Covariant action for bouncing cosmologies in modified Gauss-Bonnet gravity

Cyclic universes with bouncing solutions are candidates for solving the big bang initial singularity problem. Here we seek bouncing solutions in a modified Gauss-Bonnet gravity theory, of the type R + f(G), where R is the Ricci scalar, G is the Gauss-Bonnet term, and f some function of it. In finding such a bouncing solution we resort to a technique that reduces the order of the differential equations of the R + f(G) theory to second-order equations. As general relativity is a theory whose equations are of second-order, this order reduction technique enables one to find solutions which are perturbatively close to general relativity. We also build the covariant action of the order reduced theory. (C) 2019 Elsevier Inc. All rights reserved.