Strong gravitational lensing by wormholes

We study strong gravitational lensing by a class of static wormhole geometries. Analytical approaches to the same are developed, and the results differ substantially from strong lensing by black holes, first reported by Bozza. We consider two distinct situations, one in which the observer and the source are on the same side of the wormhole throat, and the other in which they are on opposite sides. Distinctive features in our study arise from the fact that photon and antiphoton spheres might be present on both sides of the wormhole throat, and that the throat might itself act as a photon sphere. We show that strong gravitational lensing thus opens up a rich variety of possibilities of relativistic image formation, some of which are novel, and are qualitatively distinct from black hole lensing. These can serve as clear indicators of exotic wormhole geometries.Comment: 29 pages, 7 figures, published in JCA

Why isn’t India a major global player? : the political economy of trade liberalization

The policy reforms initiated in India in the mid-1980s and expanded in 1991 helped support an expansion in India’s trade. Trade reforms since the mid-1990s have been piecemeal. This paper argues that without significant further reform and adoption of a focused trade strategy, the competitiveness of India’s industry will suffer, including in areas such as information technology and related services in which India has established a strong global niche. Critical building blocks of such strategic reforms include further reductions in tariffs, opening services sectors to foreign competition, serious initiatives to reduce trade transaction costs that prioritize integration into international supply-chains, and a greater focus on regional integration

Modelling Einstein cluster using Einasto profile

We demonstrate a general relativistic approach to model dark matter halos using the Einstein cluster, with the matter stress-energy generated by collisionless particles moving on circular geodesics in all possible angular directions and orbital radii. Such matter, as is known, allows an anisotropic pressure profile with non-zero tangential but zero radial pressure. We use the Einasto density profile for the Einstein cluster. Analytical studies on its properties (metric functions) and stability issues are investigated. Further, to establish this model (with the Einasto profile) as one for a dark matter halo, we use the SPARC galactic rotation curve data and estimate the best-fit values for the model parameters. General relativistic features (beyond the Keplerian velocities) such as the tangential pressure profile, are quantitatively explored. Thus, Einstein clusters with the Einasto profile, which tally well with observations, may be considered as a viable model for dark matter halos.Comment: 18 pages, 6 figure

Tidal disruption of white dwarfs in a modified gravity theory with SPH

Low energy imprints of modifications to general relativity are often found in pressure balance equations inside stars. These modifications are then amenable to tests via astrophysical phenomena, using observational effects in stellar astrophysics that crucially depend on such equations. One such effect is tidal disruption of stars in the vicinity of black holes. In this paper, using a numerical scheme modelled with smoothed particle hydrodynamics, we study real time tidal disruption of a class of white dwarfs by intermediate-mass black holes, in the low energy limit of a theory of modified gravity that alters the internal physics of white dwarfs, namely the Eddington inspired Born-Infeld theory. In this single parameter extension of general relativity, the mass-radius relation of white dwarfs as well as their tidal disruption radius depend on the modified gravity parameter, and these capture the effect of modifications to general relativity. Our numerical simulations incorporating these show that departure from general relativity in these scenarios might be observationally significant, and should therefore be contrasted with data. In particular, we study observationally relevant physical quantities, i.e., tidal kick velocity and trajectory deviation of the remnant core and fallback rates of the tidal debris in this theory and compare them to the Newtonian limit of general relativity. We also comment on the qualitative differences between the modified gravity theory and one with stellar rotation.Comment: 23 Page