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Natural Metric-Affine Inflation
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abstract
We consider here natural inflation in the low energy (two-derivative) metric-affine theory containing only the minimal degrees of freedom in the inflationary sector, i.e. the massless graviton and the pseudo-Nambu-Goldstone boson (PNGB). This theory contains the Ricci-like and parity-odd Holst invariants together with non-minimal couplings between the PNGB and the above-mentioned invariants. The Palatini and Einstein-Cartan realizations of natural inflation are particular cases of our construction. Explicit models of this type featuring non-minimal couplings are shown to emerge from the microscopic dynamics of a QCD-like theory with an either sub-Planckian or trans-Planckian confining scale and that is renormalizable on Minkowski spacetime. Moreover, for these models, we find regions of the parameter space where the inflationary predictions agree with the most recent observations at the $2\sigma$ level. We find that in order to enter the $1\sigma$ region it is necessary (and sufficient) to have a finite value of the Barbero-Immirzi parameter and a sizable non-minimal coupling between the inflaton and the Holst invariant (with sign opposite to the Barbero-Immirzi parameter). Indeed, in this case the potential of the canonically normalized inflaton develops a plateau as shown analytically.
Forward citations
Cited by 7 Pith papers
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Kinetic Gauge Friction in Natural Inflation
Kinetic gauge friction can sustain natural inflation with sub-Planckian f, and a Chern-Simons term stabilizes the perturbations, yielding CMB-compatible spectra.
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Inflation with Nieh-Yan-like terms in metric-affine gravity
A generalized Nieh-Yan coupling in metric-affine gravity can restore the consistency of Palatini inflation with CMB observations and produce testable tensor-to-scalar ratios.
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Torsion induced current-scalaron coupling in Einstein-Cartan gravity
In Einstein-Cartan gravity, torsion-current couplings induce scalaron-current and scalaron Chern-Simons interactions, with explicit decay constants and a classification of their asymptotic behavior.
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Friedmann cosmology with hyperfluids
For a class of hyperfluid models in metric-affine gravity, the FLRW evolution of H(t) and rho(t) keeps the general relativity form, with hypermomentum effects encoded only in modified barotropic indices.
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Symmetry-breaking inflation in non-minimal metric-affine gravity
A Holst-invariant coupling to the inflaton makes symmetry-breaking inflation viable, including the previously unattainable sub-Planckian vacuum expectation value case.
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Intrinsic Torsion, Extrinsic Torsion, and the Hubble Parameter
The second fundamental form of a spatial slice in a torsional spacetime is a sum of the Hubble term and an extrinsic torsion term, producing a negative bias in Hubble estimates when torsion is neglected.
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$\tilde\xi$-attractors in metric-affine gravity
A new class of metric-affine inflationary models, the ξ-tilde-attractors, reproduces Starobinsky inflation as a universal attractor in two strong-coupling limits.
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