Consistent modified gravity: dark energy, acceleration and the absence of cosmic doomsday

Abstract

We discuss the modified gravity which includes negative and positive powers of the curvature and which provides the gravitational dark energy. It is shown that in GR plus the term containing negative power of the curvature the cosmic speed-up may be achieved, while the effective phantom phase (with w less than −1) follows when such term contains the fractional positive power of the curvature. The minimal coupling with matter makes the situation more interesting: even 1/R theory coupled with the usual ideal fluid may describe the (effective phantom) dark energy. The account of R 2 term (consistent modified gravity) may help to escape of cosmic doomsday. * Also at TSPU, Tomsk, Russia and at IFT, UNESP, Sao Paulo (temporary)

Dark Energy

“…In [634,635], It was proposed that there is curvature singularity problem in f (R) theories, due to the dynamics of the effective scalar degree of freedom in the strong gravity regime. This problem leads to the contradiction with the existence of the relativistic stars (like neutron stars) [636].…”

Models off(R)cosmic acceleration that evade solar system tests

“…The addition of a non-linear function of the Ricci scalar R to the Einstein-Hilbert action has been demonstrated to cause acceleration for a wide variety of f (R) functions [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,…”

Ghosts, instabilities, and superluminal propagation in modified gravity models

“…Among other approaches, Modified Gravity Models have attracted great interest (see [25][26][27][28][29][30][31][32][33][34][35][36][37]) but also some criticism, partly because they were introduced as purely phenomenological models, but more seriously because it was not clear that they possessed a satisfactory Newtonian limit in the solar system, or that they were free of ghosts (see [38][39][40][41][42][43]). …”