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Gupta G.R.,Inter-University Center for Astronomy and Astrophysics
Astrophysical Journal | Year: 2017

We investigate the off-limb active-region and quiet-Sun corona using spectroscopic data. The active region is clearly visible in several spectral lines formed in the temperature range of 1.1-2.8 MK. We derive the electron number density using the line ratio method, and the nonthermal velocity in the off-limb region up to the distance of 140 Mm. We compare density scale heights derived from several spectral line pairs with expected scale heights per the hydrostatic equilibrium model. Using several isolated and unblended spectral line profiles, we estimate nonthermal velocities in the active region and quiet Sun. Nonthermal velocities obtained from warm lines in the active region first show an increase and then later either a decrease or remain almost constant with height in the far off-limb region, whereas nonthermal velocities obtained from hot lines show consistent decrease. However, in the quiet-Sun region, nonthermal velocities obtained from various spectral lines show either a gradual decrease or remain almost constant with height. Using these obtained parameters, we further calculate Alfvén wave energy flux in both active and quiet-Sun regions. We find a significant decrease in wave energy fluxes with height, and hence provide evidence of Alfvén wave damping. Furthermore, we derive damping lengths of Alfvén waves in the both regions and find them to be in the range of 25-170 Mm. Different damping lengths obtained at different temperatures may be explained as either possible temperature-dependent damping or by measurements obtained in different coronal structures formed at different temperatures along the line of sight. Temperature-dependent damping may suggest some role of thermal conduction in the damping of Alfvén waves in the lower corona. © 2017. The American Astronomical Society. All rights reserved..

Gannouji R.,Inter-University Center for Astronomy and Astrophysics | Sami M.,Jamia Millia Islamia University
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2010

We consider the covariant Galileon gravity taking into account the third order and fourth order scalar field Lagrangians L3(π) and L4(π), consisting of three and four π's with four and five derivatives acting on them, respectively. The background dynamical equations are set up for the system under consideration and the stability of the self-accelerating solution is demonstrated in a general setting. We extended this study to the general case of the fifth order theory. For the spherically symmetric static background, we spell out conditions for the suppression of fifth force effects mediated by the Galileon field π. We study field perturbations in the fixed background and investigate the conditions for their causal propagation. We also briefly discuss metric fluctuations and derive an evolution equation for matter perturbations in Galileon gravity. © 2010 The American Physical Society.

Gangopadhyay S.,West Bengal State University | Gangopadhyay S.,Inter-University Center for Astronomy and Astrophysics
Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics | Year: 2013

In this Letter, based on the Sturm-Liouville eigenvalue approach, we analytically investigate the properties of holographic superconductors in the background of pure Einstein and Gauss-Bonnet gravity taking into account the backreaction of the spacetime. Higher value of the backreaction parameter results in a harder condensation to form in both cases. The analytical results obtained are found to be in good agreement with the existing numerical results. © 2013 Elsevier B.V.

Padmanabhan T.,Inter-University Center for Astronomy and Astrophysics | Kothawala D.,Indian Institute of Technology Madras
Physics Reports | Year: 2013

Lanczos-Lovelockmodels of gravity represent a natural and elegant generalization of Einstein's theory of gravity to higher dimensions. They are characterized by the fact that the field equations only contain up to second derivatives of the metric even though the action functional can be a quadratic or higher degree polynomial in the curvature tensor. Because these models share several key properties of Einstein's theory they serve as a useful set of candidate models for testing the emergent paradigm for gravity. This review highlights several geometrical and thermodynamical aspects of Lanczos-Lovelockmodels which have attracted recent attention. © 2013 Elsevier B.V.

