National Defence Academy

About: National Defence Academy is a education organization based out in Pune, India. It is known for research contribution in the topics: Thin film & General relativity. The organization has 979 authors who have published 1751 publications receiving 39253 citations.

Introduction to modified gravity and gravitational alternative for dark energy

Abstract: We review various modified gravities considered as gravitational alternative for dark energy. Specifically, we consider the versions of f(R), f(G) or f(R, G) gravity, model with non-linear gravitational coupling or string-inspired model with Gauss-Bonnet-dilaton coupling in the late universe where they lead to cosmic speed-up. It is shown that some of such theories may pass the Solar System tests. On the same time, it is demonstrated that they have quite rich cosmological structure: they may naturally describe the effective (cosmological constant, quintessence or phantom) late-time era with a possible transition from decceleration to acceleration thanks to gravitational terms which increase with scalar curvature decrease. The possibility to explain the coincidence problem as the manifestation of the universe expansion in such models is mentioned. The late (phantom or quintessence) universe filled with dark fluid with inhomogeneous equation of state (where inhomogeneous terms are originated from the modifi...

Modified gravity with negative and positive powers of curvature: Unification of inflation and cosmic acceleration

Abstract: A modified gravity, which eliminates the need for dark energy and which seems to be stable, is considered. The terms with positive powers of curvature support the inflationary epoch while the terms with negative powers of curvature serve as effective dark energy, supporting current cosmic acceleration. The equivalent scalar-tensor gravity may be compatible with the simplest solar system experiments.

Properties of singularities in the (phantom) dark energy universe

Abstract: The properties of future singularities are investigated in the universe dominated by dark energy including the phantom-type fluid. We classify the finite-time singularities into four classes and explicitly present the models which give rise to these singularities by assuming the form of the equation of state of dark energy. We show the existence of a stable fixed point with an equation of state $wl\ensuremath{-}1$ and numerically confirm that this is actually a late-time attractor in the phantom-dominated universe. We also construct a phantom dark energy scenario coupled to dark matter that reproduces singular behaviors of the Big Rip type for the energy density and the curvature of the universe. The effect of quantum corrections coming from conformal anomaly can be important when the curvature grows large, which typically moderates the finite-time singularities.

Late-time cosmology in (phantom) scalar-tensor theory: dark energy and the cosmic speed-up

Abstract: We consider late-time cosmology in a (phantom) scalar-tensor theory with an exponential potential, as a dark energy model with equation of state parameter close to -1 (a bit above or below this value). Scalar (and also other kinds of) matter can be easily taken into account. An exact spatially-flat FRW cosmology is constructed for such theory, which admits (eternal or transient) acceleration phases for the current universe, in correspondence with observational results. Some remarks on the possible origin of the phantom, starting from a more fundamental theory, are also made. It is shown that quantum gravity effects may prevent (or, at least, delay or soften) the cosmic doomsday catastrophe associated with the phantom, i.e. the otherwise unavoidable finite-time future singularity (Big Rip). A novel dark energy model (higher-derivative scalar-tensor theory) is introduced and it is shown to admit an effective phantom/quintessence description with a transient acceleration phase. In this case, gravity favors that an initially insignificant portion of dark energy becomes dominant over the standard matter/radiation components in the evolution process.