Novel CMB constraints on the α parameter in alpha-attractor models
Laura Iacconi, Matteo Fasiello, J. Väliviita, David Wands
Abstract
Abstract Cosmological α -attractors are a compelling class of inflationary models. They lead to universal predictions for large-scale observables, broadly independent from the functional form of the inflaton potential. In this work we derive improved analytical predictions for the large-scale observables, whose dependence on the duration of reheating and the parameter α is made explicit. We compare these with Planck and BICEP/Keck 2018 data in the framework of a Bayesian study, employing uniform logarithmic and linear priors for α . Our improved universal predictions allow direct constraints on the duration of reheating. Furthermore, while it is well-known that CMB constraints on the tensor-to-scalar ratio can be used to place an upper bound on the α parameter, we demonstrate that including the α -dependence of the scalar spectral tilt yields novel constraints on α . In particular, for small α , the scalar spectral tilt scales with log 10 α , regardless of the specific potential shape. For decreasing α , this eventually puts the models in tension with CMB measurements, bounding the magnitude of α from below. Therefore, in addition to the upper bound from the tensor-to-scalar ratio, we derive the first lower bound on the magnitude of α for α -attractor T-models, log 10 α = -4.2 +5.4 -8.6 at 95% C.L.