Dark Energy as a Post-Inflation Effect in Quadratic Gravity

We analytically and numerically show that the acceleration of the cosmic expansion could be explained by a Quadratic Gravity model which is known to be able to trigger sufficient inflation, with neither negative pressure matter nor cosmological constant. Furthermore, we exactly show that this model differs greatly from Einstein's gravity in radiation and matter dominant era, giving a non-shrinking Hubble horizon and a smooth expansion history of the universe. Accordingly, it suggests that the Dark Energy could possibly be a post-inflation effect in Quadratic Gravity. We also show that this model admits all Einstein metrics as its solutions. Consequently, classic tests of Einstein's Gravity cannot falsify this model.