2002 (IPP)

Permanent URI for this collectionhttp://localhost:4000/handle/11007/627

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Start date: 2010End date: 2012

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Now showing 1 - 10 of 17
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    OVRO CMB anisotropy measurement constraints on flat - ^ and open CDM cosmogonies
    (2012-03-13) Mukherjee, Pia; Souradeep, Tarun; Ratra, Bharat; et al.
    We use Owens Valley Radio Observatory (OVRO) cosmic microwave backgroun (CMB) anisotropy data to constrain cosmological parameters. We account for th OVRO beamwidth and calibration uncertainties, as well as the uncertainty induced b the removal of non-CMB foreground contamination. We consider open and spatially flat-Λ cold dark matter cosmogonies, with nonrelativistic-mass density parameter Ω0 the range 0.1–1, baryonic-mass density parameter ΩB in the range (0.005–0.029)h− and age of the universe t0 in the range (10–20) Gyr. Marginalizing over all paramete but Ω0, the OVRO data favors an open (spatially-flat-Λ) model with Ω0 ≃ 0.33 (0.1 At the 2 σ confidence level model normalizations deduced from the OVRO data a mostly consistent with those deduced from the DMR, UCSB South Pole 1994, Pytho I-III, ARGO, MAX 4 and 5, White Dish, and SuZIE data sets.
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    Stability of accretion disk around rotating black holes: a pseudo-general-relativistic fluid dynamical study
    (2012-03-13) Mukhopadhyay, B.
    We discuss the solution of accretion disk when the black hole is chosen to be rotating. We study, how the fluid properties get affected for different rotation parameters of the black hole. We know that no cosmic object is static in Universe. Here the effect of the rotation of the black hole to the space-time is considered following an earlier work of the author, where the pseudo-Newtonian potential was prescribed for the Kerr geometry. We show that, with the inclusion of rotation of the black hole, the valid disk parameter region dramatically changes and disk becomes unstable. Also we discuss about the possibility of shock in accretion disk around rotating black holes. When the black hole is chosen to be rotating, 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
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    Cosmology with tachyon field as dark energy
    (2011-07-06) Bagla, J. S.; Jassal, H. K.; Padmanabhan, T.
    We present a detailed study of cosmological effects of homogeneous tachyon matter coexisting with non-relativistic matter and radiation, concentrating on the inverse square potential and the expo- nential potential for the tachyonic scalar field. A distinguishing feature of these models (compared to other cosmological models) is that the matter density parameter and the density parameter for tachyons remain comparable even in the matter dominated phase. For the exponential potential, the solutions have an accelerating phase, followed by a phase with a(t) ∝ t 2/3 as t → ∞. This elimi- nates the future event horizon present in ΛCDM models and is an attractive feature from the string theory perspective. A comparison with supernova Ia data shows that for both the potentials there exists a range of models in which the universe undergoes an accelerated expansion at low redshifts and are also consistent with requirements of structure formation. They do require fine tuning of parameters but not any more than in the case of ΛCDM or quintessence models.
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    Cosmological constant - The weight of the vacuum
    (2011-07-06) Padmanabhan, T.
    Recent cosmological observations suggest the existence of a positive cosmological constant Λ with the magnitude Λ(G~/c3) ≈ 10−123. This review discusses several aspects of the cosmological constant both from the cosmological (sections 1–6) and field theoretical (sections 7–11) perspectives. After a brief introduction to the key issues related to cosmological constant and a historical overview, a summary of the kinematics and dynamics of the standard Friedmann model of the universe is provided. The observational evidence for cosmological constant, especially from the supernova results, and the constraints from the age of the universe, structure for- mation, Cosmic Microwave Background Radiation (CMBR) anisotropies and a few others are described in detail, followed by a discussion of the theoretical models (quintessence, tachyonic scalar field, ...) from different perspectives. The latter part of the review (sections 7–11) concentrates on more conceptual and fundamental aspects of the cosmological constant like some alternative interpretations of the cosmological constant, relaxation mechanisms to reduce the cosmological constant to the currently observed value, the geometrical structure of the de Sitter space- time, thermodynamics of the de Sitter universe and the role of string theory in the cosmological constant problem.
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    Core-envelope model of compact stars
    (2011-07-06) Paul, B. C.; Tikekar, R.
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    Cosmological constant problem and quintessence
    (2011-07-06) Sahni, Varun
