Talk abstract details

The quest for theories of gravity which can serve as an alternative to General Relativity has become more intense due to recent feedback from cosmology. The energy density of the universe appears to be currently dominated by a cosmological constant, or by an unknown form of energy (dark energy) that is mimicking the behaviour of a cosmological constant. The problems connected with the inclusion of such a constant in Einstein\'s equations have triggered research and the proposal of alternatives, one of which could be to modify General Relativity. One of the alternative theories which have been considered is f(R) gravity in the Palatini formalism: we show that for generic choices of f(R), there are natural matter configurations (such as spherical systems consisting of an isentropic monatomic gas or a nonrelativistic degenerate electron gas) for which no satisfactory physical solution of the field equations can be found within this framework, apart from in the special case of General Relativity, casting doubt on whether Palatini f(R) gravity can be considered as giving viable alternatives to General Relativity.