Bhat P.,Inter-University Center for Astronomy and Astrophysics | Subramanian K.,Inter-University Center for Astronomy and Astrophysics
Monthly Notices of the Royal Astronomical Society | Year: 2013

Turbulence is ubiquitous in many astrophysical systems like galaxies, galaxy clusters and possibly even the filaments in the intergalactic medium. We study fluctuation dynamo action in turbulent systems focusing on one observational signature, the random Faraday rotation measure (RM) from radio emission of background sources seen through the intermittent magnetic field generated by such a dynamo. We simulate the fluctuation dynamo in periodic boxes up to resolutions of 5123, with varying fluid and magnetic Reynolds numbers, and measure the resulting random RMs. We show that even though the magnetic field generated is intermittent, it still allows for contributions to the RM to be significant. When the dynamo saturates, the rms value of RMis of the order of 40-50 per cent of the value expected in a model where fields of strength Brms uniformly fill cells of the largest turbulent eddy but are randomly oriented from one cell to another. This level of RM dispersion is obtained across different values of magnetic Reynolds number and Prandtl number explored. We also use the random RMs to probe the structure of the generated fields to distinguish the contribution from intense and diffuse field regions. We find that the strong field regions (say with B > 2Brms) contribute only of the order of 15-20 per cent to the RM. Thus, rare structures do not dominate the RM; rather, the general 'sea' of volume filling fluctuating fields are the dominant contributors. We also show that the magnetic integral scale, Lint, which is directly related to the RM dispersion, increases in all the runs, as Lorentz forces become important to saturate the dynamo. It appears that due to the ordering effect of the Lorentz forces, Lint of the saturated field tends to a modest fraction, 1/2-1/3 of the integral scale of the velocity field, for all our runs. These results are then applied to discuss the Faraday rotation signatures of fluctuation dynamo generated fields in young galaxies, galaxy clusters and intergalactic filaments. © 2013 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Subramanian K.,Inter-University Center for Astronomy and Astrophysics
Reports on Progress in Physics | Year: 2016

The universe is magnetized on all scales probed so far. On the largest scales, galaxies and galaxy clusters host magnetic fields at the micro Gauss level coherent on scales up to ten kpc. Recent observational evidence suggests that even the intergalactic medium in voids could host a weak ∼ 10-16 Gauss magnetic field, coherent on Mpc scales. An intriguing possibility is that these observed magnetic fields are a relic from the early universe, albeit one which has been subsequently amplified and maintained by a dynamo in collapsed objects. We review here the origin, evolution and signatures of primordial magnetic fields. After a brief summary of magnetohydrodynamics in the expanding universe, we turn to magnetic field generation during inflation and phase transitions. We trace the linear and nonlinear evolution of the generated primordial fields through the radiation era, including viscous effects. Sensitive observational signatures of primordial magnetic fields on the cosmic microwave background, including current constraints from Planck, are discussed. After recombination, primordial magnetic fields could strongly influence structure formation, especially on dwarf galaxy scales. The resulting signatures on reionization, the redshifted 21 cm line, weak lensing and the Lyman-α forest are outlined. Constraints from radio and γ-ray astronomy are summarized. Astrophysical batteries and the role of dynamos in reshaping the primordial field are briefly considered. The review ends with some final thoughts on primordial magnetic fields. © 2016 IOP Publishing Ltd.

Chakraborty S.,Inter-University Center for Astronomy and Astrophysics
Journal of High Energy Physics | Year: 2015

Abstract: The deep connection between gravitational dynamics and horizon thermodynamics leads to several intriguing features both in general relativity and in Lanczos-Lovelock theories of gravity. Recently in arXiv:1312.3253 several additional results strengthening the above connection have been established within the framework of general relativity. In this work we provide a generalization of the above setup to Lanczos-Lovelock gravity as well. To our expectation it turns out that most of the results obtained in the context of general relativity generalize to Lanczos-Lovelock gravity in a straightforward but non-trivial manner. First, we provide an alternative and more general derivation of the connection between Noether charge for a specific time evolution vector field and gravitational heat density of the boundary surface. This will lead to holographic equipartition for static spacetimes in Lanczos-Lovelock gravity as well. Taking a cue from this, we have introduced naturally defined four-momentum current associated with gravity and matter energy momentum tensor for both Lanczos-Lovelock Lagrangian and its quadratic part. Then, we consider the concepts of Noether charge for null boundaries in Lanczos-Lovelock gravity by providing a direct generalization of previous results derived in the context of general relativity. Another very interesting feature for gravity is that gravitational field equations for arbitrary static and spherically symmetric spacetimes with horizon can be written as a thermodynamic identity in the near horizon limit. This result holds in both general relativity and in Lanczos-Lovelock gravity as well. In a previous work [arXiv:1505.05297] we have shown that, for an arbitrary spacetime, the gravitational field equations near any null surface generically leads to a thermodynamic identity. In this work, we have also generalized this result to Lanczos-Lovelock gravity by showing that gravitational field equations for Lanczos-Lovelock gravity near an arbitrary null surface can be written as a thermodynamic identity. Our general expressions under appropriate limits reproduce previously derived results for both the static and spherically symmetric spacetimes in Lanczos-Lovelock gravity. Also by taking appropriate limit to general relativity we can reproduce the results presented in arXiv:1312.3253 and arXiv:1505.05297. © 2015, The Author(s).

