PAOLETTI, DANIELADANIELAPAOLETTIBraglia, M.M.BragliaFINELLI, FABIOFABIOFINELLIBallardini, M.M.BallardiniUmiltà, C.C.Umiltà2021-01-272021-01-2720192212-6864http://hdl.handle.net/20.500.12386/30025We acknowledge support by the "ASI/INAF Agreement 2014-024-R.0 for the Planck LFI Activity of Phase E2’’. We also acknowledge financial contribution from the agreement ASI n. I/023/12/0 "Attivitá relative alla fase B2/C per la missione Euclid". DP and FF acknowledge financial support by ASI Grant 2016-24-H.0. MB is supported by the South African SKA Project and by the Claude Leon Foundation, South Africa.We present a new isocurvature mode present in scalar-tensor theories of gravity that corresponds to a regular growing solution in which the energy of the relativistic degrees of freedom and the scalar field that regulates the gravitational strength compensate during the radiation dominated epoch on scales much larger than the Hubble radius. We study this isocurvature mode and its impact on anisotropies of the cosmic microwave background for the simplest scalar-tensor theory, i.e. the extended Jordan-Brans-Dickegravity, in which the scalar field also drives the acceleration of the Universe. We use Planck data to constrain the amplitude of this isocurvature mode in the case of fixed correlation with the adiabatic mode and we show how this mode could be generated in a simple two field inflation model.ELETTRONICOenArticle10.1016/j.dark.2019.1003072-s2.0-85064831552000477764100004https://www.sciencedirect.com/science/article/pii/S2212686418302267?via%3Dihub2019PDU....25..307PFIS/05 - ASTRONOMIA E ASTROFISICAERC sectors::Physical Sciences and Engineering::PE9 Universe sciences: astro-physics/chemistry/biology; solar systems; stellar, galactic and extragalactic astronomy, planetary systems, cosmology, space science, instrumentation