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

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

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

a r X i v : h e p - t h / 0 4 0 9 1 7 7 v 3 3 1 J a n 2 0 0 5 Preprint typeset in JHEP style - HYPER VERSION Consistent modified gravity: dark energy, acceleration and the absence of cosmic doomsday M.C.B. Abdalla Inst. Fisica Teorica, Universidade Estadual Paulista, Sao Paulo, Brazil email: mabdalla@ift.unesp.br Shin’ichi Nojiri Department of Applied Physics, National Defence Academy, Hashirimizu Yokosuka 239-8686, Japan email: nojiri@nda.ac.jp, snojiri@yukawa.kyoto-u.ac.jp Sergei D.Odintsov? Institucio? Catalana de Recerca i Estudis Avanc?ats (ICREA) and Institut d’Estudis Espacials de Catalunya (IEEC), Edifici Nexus, Gran Capita? 2-4, 08034 Barcelona, Spain email: odintsov@ieec.fcr.es, odintsov@ieec.uab.es 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 R2 term (consistent modified gravity) may help to escape of cosmic doomsday. ?Also at TSPU, Tomsk, Russia and at IFT, UNESP, Sao Paulo (temporary) Contents 1. Introduction 1 2. Sudden future singularity in modified gravity 2 3. Modified gravity coupled with matter 6 4. Discussion 7 1. Introduction The interpretation of the very recent observational data indicates that current uni- verse is flat and is in accelerating expansion which started about five billion years ago. The important characteristic of the (accelerated) expansion is so-called “equa- tion of state” parameter w (for dominant energy density contribution) which is the relation between the pressure and the energy