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 nonlinear gravitational coupling or stringinspired model with GaussBonnetdilaton coupling in the late universe where they lead to cosmic speedup. 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) latetime 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 modified gravity) is also described.
 Publication:

arXiv eprints
 Pub Date:
 January 2006
 arXiv:
 arXiv:hepth/0601213
 Bibcode:
 2006hep.th....1213N
 Keywords:

 High Energy Physics  Theory;
 Astrophysics;
 General Relativity and Quantum Cosmology
 EPrint:
 LaTeX file, 21 pages, references are added, lectures for 42 Karpacz Winter School on Theor Physics