Mukherjee S.,Inter-University Center for Astronomy and Astrophysics
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2015

Measurement of cosmic microwave background (CMB) temperature by Planck has resulted in extremely tight constraints on the ΛCDM model. However the data indicate evidence of dipole modulated temperature fluctuations at large angular scale which is beyond the standard statistically isotropic (SI) ΛCDM model. The signal measured by Planck requires a scale dependent modulation amplitude that is beyond the scope of the phenomenological model considered by Planck. We propose a phenomenological model with mixed modulation field for scalar and tensor perturbations which affect the temperature fluctuations at large angular scales. Hence this model is a possible route to explain the scale dependent nature of the modulation field. The salient prediction of this model is the direction dependent tensor to scalar ratio which results in an anisotropic stochastic gravitational wave background (SGWB). This feature is potentially measurable from the B-mode polarization map of Planck and BICEP-2 and leads to determination of the modulation strength. Measurability of SI violated polarization field due to this model is estimated for Planck and PRISM. Absence of the signal in the polarization field can restrict the viability of the model. © 2015 American Physical Society.

Padmanabhan T.,Inter-University Center for Astronomy and Astrophysics
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2010

The calculation of entanglement entropy S of quantum fields in spacetimes with horizon shows that, quite generically, S is (a) proportional to the area A of the horizon and (b) divergent. I argue that this divergence, which arises even in the case of Rindler horizon in flat spacetime, is yet another indication of a deep connection between horizon thermodynamics and gravitational dynamics. In an emergent perspective of gravity, which accommodates this connection, the fluctuations around the equipartition value in the area elements will lead to a minimal quantum of area O(1)LP2, which will act as a regulator for this divergence. In a particular prescription for incorporating the LP2 as zero-point-area of spacetime, this does happen and the divergence in entanglement entropy is regularized, leading to S?A/LP2 in Einstein gravity. In more general models of gravity, the surface density of microscopic degrees of freedom is different which leads to a modified regularization procedure and the possibility that the entanglement entropy-when appropriately regularized-matches the Wald entropy. © 2010 The American Physical Society.

Padmanabhan T.,Inter-University Center for Astronomy and Astrophysics
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2011

It has been known for several decades that Einstein's field equations, when projected onto a null surface, exhibit a structure very similar to the nonrelativistic Navier-Stokes equation. I show that this result arises quite naturally when gravitational dynamics is viewed as an emergent phenomenon. Extremizing the spacetime entropy density associated with the null surfaces leads to a set of equations which, when viewed in the local inertial frame, becomes identical to the Navier-Stokes equation. This is in contrast to the usual description of the Damour-Navier-Stokes equation in a general coordinate system, in which there appears a Lie derivative rather than a convective derivative. I discuss this difference, its importance, and why it is more appropriate to view the equation in a local inertial frame. The viscous force on fluid, arising from the gradient of the viscous stress-tensor, involves the second derivatives of the metric and does not vanish in the local inertial frame, while the viscous stress-tensor itself vanishes so that inertial observers detect no dissipation. We thus provide an entropy extremization principle that leads to the Damour-Navier-Stokes equation, which makes the hydrodynamical analogy with gravity completely natural and obvious. Several implications of these results are discussed. © 2011 American Physical Society.

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