Narrow Results

We investigate the general approach to finding exact cosmological solutions in f (R) Hořava-Lifshitz gravity, based on Noether’s theorem. A feature of this approach is that it uses the behavior of an effective Lagrangian under infinitesimal transformations of the desired symmetry, explicitly determining the form f (R) for which such symmetries exist. It is shown that the dynamics of the scale factor changes according to either a exponential function of time.

In this work, we study the F (R) gravity with f -essence for the flat and homogeneous Friedman-Robertson-Walker universe. For this model, we have presented the point-like Lagrangian and the corresponding field equations. To describe the dynamics of the universe, we have investigated some cosmological solutions for K, F and h functions. It is shown that these solutions describe the late time accelerated expansion of the Universe.

Although General Relativity (GR) is an extremely successful theory, at least for weak gravitational fields, it breaks down at very high energies. For example, extrapolating the expansion of the Universe backwards in time yields an infinite energy density, which is referred to as the initial singularity problem. Quantum Gravity is expected to provide a solution to this open question. In fact, one alternative scenario to the Big Bang, that avoids the singularity, is offered by Loop Quantum Cosmology (LQC), which predicts that the Universe undergoes a collapse to an expansion through a bounce. In this work we use metric f(R) gravity to reproduce the modified Friedmann equations, which have been obtained in the context of modified loop quantum cosmologies (mLQC). Using a order reduction method, we obtain covariant effective actions that lead to a bounce, for specific models of mLQC, considering a massless scalar field.