⊙. The probability of detecting a cloud in- creases if they have larger HI masses, except in the cases where the number of HI clouds in the cluster ﬁeld becomes very small. The probability of a detection at 610 MHz and 233 MHz is comparable to that at 320 MHz, though a deﬁnitive statement is diﬃcult owing to uncertainties in the HI content at the redshifts corresponding to these fre- quencies. Observations at a detection threshold of 2 Jy (possible in the future with the SKA) are expected to detect a few HI clouds in the ﬁeld of every cluster provided the clouds have HI masses in the range 2 × 107 M⊙ ≤ MHI ≤ 109 M⊙. Even if such observations do not result in the detection of HI clouds, they will be able to put useful constraints on the HI content of the clouds.
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.
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.
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.
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.
Structure of the Mg II and damped Lyman- systems along the line of sight to APM 08279+5255
(2000-05-24) Petitjean, Patrick; Aracil, Bastien; Srianand, R.; et al.
A study of the absorption systems toward the gravitationally lensed quasar APM 08279+5255 is pre-sented.Most of the Mg ii systems in the redshift range z ∼ 1.2– 2.07, although saturated, show large residuals at the bot- tom of the lines. The most likely interpretation is that individual clouds within Mg ii halos do cover only one of the two brightest QSO images. The separation between the two lines of sight decreases from 1.7 to 0.7 h−1 75 kpc (qo = 0.5, zlens = 1) between z = 1.22 and z = 2.07. This reveals that Mg ii halos are made of a collection of clouds of radius smaller than about 1 h−1 75 kpc. Two strong Mg ii absorbers at zabs = 1.062 and 1.181 are studied in detail. This is the ﬁrst time that the Na iλ3303 doublet is detected in such high redshift systems. To- gether with the detection of the Mg iλ2852 transition, this strongly constrains the physical characteristics of the gas. The N(Na i)/N(Mg i) ratio is found to be larger than unity, implying that the gas is cool and neutral. The Doppler parameters measured in individual and well de- tached components is probably as small as 1 km s−1. The column densities of Na i, Ca ii, Mg i, Ti ii, Mn ii and Fe ii observed at zabs = 1.1801 are very close to that observed along the line of sight towards 23 Ori in our Galaxy. The shape of the QSO continuum is consistent with attenu- ation by dust at z ∼ 1 (AV ∼ 0.5 mag). Altogether it is found that the H i column density at z = 1 is of the order of 1 to 5 1021 cm−2, the corresponding metallicity is in the range 1–0.3 Z⊙, the overall dust-to-metal ratio is about half that in our Galaxy and the relative deple- tion of iron, titanium, manganese and calcium is similar to what is observed in cool gas in the disk of our Galaxy. The objects associated with these two systems could both con-tribute to the lens together with another possible strong system at zabs = 1.1727 and the strong Lyman-α system at zabs = 2.974. The probable damped Lyman-α system at zabs = 2.974 has 19.8 < log N(H i) < 20.3. The transverse dimension of the absorber is larger than 200 h−1 75 pc. Column densities of Al ii, Fe ii, Si ii, C ii and O i indicate abundances rela- tive to solar of −2.31, −2.26, −2.10, −2.35 and −2.37 for, respectively, Fe, Al, Si, C and O (for log N(H i) = 20.3). These surprizingly similar values indicate that the amount of dust in the cloud is very small as are any deviations from relative solar abundances. It seems likely that the upper limits found for the zinc metallicity of several damped Lyman-α systems at z > 3 in previous surveys is indica- tive of a true cosmological evolution of the metallicity in individual systems.
Simultaneous solution scheme for coupled transonic accretion-wind systems
(2000-06-15) Das, Tapas K.
We discuss a non-self-similar analytical model capable of explaining the formation of accretion-powered galactic and extra-galactic jets.