doi:10.1103/PhysRevD.111.064020engAsorey, ManuelEzquerro, FernandoPardina, MiguelStability of cosmological singularity-free solutions in quadratic gravityART-2025-143445We introduce a large family of homogeneous and isotropic cosmological solutions in quadratic gravity which are singularity-free at early and late times. This kind of smooth solutions only emerge beyond the nonstandard de Sitter branch 3⁢ <, and being the couplings of the 2 and −2 ⁢ terms. We have analyzed the stability of these singularity-free solutions by computing the second-order variation of the action. We show that the expected instability is similar to that of the standard Λ Cold Dark Matter (Λ CDM) scenario. Thus, they provide cosmological background scenarios as legitimate as that of the Standard Model. The complete analysis shows that a singular big bang scenario can emerge from a singularity-free cosmological scenario when the parameters of the theory are slightly modified, which provides a bridge between the two very different cosmological scenarios with similar instabilities.2025http://zaguan.unizar.es/record/15217410.1103/PhysRevD.111.064020http://zaguan.unizar.es/record/152174oai:zaguan.unizar.es:152174info:eu-repo/grantAgreement/ES/DGA-FSE/E21-17Rinfo:eu-repo/grantAgreement/ES/DGIID-DGA/2020-E21-17Rinfo:eu-repo/grantAgreement/ES/MICINN/PGC2022-126078NB-C21info:eu-repo/grantAgreement/ES/NextGenerationEU/PRTRPhysical Review D 111, 6 (2025), 9 pp.byhttps://creativecommons.org/licenses/by/4.0/deed.esinfo:eu-repo/semantics/closedAccess