Dynamical Relaxation of Dark Energy: A Solution to Early Inflation,   Late-time Acceleration and the Cosmological Constant Problem
  In recent years different explanations are provided for both an inflation and a recent acceleration in the expansion of the universe. In this Letter we show that a model of physical interest is the modification of general relativity with a Gauss-Bonnet term coupled to a dynamical scalar-field as predicted by certain versions of string theory. This construction provides a model of evolving dark energy that naturally explains a dynamical relaxation of the vacuum energy (gravitationally repulsive potential) to a small value (exponentially close to zero) after a sufficient number of e-folds. The model also leads to a small deviation from the $w=-1$ prediction of non-evolving dark energy. 
Ishwaree P. Neupane and Benedict M.N. Carter