We consider an Einstein-scalar-Gauss-Bonnet gravitational theory, and argue that at early times the Ricci scalar can be safely ignored. We then demonstrate that the pure scalar-Gauss-Bonnet theory, with a quadratic coupling function, naturally supports inflationary—de Sitter—solutions. During inflation, the scalar field decays exponentially and its effective potential remains always bounded. The theory also contains solutions where these de Sitter phases possess a natural exit mechanism and are replaced by linearly expanding—Milne—phases.

• Received 5 March 2015

DOI:https://doi.org/10.1103/PhysRevD.92.041302