2025/12/24 by Andrea Addazi, Addazi, Andrea, Yermek Aldabergenov +5 · 1 citation
Physics and Astronomy · #gr-qc #astro-ph.CO #hep-th
paper · pdf · doi:10.1016/j.physletb.2026.140746
We revisit the Starobinsky model of inflation in light of recent data from the Atacama Cosmology Telescope (ACT), which indicates a potential preference for a slightly larger scalar spectral index ns than predicted by the standard R2 scenario. We demonstrate that a natural one-parameter generalization to a quadratic model ∼ L+L2 in the Lovelock invariant L=R+\fracα4\cal G (\cal G is the Gauss--Bonnet term), can effectively resolve this minor tension. Scalar-tensor formulation of this theory yields an Einstein-frame Starobinsky-type scalar potential augmented by Gauss--Bonnet and derivative couplings, which modify the inflationary slow-roll dynamics. We show that a non-zero coupling α for the Gauss-Bonnet term can shift (ns, r) along a trajectory that brings the predictions into better agreement with the ACT likelihood. We also find that L+L2 gravity, in its scalar-tensor formulation, is equivalent to Higgs inflation coupled to the Gauss--Bonnet term, and belongs to the Horndeski/galileon class of modified gravities. This work establishes the quadratic f(L) gravity as a compelling and physically motivated extension that preserves the successes of Starobinsky inflation while improving its fit to modern precision cosmological data.