${\mathcal{R}}^2$ effectively from Inflation to Dark Energy

We consider the single-parameter $\mathcal{R}+c{\mathcal{R}}^2$ gravitational action and use constraints from astrophysics and the laboratory to derive a natural relation between the coefficient $c$ and the value of the cosmological constant. We find that the renormalisation of $c$ from the energy of the inflationary phase to the infrared, where the acceleration of the expansion of the Universe takes place, is correlated with the evolution of the vacuum energy. Our results suggest that the coefficient of the ${\mathcal{R}}^2$ term may provide an unexpected bridge between high-energy physics and cosmological phenomena such as inflation and dark energy.