2002 (IPP)

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Probing the dark ages with redshift distribution of GRBs
(2002-01-01) Roy Choudhury, T.; Srianand, R.
In this article, we explore the possibility of using the properties of gamma ray bursts (GRBs) to probe the physical conditions in the epochs prior to reionization. The redshift distribution of GRBs is modelled using the Press-Schechter formalism with an assumption that they follow the cosmic star formation history. We reproduce the observed star formation rate obtained from galaxies in the redshift range 0 < z < 5, as well as the redshift distribution of the GRBs inferred from the luminosity-variability correlation of the burst light curve.We show that the fraction of GRBs at high redshifts, whose afterglows cannot be observed in R and I band due to HI Gunn Peterson optical depth can, at the most, account for one third of the dark GRBs. The observed redshift distribution of GRBs, with much less scatter than the one available today, can put stringent constraints on the epoch of reionization and the nature of gas cooling in the epochs prior to reionization
Pseudo-Newtonian Potentials to Describe the Temporal Effects on Relativistic Accretion Disks around Rotating Black Holes and Neutron Stars
(2002-01-02) Mukhopadhyay, B.; Misra, Ranjeev
Two pseudo-Newtonian potentials, which approximate the angular and epicyclic frequencies of the relativistic accretion disk around rotating (and counter rotating) compact objects, are presented. One of them, the Logarithmically Modiﬁed Potential, is a better approximation for the frequencies while the other, the Second-order Expanded potential, also reproduces the speciﬁc energy for circular orbits in close agreement with the General Relativistic values. These potentials may be included in time dependent hydrodynamical simulations to study the temporal behavior of such accretion disks
New cosmological singularities in braneworld models
(2002-01-02) Yuri, Shtanov; Sahni, Varun
Higher-dimensional braneworld models which contain both bulk and brane curvature terms in the action admit cosmological singularities of rather un- usual form and nature. These ‘quiescent’ singularities, which can occur both during the contracting as well as the expanding phase, are characterised by the fact that while the matter density and Hubble parameter remain ﬁnite, all higher derivatives of the scale factor ( .. a, ... a etc.) diverge as the cosmological singularity is approached. The singularities are the result of the embedding of the (3+1)-dimensional brane in the bulk and can exist even in an empty homogeneous and isotropic (FRW) universe. The possibility that the present universe may expand into a singular state is discussed.
THERMODYNAMICS AND/OF HORIZONS: A COMPARISION OF SCHWARZSCHILD, RINDER AND de SITTER SPACETIMES
(2002-02-01) Padmanabhan, T.
The notions of temperature, entropy and ‘evaporation’, usually associated with space- times with horizons, are analyzed using general approach and the following results, ap- plicable to diﬀerent spacetimes, are obtained at one go. (i) The concept of temperature associated with the horizon is derived in a uniﬁed manner and is shown to arise from purely kinematic considerations. (ii) QFT near any horizon is mapped to a conformal ﬁeld theory without introducing concepts from string theory. (iii) For spherically sym- metric spacetimes (in D = 1 + 3) with a horizon at r = l, the partition function has the generic form Z ∝ exp[S − βE], where S = (1/4)4πl 2 and |E| = (l/2). This analysis reproduces the conventional result for the blackhole spacetimes and provides a simple and consistent interpretation of entropy and energy for deSitter spacetime. (iv) For the Rindler spacetime the entropy per unit transverse area turns out to be (1/4) while the energy is zero. (v) In the case of a Schwarzschild black hole there exist quantum states (like Unruh vacuum) which are not invariant under time reversal and can describe blackhole evaporation. There also exist quantum states (like Hartle-Hawking vacuum) in which temperature is well-deﬁned but there is no ﬂow of radiation to inﬁnity. In the case of deSitter universe or Rindler patch in ﬂat spacetime, one usually uses quantum states analogous to Hartle-Hawking vacuum and obtains a temperature without the cor- responding notion of evaporation. It is, however, possible to construct the analogues of Unruh vacuum state in the other cases as well. Associating an entropy or a radiating vacuum state with a general horizon raises conceptual issues which are brieﬂy discussed.
On the spin of gravitational bosons
(2002-02-01) Ahluwalia, D.V.; Dadhich, Naresh; Kirchbach, M.
