Rituparno Goswami

Why do naked singularities form in gravitational collapse? II

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.

Black Hole Mass Threshold from Nonsingular Quantum Gravitational Collapse

Quantum gravity is expected to remove the classical singularity that arises as the end state of gravitational collapse. To investigate this, we work with a toy model of a collapsing homogeneous scalar field. We show that nonperturbative semiclassical effects of loop quantum gravity cause a bounce and remove the black hole singularity. Furthermore, we find a critical threshold scale below which no horizon forms: quantum gravity may exclude very small astrophysical black holes.

On the LCDM Universe in f(R) gravity

Several different explicit reconstructions of f(R) gravity are obtained from the background Friedmann-Laimatre-Robertson-Walker expansion history. It is shown that the only theory whose Lagrangian is a simple function of the Ricci scalar R, that admits an exact {Lambda}CDM expansion history, is standard general relativity with a positive cosmological constant and the only way to obtain this behavior of the scale factor for more general functions of R is to add additional degrees of freedom to the matter sector.

Coexistence of matter dominated and accelerating solutions in f ( G ) gravity

Working within the theory of modified Gauss-Bonnet gravity, we show that Friedmann-Lema\^{\i}tre-Robertson-Walker--like power-law solutions only exist for a very special class of

Dynamics of f(R)-cosmologies containing Einstein static models

f(\mathcal{G})

Existence of Einstein static universes and their stability in fourth-order theories of gravity

theories. Furthermore, we point out that any transition from decelerated to accelerated expansion must pass through

Collapsing spherical stars in f(R) gravity

\mathcal{G}=0

Neutron stars in the Starobinsky model

, and no function

A new approach to reconstruction methods in f (R) gravity

f(\mathcal{G})

Quantum Evaporation of a Naked Singularity

that is differentiable at this point can admit both a decelerating power-law solution and any accelerating solution. This strongly constrains the cosmological viability of