ACT stands for Awkward Cosmology Theories

@misc{grosso2026stands,
  title = {{ACT stands for Awkward Cosmology Theories}},
  author = {Loris Del Grosso and Alfredo Urbano and Marcello Ziccolella},
  howpublished = {arXiv},
  year = {2026},
  month = oct,
  doi = {10.48550/arxiv.2607.09808},
  eprint = {2607.09808},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2607.09808},
}

TY  - UNPB
TI  - ACT stands for Awkward Cosmology Theories
AU  - Loris Del Grosso
AU  - Alfredo Urbano
AU  - Marcello Ziccolella
PY  - 2026
DA  - 2026/10/14
PB  - arXiv
DO  - 10.48550/arxiv.2607.09808
UR  - https://arxiv.org/abs/2607.09808
AB  - 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 post-inflationary reheating dynamics. We critically re-examine 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 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 re-enter the horizon before recombination leads to stringent constraints on the reheating equation of state, and fitting the ACT data favors an exotic reheating phase with ω_reh > 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 post-inflationary epoch. We find that accommodating ACT-preferred n_s in this setup requires a tower of states with negative energy density. Taken together, our results suggest that reconciling Starobinsky inflation with the ACT data demands ingredients that are challenging to motivate from the perspective of UV-complete theories, undermining the minimality of the original Starobinsky model.
ER  -