We unearth spacetime structure of massive vector bosons, gravitinos, and gravitons. While the curvatures associated with these particles carry a deﬁnite spin, the un- derlying potentials cannot be, and should not be, interpreted as single spin objects. For instance, we predict that a spin measurement in the rest frame of a massive gravitino will yield the result 3/2 with probability one half, and 1/2 with probabil- ity one half. The simplest scenario leaves the Riemannian curvature unaltered; thus avoiding conﬂicts with classical tests of the theory of general relativity. However, the quantum structure acquires additional contributions to the propagators, and it gives rise to additional phases
Statefinder - anew geometrical diagnostic of dark energy
(2002-02-21) Sahni, Varun; Saini, Tarun Deep; Starobinsky, A. A.
We introduce a new cosmological diagnostic pair {r, s} called Stateﬁnder. The Stateﬁnder is a geometrical diagnostic and allows us to characterize the properties of dark energy in a model independent manner. The Stateﬁnder is dimensionless and is constructed from the scale factor of the Universe and its time derivatives only. The parameter r forms the next step in the hierarchy of geometrical cosmological parameters after the Hubble parameter H and the deceleration parameter q, while s is a linear combination of q and r chosen in such a way that it does not depend upon the dark energy density. The Stateﬁnder pair {r, s} is algebraically related to the equation of state of dark energy and its ﬁrst time derivative. The Stateﬁnder pair is calculated for a number of existing models of dark energy having both constant and variable w. For the case of a cosmological constant the Stateﬁnder acquires a particularly simple form. We demonstrate that the Stateﬁnder diagnostic can eﬀectively diﬀerentiate between diﬀerent forms of dark energy. We also show that the mean Stateﬁnder pair can be determined to very high accuracy from a SNAP-type experiment.
Non-marginally bound inhomogeneous dust collapse in higher dimensional space-time
(2002-03-01) Ghosh, S. G.; Banerjee, A.
We investigate the occurrence and nature of a naked singularity in the gravitational collapse of an inhomogeneous dust cloud described by a self-similar higher dimensional Tolman-Bondi space-time. Bound, marginally bound and unbound space-times are analyzed. The degree of inhomogeneity of the collapsing matter necessary to form a naked singularity is given.
Plane-symmetric inhomogeneous bulk viscous cosmological models with variables /\
(2002-03-01) Pradhan, A.; Pandey, H. R.
A plane-symmetric non-static cosmological model representing a bulk viscous ﬂuid distribution has been obtained which is inhomogeneous and anisotropic and a particular case of which is gravitationally radiative. Without assuming any adhoc law, we obtain a cosmological constant as a decreasing function of time. The physical and geometric features of the models are also discussed
Difference boosting neural network for automated star-galaxy classiﬁcation
(2002-03-01) Philip, Ninan S.; Wadadekar, Yogesh; Kembhavi, A.K.; et al.
In this paper we describe the use of a new artiﬁcial neural network, called the diﬀerence boosting neural network (DBNN), for automated classiﬁcation problems in astronomical data analysis. We illustrate the capabilities of the network by applying it to star galaxy classiﬁcation using recently released, deep imaging data. We have compared our results with classiﬁcation made by the widely used Source Extractor (SExtractor) package. We show that while the performance of the DBNN in star-galaxy classiﬁcation is comparable to that of SExtractor, it has the advantage of signiﬁcantly higher speed and ﬂexibility during training as well as classiﬁcation.
Statistical mechanics of gravitating systems in static and cosmological backgrounds
(2002-03-01) Padmanabhan, T.
This pedagogical review addresses several issues related to statistical de- scription of gravitating systems in both static and expanding backgrounds, focusing on the latter. After brieﬂy reviewing the results for the static background, I describe the key issues and recent progress in the context of gravitational clustering of collision-less particles in an expanding universe. The questions addressed include: (a) How does the power injected into the system at a given wave number spread to smaller and larger scales? (b) How does the power spectrum of density ﬂuctuations behave asymptotically at late times? (c) What are the universal characteristics of gravitational clustering that are independent of the initial conditions and manifest at the late time evolution of the system? The review is intended for non cosmologists and will be of interest to people working in ﬂuid mechanics, non linear dynamics and condensed matter physics.
