2003 (IPP)
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Item Traversable wormholes with arbitrarily small energy condition violations(2011-07-05) Matt, Visser; Kar, Sayan; Dadhich, NareshTraversable wormholes necessarily require violations of the averaged null energy condition; this being the definition of “exotic matter”. However, the theorems which guarantee the energy condi- tion violation are remarkably silent when it comes to making quantitative statements regarding the “total amount” of energy condition violating matter in the spacetime. We develop a suitable mea- sure for quantifying this notion, and demonstrate the existence of spacetime geometries containing traversable wormholes that are supported by arbitrarily small quantities of “exotic matter”.Item VLT-UVEs survey for molecular hydrogen in high-redshift damped lyman-alfa system(2011-07-05) Ledoux, C.; Petitjean, Patrick; Srianand, R.We have searched for molecular hydrogen in damped Lyman-α (DLA) and sub- DLA systems at high redshift (zabs > 1.8) using UVES at the VLT down to a detection limit of typically N(H2) = 2×1014 cm−2. Out of the 33 systems in our sample, 8 have firm and 2 have tentative detections of associated H2 absorption lines. Considering that 3 detections were already known from past searches, molecular hydrogen is detected in 13 to 20 percent of the newly-surveyed systems.We report new detections of molecular hydrogen at zabs = 2.087 and 2.595 toward, respectively, Q1444+014 and Q0405−443, and also reanalyse the system at zabs = 3.025 toward Q0347−383. In all of the systems, we measure metallicities relative to Solar, [X/H] (with either X=Zn, or S, or Si), and depletion factors of Fe, [X/Fe], supposedly onto dust grains, and compare the characteristics of our sample with those of the global population of DLA systems (60 systems in total). We find that there is a correlation between metallicity and depletion factor in both our sample and also the global population of DLA systems. Although H2 molecules are detected in systems with [Zn/Fe] as small as 0.3, the DLA and sub-DLA systems where H2 is detected are usually amongst those having the highest metallicities and the largest depletion factors. In particular, H2 is detected in the five systems having the largest depletion factors. Moreover, the individual components where H2 is detected have depletion factors systematically larger than other components in the profiles. In two different systems, one of the H2- detected components even has [Zn/Fe] > 1.4. These are the largest depletion factors ever seen in DLA systems. All this clearly demonstrates the presence of dust in a large fraction of the DLA systems. The mean H2 molecular fraction, f = 2N(H2)/[2N(H2) + N(Hi)], is generally small in DLA systems (typically log f < −1) and similar to what is observed in the Magellanic Clouds. There is no correlation between the observed amount of H2 and the Hi column density. In fact, two systems where H2 is detected have logN(Hi) < 20.3 and, therefore, are sub-DLA systems. From 58 to 75 percent of the DLA systems have log f < −6. This can be explained if the formation rate of H2 onto dust grains is reduced in those systems, probably because the gas is warm (T > 1000 K) and/or the ionizing flux is enhanced relative to what is observed in our Galaxy.Item viable cosmology with a scalar field coupled to the trace of the stress-tensor(2011-07-05) Sami, S.; Padmanabhan, T.We study the cosmological evolution of a scalar field that couples to the trace T = Ta a of energy momentum tensor of all the fields (including itself). In the case of a shallow exponential potential, the presence of coupling to the trace T in the field equation makes the energy density of the scalar field decrease faster thereby hastening the commencement of radiation domination. This effect gradually diminishes at later epochs allowing the scalar field to dominate the energy density again. We interpret this phase as the current epoch of cosmic acceleration with Ωφ = 0.7. A variant of this model can lead to accelerated expansion at the present epoch followed by a a(t) ∝ t 2/3 behaviour as t → ∞, making the model free from future event horizon. The main features of the model are independent of initial conditions. However, fine tuning of parameters is necessary for viable evolution.Item Variational monte carlo calculations of 5/\He hypernucleus(2011-07-05) Usmani, A . A.; Murtaza, S.We perform a realistic study of L 5 He hypernucleus using variational Monte Carlo technique. The Hamil- tonian for 4 He nuclear core of the hypernucleus is written using Argonne v18 NN potential and Urbana model-IX NNN potential, where N stands for nucleon. For the strange sector, we use phenomenological LN potential having central, spin, and exchange components and LNN potential which includes spin-dependent dispersive force and two-pion exchange force. Using this Hamiltonian and a fully correlated variational wave function, we reproduce the experimental L binding energy. Without three-body LNN potential in the Hamil- tonian and its corresponding correlation in the wave function, the hypernucleus is overbound by 0.56~4! MeV, which is about a quarter of the previous reported values of 2–3 MeV due to the use of central forces. We present the detailed energy breakdown of the hypernucleus and also show the effect of LNN correlation on it. The one-body density profiles for nucleon and L in the hypernucleus and in its nuclear core have been critically examined. The nuclear core polarization due to presence of L is precisely determined.Item Unification to the Pseudo-General-Relativistic Analysis of Accretion Disks around rotating Black Holes and Neutron Stars(2011-07-05) Mukhopadhyay, B.I analyse the relativistic accretion phenomena around rotating black holes and neutron stars and show both the kinds of disk can be treated in an unified manner. The corresponding accretion disks are described by pseudo-Newtonian approach. For this purpose, number of pseudo-Newtonian potentials are in literature, applicable to describe the relativistic properties of accretion disk. While, Kerr metric is used to describe the pseudo-Newtonian potential for accretion disk around black hole, the Hartle-Thorne metric is considered to describe disk around neutron star as the metric can describe continuously the space-time, inside the star as well as out-side of it. Two other potentials were proposed to describe the temporal effects of the accretion disk. All the potentials reproduce the marginally stable and bound orbits approximately or exactly