2003 (IPP)

Permanent URI for this collectionhttp://localhost:4000/handle/11007/626

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Bouncing Braneworlds
(2011-07-05) Shtanov, Yuri; Sahni, Varun
We study cosmological braneworld models with a single timelike extra di- mension. Such models admit the intriguing possibility that a contracting braneworld experiences a natural bounce without ever reaching a singular state. This feature persists in the case of anisotropic braneworlds under some additional and not very restrictive assumptions. Generalizing our study to braneworld models containing an induced brane curvature term, we ﬁnd that a FRW-type singularity is once again absent if the bulk extra dimension is timelike. In this case, the universe either has a non-singular origin or com- mences its expansion from a quasi-singular state during which both the Hubble parameter and the energy density and pressure remain ﬁnite while the cur- vature tensor diverges. The non-singular and quasi-singular behaviour which we have discovered diﬀers both qualitatively and quantitatively from what is usually observed in braneworld models with spacelike extra dimensions and could have interesting cosmological implications.
Brane corresponding to the nariai bulk
(2011-07-05) Dadhich, Naresh; Shtanov, Yuri
We consider the ﬁve-dimensional bulk spacetime with negative Λ described by the Nariai metric (which is not conformally ﬂat) and match it with a vacuum brane satisfying the proper boundary conditions. It is shown that the brane metric corresponds to a cloud of string dust of constant energy density.
Can brane cosmology with a vanishing /\ explain the observations?
(2011-07-05) Vishwakarma, R. G.; Singh, Parampreet
A plethora of models of the universe have been proposed in recent years claiming that the present universe is accelerating, being driven by some hypothetical source with negative pressure collectively known as dark energy which though do not appear to resemble any known form of matter tested in the laboratory. These models are motivated by the high redshift supernovae Ia observations. Though low density models, without dark energy, also appear to ﬁt the SN Ia data reasonably well, however, they are ruled out by the CMB observations. In this paper, we present a warped brane model with an additional surface term of brane curvature scalar in the action. This results in shifting the dynamical curvature of the model from its geometrical counterpart, which creates profound consequences. Even for Λ = 0, the low energy decelerating model successfully explains the observed locations of the peaks in the angular power spectrum of CMB. This model also ﬁts the high redshift supernovae Ia observations, taken together with the recently observed SN 1997ﬀ at z ≈ 1.7, very well. Additionally, it also ﬁts the data on the angular size and redshift of the compact radio sources very well.
Can dark energy be decaying?
(2011-07-05) Ujjaini, Alam; Sahni, Varun; Starobinsky, A. A.
We explore the fate of the universe given the possibility that the density associated with ‘dark energy’ may decay slowly with time. Decaying dark energy is modeled by a homogeneous scalar ﬁeld which couples minimally to gravity and whose potential has at least one local quadratic maximum. Dark energy decays as the scalar ﬁeld rolls down its potential, consequently the current acceleration epoch is a transient. We examine two models of decaying dark energy. In the ﬁrst, the dark energy potential is modeled by an analytical form which is generic close to the potential maximum. The second potential is the cosine, which can become negative as the ﬁeld evolves, ensuring that a spatially ﬂat universe collapses in the future. We examine the feasibility of both models using observations of high redshift type Ia supernovae. A maximum likelihood analysis is used to ﬁnd allowed regions in the {m, φ0} plane (m is the tachyon mass modulus and φ0 the initial scalar ﬁeld value; m ∼ H0 and φ0 ∼ MP by order of magnitude). For the ﬁrst model, the time for the potential to drop to half its maximum value is larger than ∼ 8 Gyrs. In the case of the cosine potential, the time left until the universe collapses is always greater than ∼ 18 Gyrs (both estimates are presented for Ω0m = 0.3, m/H0 ∼ 1, H0 ≃ 70 km/sec/Mpc, and at the 95.4% conﬁdence level).