Diamagnetic screening of the magnetic field in accreting neutron stars
(2002-03-01) Konar, Sushan; Choudhuri, Arnab Rai
A possible mechanism for screening of the surface magnetic ﬁeld of an accreting neutron star, by the accreted material, is investigated. In particular, we investigate the nature of the evolution of the internal ﬁeld conﬁguration in the case of a) a polar cap accretion and b) a spherical accretion.
Stability of strange stars (SS) derived from a realistic equation of state
(2002-03-01) Sinha, Monika; Dey, Jishnu; Dey, Mira
A realistic EOS (equation of state) leads to strange stars (ReSS) which are compact in the mass radius plot, close to the Schwarzchild limiting line [1]. Many of the observed stars ﬁt in with this kind of compactness, irrespective of whether they are X-ray pulsars, bursters or soft γ repeaters or even radio pulsars. We point out that a change in the radius of a star can be small or large, when its mass is increasing and this depends on the position of a particular star on the mass radius curve. We carry out a stability analysis against radial oscillations and compare with the EOS of other SS models. We ﬁnd that the ReSS is stable and an M-R region can be identiﬁed to that eﬀect
Entropy and energy of a class of spacetimes with horizon : a general derivation
(2002-03-01) Padmanabhan, T.
Euclidean continuation of several Lorentzian spacetimes with horizons requires treating the Eu- clidean time coordinate to be periodic with some period β. Such spacetimes (Schwarzschild, de- Sitter,Rindler .....) allow a temperature T = β−1 to be associated with the horizon. I construct a canonical ensemble of a subclass of such spacetimes with a ﬁxed value for β and evaluate the par- tition function Z(β). For spherically symmetric spacetimes with a horizon at r = a, the partition function has the generic form Z ∝ exp[S −βE], where S = (1/4)4πa2 and |E| = (a/2). Both S and E are determined entirely by the properties of the metric near the horizon. This analysis reproduces the conventional result for the blackhole spacetimes and provides a simple and consistent interpre- tation of entropy and energy for deSitter spacetime. For the Rindler spacetime the entropy per unit transverse area turns out to be (1/4) while the energy is zero. The implications are discussed.
Relativistic world : A common sense perspective
(2002-03-01) Dadhich, Naresh
Machian model of dark energy
(2002-03-02) Vishwakarma, R. G.
Einstein believed that Mach’s principle should play a major role in ﬁnding a meaningful spacetime geometry, though it was discovered later that his ﬁeld equations gave some solutions which were not Machian. It is shown, in this essay, that the kinematical Λ mod- els, which are invoked to solve the cosmological constant problem, are in fact consistent with Mach’s ideas. One particular model in this category is described which results from the microstructure of space- time and seems to explain the current observations successfully and also has some beneﬁts over the conventional models. This forces one to think whether the Mach’s ideas and the cosmological constant are interrelated in some way.
Neutrino propagation in a weakly magnetized medium
(2002-03-02) Konar, Sushan; Das, Subinoy
Neutrino-photon processes, forbidden in vacuum, can take place in the presence of a thermal medium and/or an external electro-magnetic ﬁeld, mediated by the corresponding charged leptons (real or virtual). Such interactions affect the propagation of neutrinos through a magnetized plasma. We investigate the neutrino-photon absorptive processes, at the one-loop level, for massless neutrinos in a weakly magnetized plasma. We ﬁnd that there is no correction to the absorptive part of the axial-vector–vector amplitude due to the presence of a magnetic ﬁeld to the linear order in the ﬁeld strength.
New measurement of sinc metallicity in a DLA at z ~ 3.35
(2002-03-02) Peroux, C.; Petitjean, Patrick; Aracil, Bastien
We present chemical abundance measurements in the zabs = 3.35045 Damped Lyman-α (DLA) system observed in the UVES spectrum of the BAL quasar BR 1117−1329. We measure a neutral hydrogen column density N(HI) = 6.9±1.7×1020 atoms cm−2 and derive mean abundances relative to solar: [Si/H] = −1.26 ± 0.13, [Fe/H] = −1.51±0.13, [Ni/H] = −1.57±0.13, [Cr/H] = −1.36±0.13, [Zn/H] = −1.18±0.13, [Al/H] > −1.25, [O/H] > −1.25 and [N/H] < −2.24. This is the third measurement of Zn, an element mildly depleted onto dust grain, at zabs > 3. The iron to zinc and chromium to zinc ratios, [Fe/Zn] = −0.33 ± 0.05 and [Cr/Zn] = −0.18 ± 0.05 demonstrate that the absorber has a low dust content. The nitrogen ratio [N/Si] < −0.98 suggests that the “secondary” N production process is taking place in this DLA. Finally, this absorber does not seem to present a convincing α- enhancement as shown by the α over Fe-peak element ratios: [Si/Fe] = 0.25±0.06, [Si/Cr] = 0.10 ± 0.06 and [Si/Zn] = −0.08 ± 0.06.
