2002 (IPP)

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Accelerated expansion of the universe driven by tachyonic matter
(2011-07-06) Padmanabhan, T.
It is an accepted practice in cosmology to invoke a scalar ﬁeld with potential V (φ) when observed evolution of the universe cannot be reconciled with theoretical prejudices. Since one function-degree- of-freedom in the expansion factor a(t) can be traded oﬀ for the function V (φ), it is always possible to ﬁnd a scalar ﬁeld potential which will reproduce a given evolution. I provide a recipe for determining V (φ) from a(t) in two cases: (i) Normal scalar ﬁeld with Lagrangian L = (1/2)∂aφ∂aφ−V (φ) used in quintessence/dark energy models; (ii) A tachyonic ﬁeld with Lagrangian L = −V (φ)[1−∂aφ∂aφ] 1/2 , motivated by recent string theoretic results. In the latter case, it is possible to have accelerated expansion of the universe during the late phase in certain cases. This suggests a string theory based nterpretation of the current phase of the universe with tachyonic condensate acting as eﬀective cosmological constant.
Aspects of Tachyonic inflation with exponential potential
(2011-07-06) Sami, M.; Chingangbam, Pravabati; Qureshi, Tabish
We consider issues related to tachyonic inﬂation with exponential potential. We ﬁnd exact solution of evolution equations in the slow roll limit in FRW cosmology. We also carry out similar analysis in case of Brane assisted tachyonic inﬂation. We investigate the phase space behavior of the system and show that the dust like solution is a late time attractor. The diﬃculties associated with reheating in the tachyonic model are also indicated.
Braneworld models of dark energy
(2011-07-06) Sahni, Varun; Shtanov, Yuri
We explore a new class of braneworld models in which the scalar curvature of the (induced) brane metric contributes to the brane action. The scalar curvature term arises generically on account of one-loop eﬀects induced by matter ﬁelds residing on the brane. Spatially ﬂat braneworld models can en- ter into a regime of accelerated expansion at late times. This is true even if the brane tension and the bulk cosmological constant are tuned to satisfy the Randall–Sundrum constraint on the brane. Braneworld models admit a wider range of possibilities for dark energy than standard LCDM. In these models the luminosity distance can be both smaller and larger than the lu- minosity distance in LCDM. Whereas models with dL ≤ dL(LCDM) imply w = p/ρ ≥ −1 and have frequently been discussed in the literature, mod- els with dL > dL(LCDM) have traditionally been ignored, perhaps because within the general-relativistic framework, the luminosity distance has this property only if the equation of state of matter is strongly negative (w < −1). Within the conventional framework, ‘phantom energy’ with w < −1 is beset with a host of undesirable properties, which makes this model of dark en- ergy unattractive. Braneworld models, on the other hand, have the capacity to endow dark energy with exciting new possibilities (including w < −1) without suﬀering from the problems faced by phantom energy. For a subclass of parameter values, braneworld dark energy and the acceleration of the universe are transient phenomena. In these models, the universe, after the current period of acceleration, re-enters the matter-dominated regime so that the deceleration parameter q(t) → 0.5 when t ≫ t0, where t0 is the present epoch. Such models could help reconcile an accelerating universe with the requirements of string/M-theory.
Bulk viscous solutions to the field equations and the deceleration parameter - revisited
(2011-07-06) Pradhan, Anirudh; Lotemshi, I.
We utilise a form for the Hubble parameter to generate a number of solutions to the Einstein ﬁeld equations with variable cosmological constant and variable gravitational constant in the presence of a bulk viscous ﬂuid. The Hubble law utilised yields a constant value for the deceleration parameter. A new class of solutions is presented in the Robertson-Walker spacetimes. The coefﬁcient of bulk vis- cosity is assumed to be a power function of the mass density. For a class of solutions, the deceleration parameter is negative which is consistent with the supernovae Ia observations.
Can the clustered dark matter and the smooth dark energy arise from the same scalar field?
(2011-07-06) Padmanabhan, T.; Choudhury, T. Roy
Cosmological observations suggest the existence of two diﬀerent kinds of energy densities domi- nating at small (< ∼ 500 Mpc) and large (> ∼ 1000 Mpc) scales. The dark matter component, which dominates at small scales, contributes Ωm ≈ 0.35 and has an equation of state p = 0, while the dark energy component, which dominates at large scales, contributes ΩV ≈ 0.65 and has an equation of state p ≃ −ρ. It is usual to postulate weakly interacting massive particles (WIMPs) for the ﬁrst component and some form of scalar ﬁeld or cosmological constant for the second component. We explore the possibility of a scalar ﬁeld with a Lagrangian L = −V (φ) p1 − ∂iφ∂iφ acting as both clustered dark matter and smoother dark energy and having a scale-dependent equation of state. This model predicts a relation between the ratio r = ρV /ρDM of the energy densities of the two dark components and expansion rate n of the universe [with a(t) ∝ t n] in the form n = (2/3)(1 +r). For r ≈ 2, we get n ≈ 2 which is consistent with observations.
Classical and quantum thermodynamics of horizons in spherically symmetric spacetimes
(2011-07-06) Padmanabhan, T.
