we investigate the growth of matter density perturbations as well as the generalized second law (gsl) of thermodynamics in the framework of f(r)-gravity. we consider a spatially flat frw universe filled with the pressureless matter and radiation which is enclosed by the dynamical apparent horizon with the hawking temperature. for some viable f(r) models containing the starobinsky, hu-sawicki, exp...

Here, dark energy is obtained using dual roles of the Ricci scalar (as a physical field as well as geometry). Dark energy density, obtained in this model, mimics phantom and the derived Friedmann equation contains a term ρde/2λ with ρde being the dark energy density and λ, called as cosmic tension. It is like brane-gravity inspired Friedmann equation, which arises here without using the brane-g...

It is demonstrated that entropy and its density play a significant role in solving the problem of the vacuum energy density (cosmological constant) of the Universe and hence the dark energy problem. Taking this in mind, two most popular models for dark energy—Holographic Dark Energy Model and Agegraphic Dark Energy Model—are analysed. It is shown that the fundamental quantities in the first of ...

Gravity probes only the total energy-momentum tensor, which leads to a perfect degeneracy for generalized dark energy models. Because of this degeneracy, m cannot be measured. We demonstrate this explicitly by showing that the combination of cosmic microwave background and supernova data is compatible with very large and very small values of m for a specific family of dark energy models. We als...

The recently observed accelerating cosmological expansion or ”dark energy” may be either a negative pressure constituent in General Relativity (Dark Energy) or modified gravity (Dark Gravity) without any constituent Dark Energy. Lowor high-curvature modifications of Einstein gravity are distinguished by the spacetime (Ricci) curvature of their vacua. If constituent Dark Energy does not exist, s...