Abstract
The starting point in this note is $f(R)$ modified gravity in a cosmological setting. We assume a spatially flat Universe to describe late-time cosmology and the perfect-fluid equation of state $p=\omega\rho$ to model the hypothesized dark energy. Given that on a cosmological scale, $f(R)$ modified gravity must remain close to Einstein gravity to be consistent with observation, it was concluded that either (1) Einstein's cosmological constant is the only acceptable model for the accelerated expansion or (2) that the equation of state for dark energy is far more complicated than the perfect-fluid model and may even exclude a constant $\omega$.
Highlights
The discovery that our universe is undergoing an accelerated expansion [1] [2] has led to a renewed interest in modified theories of gravity
Given that on a cosmological scale, f ( R) modified gravity must remain close to Einstein gravity to be consistent with observation, it was concluded that either (1) Einstein’s cosmological constant was the only acceptable model for the accelerated expansion, or that (2) the equation of state for dark energy was far more complicated than the perfect-fluid model and might even exclude a constant ω
Since this note deals with rather reasonable assumptions, the only plausible objection to this conclusion is that the equation of state for dark energy is much more complicated than the perfect-fluid equation of state p = ωρ
Summary
The discovery that our universe is undergoing an accelerated expansion [1] [2] has led to a renewed interest in modified theories of gravity. ) In the equation of state p = ωρ , a > 0 corresponds to the range of values ω < −1 3 , referred to as quintessence dark energy. The case ω = −1 is equivalent to assuming Einstein’s cosmological constant It has been forcefully argued by Bousso [6] that the cosmological constant is the best model for dark energy. We go a step further and propose that f ( R) modified gravity implies that ω = −1 is the only allowed value in the equation of state p = ωρ
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
