Browsing by Author "Joshi, P. S."

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Black hole physics in globally hyperbolic space-times
(Indian Academy of Sciences, 1982-02-16) Joshi, P. S.; Narlikar, J. V.
The usual definition of a black hole is modified to make it applicable in a globally hyperbolic space-time. It is shown that in a closed globally hyperbolic universe the surface area of a black hole must eventually decrease. The implications of this breakdown of the black hole area theorem are discussed in the context of thermodynamics and cosmology. A modified definition of surface gravity is also given for non-stationary universes. The limitations of these concepts are illustrated by the explicit example of the Kerr-Vaidya metric.
Bounds on vacuum energy density in a general cosmological scenario
(Elsevier Science Publishers, 1987-02-09) Joshi, P. S.; Padmanabhan, T.; Chitre, S. M.
General limits on the cosmological constant (or equivalently the vacuum energy density) are derived for an inflationary universe. This is accomplished under the general assumption of global hyperbolicity and without the use of any special properties like spherical symmetry or homogeneity ofthe underlying spacetime. Aclear upper limit of 1/3 is obtained for the vacuum energy density parameterQ~,while the lower limit is found to depend on the age of the oldest object in the universe.
Conformal quantization and space-time singularity
(World Scientiﬁc Publishing group, 1986-06-06) Joshi, P. S.; Narlikar, J. V.
This work generalizes earlier results of conformal quantization that within the full range of space-times conformal to any singular space-time satisfying Einstein's field equations for minimally coupled matter, the singular solutions form a set of zero-probability measure. A wider definition of space-time singularity that includes the curvature singularity assumed in the earlier work (op. cit.) is adapted and it is shown that the previous conclusion stands even when the present state of the universe is defined by wave functionals that are not necessarily wave packets. Within the present framework of quantum gravity therefore it seems extremely unlikely that the universe had a singular origin.
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