Nature of singularity in Einstein-massless scalar theory

We study the static and spherically symmetric exact solution of the Einstein-massless scalar equations given by Janis, Newman and Winicour. We find that this solution satisfies the weak energy condition and has strong globally naked singularity.Comment: 9 pages, LaTex, no figures, misprints corrected, to appear in Int. J. Mod. Phys.

Gravitational Collapse in Constant Potential Bath

We analyse here the gravitational collapse of directed null radiation in a background with a constant potential such as one produced by a star system like galaxy in which the collapsing object is immersed. Both naked singularities and black holes are shown to be developing as the final outcome of the collapse. An interesting feature that emerges is that a part of the naked singularity spectrum in collapsing Vaidya region gets covered in the corresponding dual-Vaidya region, which corresponds to the Vaidya directed null radiation sitting in constant potential bath. The implications of such a result towards the issue of stability of naked singularities are discussed.Comment: LaTex 12 Pages, 2 figures. To appear in Physical Review

Constraining f(R)f(R) gravity models with disappearing cosmological constant

The $f(R)$ gravity models proposed by Hu-Sawicki and Starobinsky are generic for local gravity constraints to be evaded. The large deviations from these models either result into violation of local gravity constraints or the modifications are not distinguishable from cosmological constant. The curvature singularity in these models is generic but can be avoided provided that proper fine tuning is imposed on the evolution of scalaron in the high curvature regime. In principle, the problem can be circumvented by incorporating quadratic curvature correction in the Lagrangian though it might be quite challenging to probe the relevant region numerically.Comment: 9 pages and 4 figures, minor clarifications and corrections added, final version to appear in PR

Improvement on the metric reconstruction scheme in Regge-Wheeler-Zerilli formalism

We have done a study on master variables in the Regge-Wheeler-Zerilli formalism. We show that a specific choice of new variables is suitable for studying perturbation theory from the view point of radiation reaction calculation. With the explicit definition of the improved master variables in terms of components of metric perturbations, we present the master equations, with source terms, and the metric reconstruction formulas. In the scheme using these new variables, we do not need any time and radial integrations except for solving the master equation. We also show that the master variable for even parity modes obtained by Chandrasekhar transformation, which satisfies the same simpler homogeneous equation as the odd-parity case, does not have the good property in this sense

Sufficient condition for Blackhole formation in spherical gravitational collapse

A sufficient condition for the validity of Cosmic Censorship in spherical gravitational collapse is formulated and proved. The condition relies on an attractive mathematical property of the apparent horizon, which holds if ''minimal'' requirements of physical reasonableness are satisfied by the matter model.Comment: 5 pages, LaTeX2