ACT-ing on inflation: Implications of non bunch-Davies initial condition and reheating on single-field slow roll models

Abstract

We investigate a class of slow roll inflationary models in light of the recent Cosmic Microwave Background constraints from Planck 2018, ACT DR6, DESI DR1, and BICEP/Keck 2018. The combined dataset favors a higher value of the scalar spectral index nS=0.9743±0.0034, which places increased pressure on several conventional inflationary scenarios. In this study, we analyze the observational viability of various well-motivated models, including the α-attractor E- and T-models, chaotic inflation, hilltop inflation, and natural inflation. We incorporate the effects of a post-inflationary phase of reheating and examine how the dynamics of reheating influence the predictions in the nS−r plane. We also impose a lower bound on the reheating temperature based on the constraint from the effective number of relativistic species (ΔNeff) arising from primordial gravitational waves. While reheating improves agreement with observations for some models, significant regions of parameter space remain disfavored. Finally, we explore the impact of a non Bunch-Davies initial state and demonstrate that it can substantially improve the fit to the nS−r data across a broader class of inflationary models, thereby offering a potentially viable mechanism for reconciling theory with the latest observations.