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How Accidental was Inflation?
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abstract
Data on the cosmic microwave background (CMB) are discriminating between different models of inflation, disfavoring simple monomial potentials whilst being consistent with models whose predictions resemble those of the Starobinsky $R + R^2$ cosmological model. However, this model may suffer from theoretical problems, since it requires a large initial field value, threatening the validity of the effective field theory. This is quantified by the Swampland Distance Conjecture, which predicts the appearance of a tower of light states associated with an effective ultra-violet cutoff. This could be lower than the inflation scale for cases with an extended period of inflation, leading to an additional problem of initial conditions. No-scale supergravity models can reproduce the predictions of the Starobinsky model and accommodate the CMB data at the expense of fine-tuning of parameters at the level of $10^{-5}$. Here, we propose a solution to this problem based on an explicit realisation of the Starobinsky model in string theory, where this `deformation' parameter is calculable and takes a value of order of the one corresponding to the Starobinsky inflaton potential. Within this range, there are parameter values that accommodate more easily the combination of Planck, ACT and DESI BAO data, while also restricting the range of possible inflaton field values, thereby avoiding the swampland problem and predicting that the initial conditions for inflation compatible with the CMB data are generic.
Forward citations
Cited by 6 Pith papers
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Gravitational waves from self-resonance during reheating with a quantum-corrected inflaton potential
A Coleman-Weinberg correction that cancels the inflaton's quadratic term at the potential minimum triggers quartic self-resonance and a peaked GW background at 10^8-10^10 Hz; a negative quadratic term instead gives a ...
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ACT stands for Awkward Cosmology Theories
Reconciling Starobinsky inflation with ACT data requires fine-tuned higher-curvature scales, stiff reheating with ω_reh>1/3, or negative-energy towers, all hard to motivate from UV completions.
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Kinetically Modified Palatini Inflation Meets ACT Data
Palatini chaotic inflation with kinetic mixing f_K = f_R^m can shift the predicted spectral index up to the ACT DR6 value n_s = 0.974 while keeping r below current bounds.
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The BAO-CMB Tension and Implications for Inflation
The upward shift in the scalar spectral index n_s in CMB+BAO analyses is driven by the combined effects of a known CMB degeneracy and the tension between CMB and DESI BAO data, not by new information about n_s itself.
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(Lovelock)$^2$ inflation: explaining the ACT data and equivalence to Higgs-Gauss-Bonnet inflation
A quadratic f(L) gravity with a negative Gauss-Bonnet coupling shifts Starobinsky inflation's (n_s, r) predictions toward the ACT-preferred higher n_s, at the cost of a larger tensor-to-scalar ratio.
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Minimal Plateau Inflation in light of ACT DR6 Observations
A minimal plateau inflation model with exponent n=2 and matter-like reheating remains consistent with the latest CMB datasets at both 1σ and 2σ, while stiffer reheating scenarios are increasingly disfavored.
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