A general formalism for understanding the thermodynamics of horizons in spherically symmetric spacetimes is developed. The formalism reproduces known results in the case of black hole spacetimes and can handle more general situations like: (i) spacetimes which are not asymptotically ﬂat (like the de Sitter spacetime) and (ii) spacetimes with multiple horizons having diﬀerent temperatures (like the Schwarzschild-de Sitter spacetime) and provide a consistent interpretation for temperature, entropy and energy. I show that it is possible to write Einstein’s equations for a spherically symmetric spacetime in the form TdS −dE = PdV near any horizon of radius a with S = (1/4)(4πa2), |E| = (a/2) and the temperature T determined from the surface gravity at the horizon. The pressure P is provided by the source of the Einstein’s equations and dV is the change in the volume when the horizon is displaced inﬁnitesimally. The same results can be obtained by evaluating the quantum mechanical partition function without using Einstein’s equations or WKB approximation for the action. Both the classical and quantum analysis provide a simple and consistent interpretation of entropy and energy for de Sitter spacetime as well as for (1 + 2) dimensional gravity. For the Rindler spacetime the entropy per unit transverse area turns out to be (1/4) while the energy is zero. The approach also shows that the de Sitter horizon — like the Schwarzschild horizon — is eﬀectively one dimensional as far as the ﬂow of information is concerned, while the Schwarzschild-de Sitter, Reissner- Nordstrom horizons are not. The implications for spacetimes with multiple horizons are discussed.
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.
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.
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.
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.
Core-envelope model of compact stars
(2011-07-06) Paul, B. C.; Tikekar, R.
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.
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.
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.
Description of pseduo-Newtonina potential for the relativistic accretion disk around kerr black holes
(2002-04-01) Mukhopadhyay, B.
We present a pseudo-Newtonian potential for accretion disk modeling around the rotating black holes. This potential can describe the general relativistic eﬀects on accretion disk. As the inclusion of rotation in a proper way is very important at an inner edge of disk the potential is derived from the Kerr metric. This potential can reproduce all the essential properties of general relativity within 10% error even for rapidly rotating black holes.
Detection of molecular hydrogen in a near solar-metallicity damped Lyman - alpha system at Zabc ~ 2 toward Q 0551 - 366 Authors: Ledoux, C.
(2002-05-01) Ledoux, C.; Srianand, R.; Petitjean, Patrick
We report the detection of H2, C I, C 1*, C I ** and Cl I lines in a near Solar-metallicity ([Zn/H] = -0.13) damped Lyman-a (DLA) system at Zabs = 1.962 observed on the line of sight to the quasar Q 0551-366. The iron-peak elements, X = Fe, Cr and Mn are depleted compared to zinc, [X/Zn] "'" -0.8, probably because they are tied up onto dust grains. Among the three detected Hs-bearing clouds, spanning 55 km S-l in velocity space, we derive a total molecular hydrogen column density N(H2) = 2.6 X 1017 cm- 2 and a mean molecular fraction f = 2N(H2)/(2N(H2) + N(H I» = 1.7 x 10- 3. The depletion of heavy elements (S, Si, Mg, Mn, Cr, Fe, Ni and Ti) in the central component is similar to that observed in the diffuse neutral gas of the Galactic halo. This depletion is approximately the same in the six C I-detected components independently of the presence or absence of H2. The gas clouds in which H2 is detected always have large densities, nH > 30 cm- 3 , and low temperatures, T01 ~ 100 K. This shows that presence of dust, high particle density and/or low temperature are required for molecules to be present. The photo-dissociation rate derived in the components where H2 is detected suggests the existence of a local UV radiation field similar in strength to the one in the Galaxy. Star formation therefore probably occurs near these H2-bearing clouds.
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.
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.
Discovery of giant 'radio arcs' in cluster Abell 3376 : evidence for shock accleration in a violent cluster merger?
(2002-10-24) Bagchi, Joydeep
New multi-wavelength (radio, optical & X-rays) observational evidences are presented which show that the nearby (z = 0.046), rich cluster of galaxies Abell 3376 is experiencing a major event of binary subcluster merger. The key evidence is the discovery of a pair of large, optically unidentiﬁed diﬀuse radio sources (‘arcs’), symmetrically located about 2.6 h−1 50 Mpc apart at the opposite ends of the hot intra-cluster gas mapped by ROSAT in X-rays. It is argued that the gas-dynamical shock-waves, which occur naturally during cluster formation, are accelerating charged particles (cosmic rays) to relativis- tic energies, leading to synchrotron emission from the megaparsec scale radio arcs. If this is so, cluster Abell 3376 would also be a potential source capa- ble of accelerating cosmic ray particles upto ultra-high energies (UHECR) of Emax ∼ 1018−19 eV. Thus this cluster is an excellent test-bed for understand- ing the physics of merger shocks and origin of enigmatic UHECR particles in structure formation process. Hence, Abell 3376 provides unique oppurtu- nities for further multi-wavelength observations with ground and space-borne observatories
Effect of dissipative corona on the structure and stability of cold optically thick accretion disks at high accretion rates
(2002-03-20) Misra, Ranjeev; Taam, Ronald E.
The vertical structure of optically thick accretion disks is investigated in the two-zone approximation. The disk is divided into an underlying disk and a corona, where the latter is deﬁned as the upper surface layers for which the eﬀective optical depth is unity. It is found that a signiﬁcant part of the accretion ﬂow (or dissipation rate) can take place in the corona if the scale height of the magnetic ﬁeld is larger than that of the disk. The presence of such a dissipative corona leads to a modiﬁcation in the topology of local disk solutions. For example, these solutions are found from local stability analysis to be both secularly and thermally stable, for accretion rates which are a factor ≈ four higher than those inferred from the stability of standard disk solutions. Thus, the applicability of optically thick disks with dissipative coronas are not as restrictive as disks without such coronas and can provide an attractive explanation for the origin of the soft spectral component observed in black hole X-ray binary systems.