Conditions for the cosmological viability of f(R) dark energy models

Luca Amendola*, Radouane Gannouji, David Polarski, Shinji Tsujikawa

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

619 Citations (Scopus)


We derive the conditions under which dark energy models whose Lagrangian densities f are written in terms of the Ricci scalar R are cosmologically viable. We show that the cosmological behavior of f(R) models can be understood by a geometrical approach consisting of studying the m(r) curve on the (r,m) plane, where m≡Rf,RR/f,R and r≡-Rf,R/f with f,R≡df/dR. This allows us to classify the f(R) models into four general classes, depending on the existence of a standard matter epoch and on the final accelerated stage. The existence of a viable matter-dominated epoch prior to a late-time acceleration requires that the variable m satisfies the conditions m(r)≈+0 and dm/dr>-1 at r≡-1. For the existence of a viable late-time acceleration we require instead either (i) m=-r-1, (3-1)/2<m≤1 and dm/dr<-1 or (ii) 0<m≤1 at r=-2. These conditions identify two regions in the (r,m) space, one for the matter era and the other for the acceleration. Only models with an m(r) curve that connects these regions and satisfies the requirements above lead to an acceptable cosmology. The models of type f(R)=αR-n and f=R+αR-n do not satisfy these conditions for any n>0 and n<-1 and are thus cosmologically unacceptable. Similar conclusions can be reached for many other examples discussed in the text. In most cases the standard matter era is replaced by a cosmic expansion with scale factor a t1/2. We also find that f(R) models can have a strongly phantom attractor but in this case there is no acceptable matter era.

Original languageEnglish
Article number083504
JournalPhysical Review D - Particles, Fields, Gravitation and Cosmology
Issue number8
Publication statusPublished - 2007 Apr 6
Externally publishedYes

ASJC Scopus subject areas

  • Nuclear and High Energy Physics
  • Physics and Astronomy (miscellaneous)


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