Collapsing spherical stars inf(R)gravity

Abstract

We perform a careful investigation of the problem of physically realistic gravitational collapse of massive stars in f (R)-gravity. We show that the extra matching conditions that arise in the modified gravity impose strong constraints on the stellar structure and thermodynamic properties. In our opinion these constraints are unphysical. We prove that no homogeneous stars with nonconstant Ricci scalar can be matched smoothly with a static exterior for any nonlinear function f (R). Therefore, these extra constraints make classes of physically realistic collapse scenarios in general relativity, non-admissible in these theories. We also find an exact solution for an inhomogeneous collapsing star in the Starobinski model that obeys all the energy and matching conditions. However, we argue that such solutions are fine-tuned and unstable to matter perturbations. Possible consequences on black hole physics and the cosmic censorship conjecture are also discussed.

Gravitational collapse in f (R) gravity for a spherically symmetric spacetime admitting a homothetic Killing vector

“…Rigorous analytical investigation in this direction has been carried out very recently by Goswami et al [9]. This brings out some important results, for instance the existence of a regular horizon would require an inhomogeneity in the fluid distribution.…”

“…The relevance of chosing this type of functional form for a spherical star in f (R) gravity was elaborated in [9]. From (36) and (37), we can write δ(r) = r g(r)…”

Dilatonic equation of hydrostatic equilibrium and neutron star structure

“…On the other hand, if one is interested in studying the structure and evolution of stars in different gravities, one should obtain the HEE in those gravity models. In recent years, the generalizations and modifications of this equation were of special interests for many authors (Heydarifard 2009;Astashenok 2013;Orellana 2013;Arbanil 2013;Doneva 2013;Goswami 2014;Lemos 2015;Yazadjiev 2014;Capozziello 2011;Capozziello 2012;Momeni 2015a). For more examples, we may note that the HEE equation in f (R) and f (G) gravities werediscussed in (Astashenok 2015b;Momeni 2015b;Astashenok 2015a;Abbas 2015), d-dimensional HEE in EN gravity was investigated in (Ponce 2000) and HEE of EN-Λ gravity with arbitrary dimensions was obtained in (Bordbar 2015).…”

Scalar field collapse with an exponential potential

“…In modified theories of gravity an alternative scenario is treated sometimes where the exterior is non-static, however, the solar system experiments constrain heavily such a scenario. Another possible way to avoid such a scenario can perhaps be allowing jump in the curvature terms in the field equations, however, this must result in surface stress energy terms, which on realistic collapsing models must have observational signatures and can be established via experimental evidences [12].…”

“…Gravitational collapse is well studied analytically and numerically, in the framework of General Relativity [6,7,8,9,10,11], and modifications of GR as well [12,13,14,15,16,17]. The exact solution of Einsteins' field equations play a crucial role in understanding a collapsing solution in a detailed manner.…”