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Novel CMB constraints on the $\alpha$ parameter in alpha-attractor models
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abstract
Cosmological $\alpha$-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 $\alpha$ 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 $\alpha$. 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 $\alpha$ parameter, we demonstrate that including the $\alpha$-dependence of the scalar spectral tilt yields novel constraints on $\alpha$. In particular, for small $\alpha$, the scalar spectral tilt scales with $\log_{10}\alpha$, regardless of the specific potential shape. For decreasing $\alpha$, this eventually puts the models in tension with CMB measurements, bounding the magnitude of $\alpha$ 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 $\alpha$ for $\alpha$-attractor T-models, $\log_{10}{\alpha} = -4.2^{+5.4}_{-8.6}$ at $95\%$ C.L. .
Forward citations
Cited by 5 Pith papers
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Beyond monomial $\alpha$-attractors
Binomial α-attractor T-models yield non-universal ns near c≈−1/2 and a transient radiation-like reheating phase only under large quartic/quadratic hierarchies, undermining fine-tuned large-p monomial assumptions.
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Closing in on $\alpha$-attractors
Under perturbative reheating, monomial α-attractor T-models cannot make the scalar spectral index exceed 0.9682, and n_s is maximized at α≈1.
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Post-inflationary enhancement of adiabatic perturbations in modular cosmology
In modular inflation models, entropic perturbations frozen during inflation are converted into curvature perturbations after inflation, producing an enhanced power spectrum while preserving the spectral index ns.
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Induced-Gravity Palatini-Like Higgs Inflation in Supergravity Confronts ACT DR6
A Palatini-supergravity Higgs-inflation model with induced gravity predicts a scalar spectral index ns≈0.972-0.974, consistent with ACT DR6, and favors split supersymmetry with gravitino mass 40-60 PeV.
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Primordial black holes through preheating instabilities in $\alpha$-attractor models
In alpha-attractor models, preheating self-resonance can form primordial black holes of 10^2 to 10^7 g, with the Khlopov-Polnarev collapse formalism staying viable and the Press-Schechter formalism overproducing.
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