In this paper we analyze the gravitational field of a global monopole in the context of f (R) gravity. More precisely, we show that the field equations obtained are expressed in terms of F (R) = df (R) dR . Since we are dealing with a spherically symmetric system, we assume that F (R) is a function of the radial coordinate only. Moreover, adopting the weak field approximation, we can provide all components of the metric tensor. A comparison with the corresponding results obtained in General Relativity and in the Brans-Dicke theory is also made.

In this paper we suggest an approach to analyse

In this paper the f (R) global monopole is reexamined. We provide an exact solution for the modified field equations in the presence of a global monopole for regions outside its core, generalizing previous results. Additionally, we discuss some particular cases obtained from this solution. We consider a setup consisting of a possible Schwarzschild black hole that absorbs the topological defect, giving rise to a static black hole endowed with a monopole's charge. Besides, we demonstrate how the asymptotic behavior of the Higgs field far from the monopole's core is shaped by a class of spacetime metrics which includes the ones analyzed here. In order to assess the gravitational properties of this system, we analyze the geodesic motion of both massive and massless test particles moving in the vicinity of such configuration. For the material particles we set the requirements they have to obey in order to experience stable orbits. On the other hand, for the photons we investigate how their trajectories are affected by the gravitational field of this black hole.

Among many alternative gravitational theories to General Relativity (GR), f (R, T ) gravity (where R is the Ricci scalar and T the trace of the energy-momentum tensor) has been widely studied recently. By adding a matter contribution to the gravitational Lagrangian, f (R, T ) theories have become an interesting extension to GR displaying a broad phenomenology in astrophysics and cosmology. In this paper, we discuss however the difficulties appearing in explaining a viable and realistic cosmology within the f (R, T ) class of theories. Our results challenge the viability of f (R, T ) as an alternative modification of gravity.PACS numbers: 04.50. Kd, 95.36.+x,

We formulate a theory combining the principles of scalar-tensor gravity and Rastall's proposal of a violation of the usual conservation laws. We obtain a scalar-tensor theory with two parameters ω and λ, the latter quantifying the violation of the usual conservation laws (λ = 1 corresponding to the General Relativity limit). The only exact spherically symmetric solution is that of Robinson-Bertotti besides the Schwarzschild solution. A PPN analysis reveals that General Relativity results are reproduced when λ = 0. The cosmological case displays a possibility of deceleration/acceleration or acceleration/deceleration transitions during the matter dominated phase depending on the values of the free parameters.

The possibility of dark matter being a dissipative component represents an option for the standard view where cold dark matter (CDM) particles behave on large scales as an ideal fluid. By including a physical mechanism to the dark matter description like viscosity we construct a more realistic model for the universe. Also, the known small scale pathologies of the standard CDM model either disappear or become less severe. We study clustering properties of a ΛCDM-like model in which dark matter is described as a bulk viscous fluid. The linear power spectrum, the nonlinear spherical "top hat" collapse and the mass functions are presented. We use the analysis with such structure formation tools in order to place an upper bound on the magnitude of the dark matter's viscosity.

The viability of achieving gravitational consistent braneworld models in the framework of a f (R) theory of gravity is investigated. After a careful generalization of the usual junction conditions encompassing the embedding of the 3-brane into a f (R) bulk, we provide a prescription giving the necessary constraints in order to implement the projected second order effective field equations on the brane.

Apart from the familiar structure firmly-rooted in the general relativistic field equations where the energy–momentum tensor has a null divergence i.e., it conserves, there exists a considerable number of extended theories of gravity allowing departures from the usual conservative framework. Many of these theories became popular in the last few years, aiming to describe the phenomenology behind dark matter and dark energy. However, within these scenarios, it is common to see attempts to preserve the conservative property of the energy–momentum tensor. Most of the time, it is done by means of some additional constraint that ensures the validity of the standard conservation law, as long as this option is available in the theory. However, if no such extra constraint is available, the theory will inevitably carry a non-trivial conservation law as part of its structure. In this work, we review some of such proposals discussing the theoretical construction leading to the non-conservation of the energy–momentum tensor.