188 research outputs found
Cosmology of modified (but second order) gravity
This is a brief review of modified gravity cosmologies. Generically
extensions of gravity action involve higher derivative terms, which can result
in ghosts and instabilities. There are three ways to circumvent this:
Chern-Simons terms, first order variational principle and nonlocality. We
consider recent cosmological applications of these three classes of modified
gravity models, in particular to the dark energy problem. The viable parameter
spaces can be very efficiently constrained by taking into account cosmological
data from all epochs in addition to Solar system tests and stability
considerations. We make some new remarks concerning so called algebraic
scalar-tensor theories, biscalar reformulation of nonlocal actions involving
the inverse d'Alembertian, and a possible covariant formulation holographic
cosmology with nonperturbative gravity.Comment: 18 pages, 2 figures. Based on a talk given at the 4th Gamow
conference "Astrophysics and Cosmology after Gamow: recent progress and new
horizons" at Odessa, Ukraine, 17-23 August 200
Cosmological perturbations in modified teleparallel gravity models
Cosmological perturbations are considered in and in scalar-torsion
teleparallel models of gravity. Full sets of linear perturbation
equations are accurately derived and analysed at the relevant limits.
Interesting features of generalisations to other teleparallel models, spatially
curved backgrounds, and rotated tetrads are pointed out.Comment: 18 pages without figures. v2: added reference
A note on viability of nonminimally coupled theory
Consistency conditions for nonminimally coupled theories have been
derived by requiring the absence of tachyons and instabilities in the scalar
fluctuations. This note confirms these results and clarifies a subtlety
regarding different definitions of sound speeds.Comment: 4 pages. -added acknowledgemen
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