2002 (IPP)

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Start date: 2002End date: 2009

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Now showing 1 - 10 of 42
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    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 .
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    Relativistic world : A common sense perspective
    (2002-03-01) Dadhich, Naresh
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    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
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    Magnetic helicity in galactic dynamos
    (2002-07-06) Subramanian, Kandaswamy
    Magnetic fields correlated on kiloparsec scales are seen in spiral galaxies. Their origin could be due to amplification of a small seed field by a turbulent galactic dynamo. We review the current status of the galactic dynamo, especially the constraints imposed by magnetic helicity conservation. We estimate the minimal strength of the large-scale magnetic field which could arise inspite of the helicity constraint
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    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.
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    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.
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    Is gravity an intrinsically quantum phenomenon? Dynamics of gravity from the entropy of spacetime and the principle of equivalence
    (2002-05-01) Padmanabhan, T.
    The two surprising features of gravity are (a) the principle of equivalence and (b) the connection between gravity and thermodynamics. Using principle of equivalence and special relativity in the local inertial frame, one could obtain the insight that gravity must possess a geometrical description. I show that, using the same principle of equivalence, special relativity and quantum theory in the local Rindler frame one can obtain the Einstein-Hilbert action functional for gravity and thus the dynamics of the spacetime. This approach, which essentially involves postulating that the horizon area must be proportional to the entropy, uses the local Rindler frame as a natural extension of the local inertial frame and leads to the interpretation that the gravitational action represents the free energy of the spacetime geometry. As an aside, one also obtains a natural explanation as to: (i) why the covariant action for gravity contains second derivatives of the metric tensor and (ii) why the gravitational coupling constant is positive. The analysis suggests that gravity is intrinsically holographic and even intrinsically quantum mechanical.
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    Theoretical models of dark energy
    (2002-04-01) Sahni, Varun
    Observations of high redshift type Ia supernovae indicate that the universe is accelerating, fueled by an unknown form of dark energy having large negative pressure p < 0. The simplest example of dark energy is the cosmological constant (p ¼ q K=8pGÞ. The cosmological constant arises at a fundamental level from one-loop quantum effects which generate a K-term many orders of magnitude larger than the observed value of dark energy 10 47 GeV4 . This leads to the cosmological constant problem . Dynamical models of dark energy include scalar fields with exponential and power law potentials. Dark energy can also be generated in extra-dimensional braneworld models. Model-inde- pendent methods which attempt to reconstruct dark energy from supernova observations are discussed. 2002 Elsevier Science Ltd. All rights reserved
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    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.
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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