as that of general relativity. These also reproduce the specific mechanical energy approximately. Using these potentials, I study the global parameter space of the accretion disk around black holes and neutron stars. I study, how the fluid properties get affected for different angular momentum of the compact object. I show that, for different angular momenta of the compact object, the valid disk parameter region dramatically changes and disk may become unstable in certain situations. Also I discuss about the possibility of shock in accretion disk around rotating black holes and neutron stars. When the angular momentum of compact object is chosen to be varied, the sonic locations of the accretion disk get shifted or disappear, making the disk unstable. To bring it in the stable situation, the angular momentum of the accreting matter has to be reduced/enhanced (for co/counter-rotating disk) by means of some physical process. I also study, how the fluid properties get changed with different rotations of the black holes, neutron stars and other gravitating central stars. Moreover, I show the effect of viscosity to the fluid properties of the disk. Thus, I find out the unified physical parameter regime, for which the stable accretion disk can be formed. Subsequently, a theoretical prediction of kHz QPO is given, for a fast rotating compact object as 4U 1636-53.Item Size of the longest filaments in the Universe(2011-07-05) Bharadwaj, Somnath; Bhavsar, Suketu P.; Sheth, Jatush V.We analyze the filamentarity in the Las Campanas redshift survey (LCRS) and determine the length scale at which filaments are statistically significant. The largest length-scale at which filaments are statistically significant, real objects, is between 70 to 80 h−1 Mpc, for the LCRS −3o slice. Filamentary features longer than 80 h−1 Mpc, though identified, are not statistically significant; they arise from chance alignments. For the five other LCRS slices, filaments of lengths 50 h−1 Mpc to 70 h−1 Mpc are statistically significant, but not beyond. These results indicate that while individual filaments up to 80 h−1 Mpc are statistically significant, the impression of structure on larger scales is a visual effect. On scales larger than 80 h−1 Mpc the filaments interconnect by statistical chance to form the the filament-void network. The reality of the 80 h−1 Mpc features in the −3o slice make them the longest coherent features in the LCRS. While filaments are a natural outcome of gravitational instability, any numerical model attempting to describe the formation of large scale structure in the universe must produce coherent structures on scales that match these observations.Item Tolman - Bayin type static charged fluid spheres in general relativity(2011-07-05) Ray, Saibal; Das, BasantiIn a static spherically symmetric Einstein-Maxwell spacetime the class of astrophysical solu- tion found out by Ray and Das (2002) and Pant and Sah (1979) are revisited here in connection to the phenomenological relationship between the gravitational and electromagnetic fields. It is qualitatively shown that the charged relativistic stars of Tolman (1939) and Bayin (1978) type are of purely electromagnetic origin. The existence of this type of astrophysical solutions is a probable extension of Lorentz’s conjecture that electron-like extended charged particle possesses only ‘electromagnetic mass’ and no ‘material mass’.Item Relativistic anisotropic charged fluid spheres with varying cosmological constant(2011-07-05) Ray, Saibal; Bhadra, Sumana; Sengupta, AnandStatic spherically symmetric anisotropic source has been studied for the Einstein- Maxwell field equations assuming the erstwhile cosmological constant Λ to be a space-variable scalar, viz., Λ = Λ(r). Two cases are examined out of which one reduces to isotropic sphere. The solutions thus obtained are shown to be electromagnetic in origin as a particular case. It is also shown that the generally used pure charge condition, viz., ρ+pr = 0 is not always required for constructing electromagnetic mass models.Item Time delay interferometry and LISA optimal sensitivity(2011-07-05) Pai, A.; Nayak, K. R.; Dhurandhar, Sanjeev; et al.The sensitivity of LISA depends on the suppression of several noise sources; dominant one is laser frequency noise. It has been shown that the six Doppler data streams obtained from three space-crafts can be appropriately time delayed and optimally combined to cancel this laser frequency noise. We show that the optimal data combinations when operated in a network mode improves the sensitivity over Michelson ranging from 40% to 100%. In this article, we summarize these results. We further show that the residual laser noise in the optimal data combination due to typical arm-length inaccuracy of 200 m is much below the level of optical path and the proof mass noises.Item Theoretician's analysis of the supernova data and the limitations in determining the nature of dark energy(2011-07-05) Padmanabhan, T.; Choudhury, T. RoyCurrent cosmological observations show a strong signature of the existence of a dark en-ergy component with negative pressure. The most obvious candidate for this dark energy is the cosmological constant (with the equation of state wX = p/ρ = −1), which, however,raises several theoretical ifficulties. This has led to models for dark energy componentwhich evolves with time. We discuss certain questions related to the determination of the nature of dark energy component from observations of high redshift supernova. The main results of our analysis are: (i) Even if the precise value of wX is known from observations, it is not possible to determine the nature of the unknown dark energy source using only kinematical and geometrical measurements.We have given explicit examples to show that different types of sources can give rise to a given wX. (ii) Although the full data set of supernova observa- tions (which are currently available) strongly rule out models without dark energy, the high (z > 0.25) and low (z < 0.25) redshift data sets, individually, admit decelerating models with zero dark energy. Any possible evolution in the absolute magnitude of the supernovae, if detected, might allow the decelerating models to be consistent with the data. (iii) We have in- troduced two parameters, which can be obtained entirely from theory, to study the sensitivity of the luminosity distance on wX. Using these two parameters, we have argued that although one can determine the present value of wX accurately from the data, one cannot constrain the evolution of wX.