Classical electron model with negative energy density in Einstein-Cartan theory of gravitation
(2011-07-05) Ray, Saibal; Bhadra, Sumana
Experimental result regarding the maximum limit of the radius of the electron ∼ 10−16 cm and a few of the theoretical works suggest that there might have some negative energy density regions within the particle in general theory of relativity. It is argued in the present investigation that such a negative energy density also can be obtained with a better physical interpretation in the framework of Einstein-Cartan theory.
Cosmological aspects of rolling tachyon
(2011-07-06) Sami, M.
We examine the possibility of rolling tachyon to play the dual roll of inﬂaton at early epochs and dark matter at late times. We argue that enough inﬂation can be generated with the rolling tachyon either by invoking the large number of branes or brane world assisted inﬂation. However, reheating is problematic in this model.
Cosmological Dynamics of Phantom Field
(2011-07-05) Singh, Parampreet; Sami, M.; Dadhich, Naresh
We study the general features of the dynamics of the phantom ﬁeld in the cosmological context. In the case of inverse coshyperbolic potential, we demonstrate that the phantom ﬁeld can successfully drive the observed current accelerated expansion of the universe with the equation of state parameter wφ < −1. The de-Sitter universe turns out to be the late time attractor of the model. The main features of the dynamics are independent of the initial conditions and the parameters of the model. The model ﬁts the supernova data very well, allowing for −2.4 < wφ < −1 at 95 % conﬁdence level
Effects of curvature and interactions on the dynamics of the deconfinement phase transition
(2011-07-05) Chandra, Deepak; Goyal, Ashok
We study the dynamics of ﬁrst-order coﬁnement-deconﬁnement phase transition through nucleation of hadronic bubbles in an expanding quark gluon plasma in the context of heavy ion collisions for interacting quark and hadron gas and by incorporating the eﬀects of curvature energy. We ﬁnd that the interactions reduce the delay in the phase transition whereas the curvature energy has a mixed behavior. In contrast to the case of early Universe phase transition, here lower values of surface tension increase the supercooling and slow down the hadronization process. Higher values of bag pres- sure tend to speed up the transition. Another interesting feature is the start of the hadronization process as soon as the QGP is created.
Effects of curvature and interactions on the dynamics of the deconfinement phase transition
(2011-07-06) Subramanian, Kandaswamy; Seshadri, T. R.; Barrow, John D.
Energy density in general relativity : a possible role of cosmological constant
(2011-07-05) Ray, Saibal; Bhadra, Sumana
We consider a static spherically symmetric charged anisotropic ﬂuid source of radius ∼ 10−16 cm by introducing a variable Λ dependent on the radial coordinate r under general relativity. From the solution sets a possible role of the cosmological constant is investigated which indicates the dependency of energy density on it.
Exploring the expanding universe and dark energy using the statefinder diagnostic
(2011-07-05) Ujjaini, Alam; Sahni, Varun; Saini, Tarun Deep; et al.
The coming few years are likely to witness a dramatic increase in high quality Sn data as current surveys add more high redshift supernovae to their inventory and as newer and deeper supernova experiments become operational. Given the current variety in dark energy models and the expected improvement in observational data, an accurate and versatile diagnostic of dark energy is the need of the hour. This paper examines the Stateﬁnder diagnostic in the light of the proposed SNAP satellite which is expected to observe about 2000 supernovae per year. We show that the Stateﬁnder is versatile enough to diﬀerentiate between dark energy models as varied as the cosmological constant on the one hand, and quintessence, the Chaplygin gas and braneworld models, on the other. Using SNAP data, the Stateﬁnder can distinguish a cosmological constant (w = −1) from quintessence models with w > −0.9 and Chaplygin gas models with κ 6 15 at the 3σ level if the value of Ωm is known exactly. The Stateﬁnder gives reasonable results even when the value of Ωm is known to only ∼ 20% accuracy. In this case, marginalizing over Ωm and assuming a ﬁducial LCDM model allows us to rule out quintessence with w > −0.85 and the Chaplygin gas with κ 6 7 (both at 3σ). These constraints can be made even tighter if we use the Stateﬁnders in conjunction with the deceleration parameter. The Stateﬁnder is very sensitive to the total pressure exerted by all forms of matter and radiation in the universe. It can therefore diﬀerentiate between dark energy models at moderately high redshifts of z < 10.
