Reinterpretation of the Starobinsky model

Abstract

The Starobinsky model of inflation, consistent with Planck 2015, has a peculiar form of the action, which contains the leading Einstein term $R$, the $R^2$ term with a huge coefficient, and negligible higher-order terms. We propose an explanation of this form based on compactification of extra dimensions. Once tuning of order $10^{-4}$ is accepted to suppress the linear term $R$, we no longer have to suppress higher-oder terms, which give nontrivial corrections to the Starobinsky model. We show our predictions of the spectral index, its runnings, and the tensor-to-scalar ratio. Finally, we discuss a possibility that quantum gravity may appear at the scale $\Lambda \gtrsim 5\times 10^{15}$ GeV.

13 pages, 2 figures; published version, references and discussion on the uncertainties of the predictions added

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• Rescuing Quartic and Natural Inflation in the Palatini Formalism
• Nonminimal Coleman--Weinberg Inflation with an $R^2$ term
• Keeping the relation between the Starobinsky model and no-scale supergravity ACTive
• Tideless Traversable Wormholes surrounded by cloud of strings in $f(R)$ gravity
• Higher-order modified Starobinsky inflation
• Enhancement of primordial curvature perturbations in $R^3$-corrected Starobinsky-Higgs inflation
• Trans-Planckian censorship and single-field inflaton potential
• Higher-order extension of Starobinsky inflation: initial conditions, slow-roll regime, and reheating phase
• Generalized non-local $R^2$-like inflation
• The $R^2$-Higgs inflation: $R^3$ contribution and preheating after ACT and SPT data
• Testing f(R)-gravity models with DESI DR2 2025-BAO and other cosmological data
• Deforming the Starobinsky model in ghost-free higher derivative supergravities
• Cornering Extended Starobinsky Inflation with CMB and SKA
• Inflation in Weyl Scaling Invariant Gravity with $R^3$ Extensions
• Purely gravitational dark matter production in warm inflation
• Primordial black holes save $R^2$ inflation
• Self-tuning inflation
• Graviton mass constraint from CMB
• Inflation in light of ACT/SPT: A new perspective from Weyl gravity
• Relaxation of hierarchy in higher-dimensional Starobinsky model
• Linearized Gravity in the Starobinsky Model: Perturbative Deviations from General Relativity
• Spherically Symmetric Potentials in Quadratic $f(R)$ Gravity
• Inflationary assessment of $F(\mathcal{R},\tilde{\mathcal{R}})$ Einstein-Cartan models
• Scalar-field potential for viable models in $f(R)$ theory