Why do we observe a small but non zero cosmological constant?
(2002-03-03) Padmanabhan, T.
The current observations seem to suggest that the universe has a positive cosmological constant of the order of H2 0 while the most natural value for the cosmological constant will be L−2 P where LP = (G¯ h/c3)1/2 is the Planck length. This reduction of the cosmological constant from L−2 P to L−2 P (LPH0)2 may be interpreted as due to the ability of quantum micro structure of spacetime to readjust itself and absorb bulk vacuum energy densities. Being a quantum mechanical process, such a cancellation cannot be exact and the residual quantum ﬂuctuations appear as the “small” cosmological constant. I describe the features of a toy model for the spacetime micro structure which could allow for the bulk vacuum energy densities to be canceled leaving behind a small residual value of the the correct magnitude. Some other models (like the ones based on canonical ensemble for the four volume or quantum ﬂuctuations of the horizon size) lead to an insigniﬁcantly small value of H2 0 (LPH0)n with n = 0.5 − 1 showing that obtaining the correct order of magnitude for the residual ﬂuctuations in the cosmological constant is a nontrivial task, becaue of the existence of the small dimensionless number H0LP .
Molecular hydrogen at Zabx=1.973 towards Q0013 - 004: Dust depletion pattern in damped Lyman - alpha systems
(2002-03-06) Petitjean, Patrick; Srianand, R.; Ledoux, C.
We study the dust depletion pattern in different well separated components of the Zabs = 1.973, log N(H I) = 20.83, damped Lyman-a system toward Q 0013-004. The apparent correlation between [Fe/S] and [Si/S] in the components indicates that the abundance pattern is indeed due to dust-depletion. In particular, we find evidence for depletion similar to what is observed in cold gas of the Galactic disk in one of the weakest components ([Fe/Zn] = -1.62, [Fe/S]= -1.82, [Zn/S] = -0.2, [Si/S]= -0.92) in which molecular hydrogen is detected with log N(H2) '" 16.5. This is the first time that such depletion is seen in a DLA system. Extinction due to this component is negligible owing to small total HI column density, log N(HI ) ::;;19.4.This observation supports the possibility that current samples of DLA systems might be biased against the presence of cold and dusty gas along the line of sight. The global metallicities of this peculiar DLA system in which 0 I and C II are spread over ",1050 km S-1 are [P/H] = -0.64, [Zn/H] = -0.75 and [S/H] = -0.76 relative to solar. The overall molecular fraction is in the range -2.7 < log f < -0.6. which is the highest value found for DLA systems. H2 is detected in four components at -625, -475, 0 and 80 km S-1 relative to the strongest component at Zabs = 1.97296.CO is not detected (log N(CO)/ N(H r) < -8) and HD could be present at Zabs = 1.97380. We show that the presence of H2 is closely related to the physical conditions in the gas: high particle density together with low temperature. Excitation of high J levels and molecular fraction vary largely from one component to the ot.her suggesting that the UV radiation field is highly inhomogeneous through the system. Gas pressure, estimated from C I absorptions, is larger than what is observed in the ISM of our Galaxy. This, together with the complex kinematics, suggests that part of the gas is subject to high compression due to either collapse, merging and/or supernovae explosion. This is probably a consequence of star-formation activity in the vicinity of the absorbing gas
Why do naked singularities form in gravitational
(2002-03-12) Joshi, P. S.; Dadhich, Naresh; Roy, Maartens
We investigate what are the key physical features that cause the development of a naked singularity, rather than a black hole, as the end-state of spherical gravitational collapse. We show that suﬃciently strong shearing eﬀects near the singularity delay the formation of the apparent horizon. This exposes the singularity to an external observer, in contrast to a black hole, which is hidden behind an event horizon due to the early formation of an apparent horizon