A faster implementation of the hierarchical search algorithm for detection of gravitational waves from inspiraling compact binaries
(2011-06-05) Sengupta, Anand; Dhurandhar, Sanjeev; Lazzarini, Albert
The ﬁrst scientiﬁc runs of kilometer scale laser interferometric detectors like LIGO are underway. Data from these detectors will be used to look for signatures of gravitational waves (GW) from astrophysical objects like inspiraling neutron star/blackhole binaries using matched ﬁltering. The computational resources required for online ﬂat-search implementation of the matched ﬁltering are large if searches are carried out for small total mass. Flat search is implemented by constructing a single discrete grid of densely populated template waveforms spanning the dynamical parameters - masses, spins - which are correlated with the interferometer data. The correlations over the kinematical parameters can be maximized apriori without constructing a template bank over them. Mohanty and Dhurandhar (1996) showed that a signiﬁcant reduction in computational resources can be accomplished by using a hierarchy of such template banks where candidate events triggered by a sparsely populated grid is followed up by the regular, dense ﬂat search grid. The estimated speed up in this method was a factor ∼ 25 over the ﬂat search. In this paper we report an improved implementation of the hierarchical search, wherein we extend the domain of hierarchy to an extra dimension - namely the time of arrival of the signal in the bandwidth of the interferometer. This is accomplished by lowering the Nyquist sampling rate of the signal in the trigger stage. We show that this leads to further improvement in the eﬃciency of data analysis and speeds up the online computation by a factor of ∼ 65 − 70 over the ﬂat search. We also take into account and discuss issues related to template placement, trigger thresholds and other peculiar problems that do not arise in earlier implementation schemes of the hierarchical search. We present simulation results for 2PN waveforms embedded in the noise expected for initial LIGO detectors.
Galactic foreground constraints from the Python V cosmic microwave background anisotropy data
(2011-07-05) Mukherjee, Pia; Coble, Kim; Dragovan, Mark
We constrain Galactic foreground contamination of the Python V cosmic microwave background anisotropy data by cross correlating it with foreground contaminant emis- sion templates. To model foreground emission we use 100 and 12 µm dust templates and two point source templates based on the PMN survey. The analysis takes account of inter-modulation correlations in 8 modulations of the data that are sensitive to a large range of angular scales and also densely sample a large area of sky. As a conse- quence the analysis here is highly constraining. We ﬁnd little evidence for foreground contamination in a analysis of the whole data set. However, there is indication that foregrounds are present in the data from the larger-angular-scale modulations of those Python V ﬁelds that overlap the region scanned earlier by the UCSB South Pole 1994 experiment. This is an independent consistency cross-check of ﬁndings from the South Pole 1994 data.
Giant meterwave radio telescope observations of an M2.8 flare: insights into the initiation of a flare-coronal mass ejection event
(2011-07-05) Subramanian, Prasad; Ananthakrishnan, S.; Janardhan, P.; et al.
We present the ﬁrst observations of a solar ﬂare with the GMRT. An M2.8 ﬂare observed at 1060 MHz with the GMRT on Nov 17 2001 was associated with a prominence eruption observed at 17 GHz by the Nobeyama radioheliograph and the initiation of a fast partial halo CME observed with the LASCO C2 corono- graph. Towards the start of the eruption, we ﬁnd evidence for reconnection above the prominence. Subsequently, we ﬁnd evidence for rapid growth of a vertical current sheet below the erupting arcade, which is accompanied by the ﬂare and prominence eruption.
