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Reconciling Starobinsky inflation with ACT data

Loris Del Grosso1,*, Alfredo Urbano2,3,†, and Marcello Ziccolella2,3,‡

  • *Contact author: ldelgro1@jh.edu
  • †Contact author: alfredo.urbano@uniroma1.it
  • ‡Contact author: marcello.ziccolella@uniroma1.it
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Phys. Rev. D 114, 083522 – Published 13 October, 2026

DOI: https://doi.org/10.1103/1znv-zjgd

Abstract

Recent Atacama Cosmology Telescope (ACT) observations, combined with Planck, favor a scalar spectral index in tension with Starobinsky inflation. Two main strategies have been proposed to reconcile Starobinsky inflation with these data: (i) including higher-curvature operators in the gravitational action and (ii) modifying the postinflationary reheating dynamics. We critically reexamine both approaches. When higher-curvature corrections are included, dimensional-analysis arguments allow introducing a second mass scale suppressing higher-dimensional operators, resulting in a one-coupling, two-scale effective theory. We show that fitting the Planck+ACT data drives this class of effective theories toward a regime of stronger sensitivity to UV physics, potentially implying fine-tuning of the inflationary observables. We illustrate our general point with two concrete models, namely, no-scale supergravity and metric-affine gravity, showing that they provide different realizations of the same underlying f(R) theory. On the reheating side, requiring the CMB modes to reenter the horizon before recombination leads to stringent constraints on the reheating equation of state, and fitting the Planck+ACT data favors an exotic reheating phase with ωreh>1/3, incompatible with inflaton oscillations about the minimum of the standard Starobinsky potential. Finally, we explore the possibility that a phase of cosmological stasis, driven by a tower of decaying massive states, could play the role of the postinflationary epoch. We find that accommodating ACT-preferred ns in this setup requires a tower of states with negative energy density. Taken together, our results suggest that reconciling Starobinsky inflation with the Planck+ACT data demands ingredients that are challenging to motivate from the perspective of UV-complete theories, undermining the minimality of the original Starobinsky model.

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Phys. Rev. D 114, 083522– Published 13 October, 2026
  • Received 21 July 2026
  • Accepted 17 September 2026
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