Tachyonic Encore: A universal shift of inflationary observables

Abstract

We propose a generic mechanism applicable to single-field inflaton models in which a light axion spectator, initialized near the hilltop of its potential, reshapes inflationary observables through purely gravitational multi-field dynamics. During inflation the axion is frozen and the background follows an effectively single-field trajectory. After inflation ends, the axion rolls, inducing a turn in field space and transient tachyonic phases of the isocurvature mode. The resulting ``tachyonic encore'' occurs entirely on super-horizon scales. These phases generate a nearly scale-invariant enhancement of the curvature power spectrum, suppressing the tensor-to-scalar ratio and shifting the scalar tilt to a weighted combination of adiabatic and entropic tilts at horizon crossing. We show that these effects can reconcile otherwise disfavored inflaton potentials with current CMB constraints. The same dynamics predict local non-Gaussianity, f_NL^loc.∼ 𝒪(1), within reach of upcoming surveys.

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BibTeX RIS

BibTeX

@misc{gorgulho2026tachyonic,
  title = {{Tachyonic Encore: A universal shift of inflationary observables}},
  author = {Diogo S. Gorgulho and Margherita Putti and Rodrigo Gonzalez Quaglia and Ema Dimastrogiovanni and Matteo Fasiello and Diederik Roest},
  howpublished = {arXiv},
  year = {2026},
  month = nov,
  doi = {10.48550/arxiv.2606.12549},
  eprint = {2606.12549},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2606.12549},
  note = {CC BY 4.0},
}

RIS

TY  - UNPB
TI  - Tachyonic Encore: A universal shift of inflationary observables
AU  - Diogo S. Gorgulho
AU  - Margherita Putti
AU  - Rodrigo Gonzalez Quaglia
AU  - Ema Dimastrogiovanni
AU  - Matteo Fasiello
AU  - Diederik Roest
PY  - 2026
DA  - 2026/11/04
PB  - arXiv
DO  - 10.48550/arxiv.2606.12549
UR  - https://arxiv.org/abs/2606.12549
AB  - We propose a generic mechanism applicable to single-field inflaton models in which a light axion spectator, initialized near the hilltop of its potential, reshapes inflationary observables through purely gravitational multi-field dynamics. During inflation the axion is frozen and the background follows an effectively single-field trajectory. After inflation ends, the axion rolls, inducing a turn in field space and transient tachyonic phases of the isocurvature mode. The resulting ``tachyonic encore'' occurs entirely on super-horizon scales. These phases generate a nearly scale-invariant enhancement of the curvature power spectrum, suppressing the tensor-to-scalar ratio and shifting the scalar tilt to a weighted combination of adiabatic and entropic tilts at horizon crossing. We show that these effects can reconcile otherwise disfavored inflaton potentials with current CMB constraints. The same dynamics predict local non-Gaussianity, f_NL^loc.∼ 𝒪(1), within reach of upcoming surveys.
ER  -

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