Global solution of viscous accretion disk around rotating compact objects : A pseudo-general-relativistic study
(2011-07-05) Mukhopadhyay, B.; Ghosh, Shubhrangshu
We study the solution of viscous accretion disks around rotating com-pact/central object having hard surface i.e. neutron star, strange star and any other highly gravitating objects. We choose pseudo-Newtonian approach to describe the relativistic accretion disk. For this purpose, a new pseudo- Newtonian potential is established which is applicable to describe the rela- tivistic properties of star and its disk. As we know, the Hartle-Thorne met- ric can describe geometry of star as well as the space-time out-side of it, we use this metric to establish our potential. Our potential reproduces the marginally stable orbit exactly as that of general relativity. It also reproduces the marginally bound orbit and speciﬁc mechanical energy at the marginally stable orbit with at most 4% and 10% error respectively. Using this poten- tial we study the global parameter space of the accretion disk. Thus, we ﬁnd out the physical parameter regime, for which the stable accretion disk can be formed around gravitating object with hard surface. We also study, how the ﬂuid properties get changed with diﬀerent rotations of the central star. We show that with the change of rotation to the central object, the valid disk parameter region dramatically changes.We also show the eﬀect of viscosity to the ﬂuid properties of the disk. Subsequently, we give a theoretical prediction
Gravitational collapse of perfect fluid in self-similar higher dimensional space-times
(2011-07-05) Ghosh, S. G.; Deshkar, D. W.
We investigate the occurrence and nature of naked singularities in the gravitational collapse of an adiabatic perfect ﬂuid in self-similar higher dimensional space-times. It is shown that strong curvature naked singularities could occur if the weak energy condition holds. Its implication for cosmic censorship conjecture is discussed. Known results of analogous studies in four dimensions can be recovered.
Gravitational wave data analysis for laser interferometrie space antenna
(2011-07-05) Dhurandhar, Sanjeev; Vinet, Jean-Yves
Gravitationally induced neutrino asymmetry
(2011-07-05) Singh, Parampreet; Mukhopadhyay, B.
We study the propagation of Dirac neutrinos in gravitational backgrounds and show that the gravitational interaction can lead to neutrino asymmetry due to modi cations in dispersion relation. We give some examples of spacetime geometries where such asym- metry can arise. This asymmetry would have contributed to the relic neutrino asymmetry through the interaction of neutrinos with primordial black holes before neutrinos decou- pled. In the present epoch it may be generated in accretion around high temperature rotating black holes.
Gravity and the thermodynamics of horizons
(2011-07-05) Padmanabhan, T.
Abstract Spacetimes with horizons show a resemblance to thermodynamic systems and it is possible to associate the notions of temperature and entropy with them. Several as- pects of this connection are reviewed in a manner appropriate for broad readership. The approach uses two essential principles: (a) the physical theories must be formu- lated for each observer entirely in terms of variables any given observer can access and (b) consistent formulation of quantum ﬁeld theory requires analytic continua- tion to the complex plane. These two principles, when used together in spacetimes with horizons, are powerful enough to provide several results in a uniﬁed manner. Since spacetimes with horizons have a generic behaviour under analytic continua- tion, standard results of quantum ﬁeld theory in curved spacetimes with horizons can be obtained directly (Sections III to VII). The requirements (a) and (b) also put strong constraints on the action principle describing the gravity and, in fact, one can obtain the Einstein-Hilbert action from the thermodynamic considerations (Section VIII). The review emphasises the thermodynamic aspects of horizons, which could be obtained from general principles and is expected to remain valid, independent of the microscopic description (‘statistical mechanics’) of horizons.
Hyperdiffusion in non-linear, large and small-scale turbulent dynamos
(2011-07-05) Subramanian, Kandaswamy
The generation of large-scale magnetic ﬁelds is generically accompanied by the more rapid growth of small-scale ﬁelds. The growing Lorentz force due to these ﬁelds back reacts on the turbulence to saturate the mean-ﬁeld and small-scale dynamos. For the mean-ﬁeld dynamo, in a quasi-linear treatment of this saturation, it is generally thought that, while the alpha-eﬀect gets renormalised and suppressed by non-linear eﬀects, the turbulent diﬀusion is left unchanged. We show here that this is not true and the eﬀect of the Lorentz forces, is also to generate additional non-linear hyperdiﬀusion of the mean ﬁeld. A combination of such non-linear hyperdiﬀusion with diﬀusion at small scales, also arises in a similar treatment of small-scale dynamos, and is crucial to understand its saturation.