1,721,013 research outputs found
Dark Energy: the equation of state description versus scalar-tensor or modified gravity
No full textDark energy dynamics of the universe can be achieved by equivalent mathematical descriptions taking into account generalized fluid equations of state in General Relativity, scalar-tensor theories or modified F(R) gravity in Einstein or Jordan frames. The corresponding technique trans- forming equation of state description to scalar-tensor or modified gravity is explicitly presented. We show that such equivalent pictures can be discriminated by matching solutions with data capable of selecting the true physical frame
Phantom scalar dark energy as modified gravity: Understanding the origin of the Big Rip singularity
No full textIt is shown that phantom scalar models can be mapped into a mathematically equivalent, modified F (R) gravity, which turns out to be complex, in general. Only for even scalar potentials is the ensuing modified gravity real. It is also demonstrated that, even in this case, modified gravity becomes complex at the region where the original phantom dark energy theory develops a Big Rip singularity. A number of explicit examples are presented which show that these two theories are not completely equivalent, from the physical viewpoint. This basically owes to the fact that the physical metric in both theories differ in a time-dependent conformal factor. As a result, an FRW accelerating solution, or FRW instanton, in the scalar-tensor theory may look as a decelerating FRW solution, or a non-instantonic one, in the corresponding modified gravity theory. © 2007 Elsevier B.V. All rights reserved
Magnetic neutron stars in f(R) gravity
No full textNeutron stars with strong magnetic fields are considered in the framework of f(R) gravity. In order to describe dense matter in magnetic field, the model with baryon octet interacting through σρω-fields is used. The hyperonization process results in softening the equation of state (EoS) and in decreasing the maximal mass. We investigate the effect of strong magnetic field in models involving quadratic and cubic corrections in the Ricci scalar R to the Hilbert–Einstein action. For large fields, the Mass–Radius relation differs considerably from that of General Relativity only for stars with masses close to the maximal one. Another interesting feature is the possible existence of more compact stable stars with extremely large magnetic fields (∼6×1018 G instead of ∼4×1018 G as in GR) in the central regions of the stars. Due to cubic terms, a significant increasing of the maximal mass is possible
Unified phantom cosmology: inflation, dark energy and dark matter under the same standard
No full textPhantom cosmology allows to account for dynamics and matter content of the universe tracing back the evolution to the inflationary epoch, considering the transition to the non-phantom standard cosmology (radiation/matter dominated eras) and recovering the today observed dark energy epoch. We develop the unified phantom cosmology where the same scalar plays the role of early time (phantom) inflaton and late-time dark energy. The recent transition from decelerating to accelerating phase is described too by the same scalar field. The (dark) matter may be embedded in this scheme, giving the natural solution of the coincidence problem. It is explained how the proposed unified phantom cosmology can be fitted against the observations which opens the way to define all the important parameters of the model
Erratum: Modified first-order Hořava-Lifshitz gravity: Hamiltonian analysis of the general theory and accelerating FRW cosmology in a power-law F(R) model ()
No full textCosmological viability of f(R)-gravity as an ideal fluid and its compatibility with a matter dominated phase
No full textWe show that f (R)-gravity can, in general, give rise to cosmological viable models compatible with a matter-dominated epoch evolving into a late accelerated phase. We discuss the various representations of f (R)-gravity as an ideal fluid or a scalar–tensor gravity theory, taking into account conformal transformations. We point out that mathematical equivalence does not correspond, in several cases, to the physical equivalence of Jordan frame and Einstein frame. Finally, we show that wide classes of f (R) gravity models, including matter and accelerated phases, can be phenomenologically reconstructed by means of observational data. In principle, any popular quintessence models could be “reframed” as an f (R)-gravity model
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