    I briefly review the cosmological constant problem and the issue of dark energy (or quintessence). Within the framework of quantum field theory, the vacuum expectation value of the energy momentum tensor formally diverges as k4. A cutoff at the Planck or electroweak scale leads to a cosmological constant which is, respectively, 10123 or 1055 times larger than the observed value, Λ/8πG ≃ 10−47 GeV4. The absence of a fundamental symmetry which could set the value of Λ to either zero or a very small value leads to the cosmological constant problem. Most cosmological scenario’s favour a large time-dependent Λ-term in the past (in order to generate inflation at z ≫ 1010), and a small Λ-term today, to account for the current acceleration of the universe at z < ∼ 1. Constraints arising from cosmological nucleosynthesis, CMB and structure formation constrain Λ to be sub-dominant during most of the intermediate epoch 1010 < z < 1. This leads to the cosmic coincidence conundrum which suggests that the acceleration of the universe is a recent phenomenon and that we live during a special epoch when the density in Λ and in matter are almost equal. Time varying models of dark energy can, to a certain extent, ameliorate the fine tuning problem (faced by Λ), but do not resolve the puzzle of cosmic coincidence. I briefly review tracker models of dark energy, as well as more recent brane inspired ideas and the issue of horizons in an accelerating universe. Model independent methods which reconstruct the cosmic equation of state from supernova observations are also assessed. Finally, a new diagnostic of dark energy – ‘Statefinder’, is discussed.
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    Conformally flat spherically symmetric cosmological models-revisited
    (2011-07-06) Pradhan, A.; Pandey, Om Prakash
    Conformally flat spherically symmetric cosmological models rep- resenting a charged perfect fluid as well as a bulk viscous fluid dis- tribution have been obtained. The cosmological constant Λ is found positive and is a decreasing function of time which is consistent with the recent supernovae observations. The physical and geometrical properties of the models are discussed.
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    Collapsing perfect fluid in self-similar five dimensional space-time and cosmic censorship
    (2011-07-06) Ghosh, S. G.; Sarwe, S. B.; Saraykar, R. V.
    We investigate the occurrence and nature of naked singularities in the gravitational collapse of a self-similar adiabatic perfect fluid in a five dimensional space-time. The naked singularities are found to be gravitationally strong in the sense of Tipler and thus violate the cosmic censorship conjecture.
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    Collimated flow driven by radiative pressure from the nucleus of quasar Q 1511 + 091
    (2011-07-06) Srianand, R.; Petitjean, Patrick; Ledoux, C.; et al.
    High velocity outflows from quasars are revealed by the absorption signatures they produce in the spectrum of the quasar. Clues on the nature and origin of these flows are important for our understanding of the dynamics of gas in the central regions of the Active Galactic Nucleus (AGNs) but also of the metal enrichment of the intergalactic space. Line radiation pressure has often been suggested to be an important process in driving these outflows, however no convincing evidence has been given so far. Here we report observation of a highly structured flow, toward Q 1511+091, where the velocity separations between distinct components are similar to O vi, N v and C iv doublet splittings with some of the profiles matching perfectly. This strongly favors the idea that the absorbing clumps originate at similar physical location and are driven by radiative acceleration due to resonance lines. The complex absorption can be understood if the flow is highly collimated so that the different optically thick clouds are aligned and cover the same region of the background source. One component shows saturated H i Lyman series lines together with absorptions from excited levels from C ii and Si ii but covers only 40% of the source of continuum. The fact that clouds cover only part of the small continuum source implies that the flow is located very close to it.
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    CMB Anisotropy Constraints on Flat-Lambda and Open CDM Cosmogonies from DMR, UCSB South Pole, Python, ARGO, MAX, White Dish, OVRO, and SuZIE Data
    (2011-07-06) Mukherjee, Pia; Ken, Ganga; Ratra, Bharat; et al.
    We use joint likelihood analyses of combinations of fifteen cosmic microwave back- ground (CMB) anisotropy data sets from the DMR, UCSB South Pole 1994, Python I–III, ARGO, MAX 4 and 5, White Dish, OVRO, and SuZIE experiments to constrain cosmogonies. We consider open and spatially-flat-Λ cold dark matter cosmogonies, with nonrelativistic-mass density parameter Ω0 in the range 0.1–1, baryonic-mass den- sity parameter ΩB in the range (0.005–0.029)h−2 , and age of the universe t0 in the range (10–20) Gyr. Marginalizing over all parameters but Ω0, the data favor Ω0 ≃ 0.9–1 (0.4–0.6) flat- Λ (open) models. The range in deduced Ω0 values is partially a consequence of the different combinations of smaller-angular-scale CMB anisotropy data sets used in the analyses, but more significantly a consequence of whether the DMR quadrupole moment is accounted for or ignored in the analysis. While the open model is difficult to reconcile with the results of less exact analyses of more recent CMB anisotropy data, the lower values of Ω0 found in this case are more easily reconciled with dynamical estimates of this parameter. For both flat-Λ and open models, after marginalizing over all other parameters, a lower ΩBh2 ≃ 0.005–0.009 is favored. This is also marginally at odds with estimates from more recent CMB anisotropy data and some estimates from standard nucleosynthesis theory and observed light element abundances. For both sets of models a younger universe with t0 ≃ 12–15 Gyr is favored, consistent with other recent non- CMB indicators. We emphasize that since we consider only a small number of data sets, these results are tentative. More importantly, the analyses here do not rule out the currently favored flat-Λ model with Ω0 ∼ 0.3, nor the larger ΩBh2 values favored by some other data.