Unifying Inflation with Early and Late-time Dark Energy in $F(R)$ Gravity

Abstract: In this work we shall present models of $F(R)$ gravity which realize in a unified way the inflationary era along with a post-inflationary early dark energy era, with the late-time dark energy era. We shall use two approach methods in order to realize the unified cosmological eras, firstly we specify a Hubble rate which may describe the three distinct acceleration eras, and then by using well known $F(R)$ gravity reconstruction techniques, we shall find the differential equation which may yield the $F(R)$ gravity that realizes the cosmologies. In our second approach, we shall present in a qualitative way, several $F(R)$ gravities which unify the inflationary era with the early and late-time dark energy eras, and we discuss several qualitative issues related to the terms that realize the post-inflationary early dark energy era. We quantify our analysis by numerically solving the Friedmann equation, using the redshift as the main variable, and expressing all the physical quantities as functions of the statefinder $y_\mathrm{H}(z)$, which depends on the redshift and the Hubble rate $H(z)$. For the model studied numerically, we present the behavior of some statefinder quantities, like the deceleration parameter, and we calculate the dark energy density parameter and the dark energy equation of state parameter at present time. After demonstrating that the dark energy era is viable, we investigate when the early dark energy term does not affect the late-time era, and this restricts the free parameters of the model.