Proofs of singularity-free solutions and scalarization in nonlinear Einstein-scalar-Gauss-Bonnet cosmology

Abstract: We establish the global existence and precise estimates of a class of singularity-free cosmological solutions in nonlinear Einstein-scalar-Gauss-Bonnet (ESGB) gravity with quadratic coupling, in close agreement with previous numerical results. Our analysis also yields a rigorous mathematical proof of nonlinear spontaneous scalarization, triggered by a tachyonic instability induced by the Gauss-Bonnet term. The proof is based on a set of decoupled differential inequalities for the Hubble parameter $H$, derived from a key structural identity that we refer to as the power identity.