(Lovelock)2 inflation: Explaining the ACT data and equivalence to Higgs–Gauss–Bonnet inflation

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 n s than predicted by the standard R 2 scenario. We demonstrate that a natural one-parameter generalization to a quadratic model ∼ L + L 2 in the Lovelock invariant L = R + α 4 G ( 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 ( n s , r ) along a trajectory that brings the predictions into better agreement with the ACT likelihood. We also find that L + L 2 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.