Genesis in Minimally Modified Gravity

Abstract

We construct a Genesis scenario within the framework of a type-II minimally modified gravity. The background solution is past geodesically complete and approaches Minkowski spacetime in the asymptotic past. While the Hubble expansion rate grows, the matter sector satisfies the null-energy condition. Scalar perturbations are ghost-free and have no ultraviolet gradient instability, although the squared sound speed cR² becomes negative within a finite infrared region in the Fourier space for a finite time interval. Numerical evolution, including superhorizon growth, yields a nearly scale-invariant, slightly red-tilted scalar spectrum over a broad region of parameter space. The tensor spectrum is blue, with a strongly suppressed tensor-to-scalar ratio. Finally, using the matter fluctuation δχ, whose asymptotic normalization is regular, we find that the background-dependent cubic couplings vanish in the far past at fixed nonzero momenta. The surviving matter--graviton and graviton self-interactions are Planck suppressed.

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BibTeX

@misc{ganz2026genesis,
  title = {{Genesis in Minimally Modified Gravity}},
  author = {Alexander Ganz and Paul Martens and Shinji Mukohyama and Pavel Petrov},
  howpublished = {arXiv},
  year = {2026},
  month = oct,
  doi = {10.48550/arxiv.2610.37063},
  eprint = {2610.37063},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2610.37063},
  note = {CC BY 4.0},
}

RIS

TY  - UNPB
TI  - Genesis in Minimally Modified Gravity
AU  - Alexander Ganz
AU  - Paul Martens
AU  - Shinji Mukohyama
AU  - Pavel Petrov
PY  - 2026
DA  - 2026/10/27
PB  - arXiv
DO  - 10.48550/arxiv.2610.37063
UR  - https://arxiv.org/abs/2610.37063
AB  - We construct a Genesis scenario within the framework of a type-II minimally modified gravity. The background solution is past geodesically complete and approaches Minkowski spacetime in the asymptotic past. While the Hubble expansion rate grows, the matter sector satisfies the null-energy condition. Scalar perturbations are ghost-free and have no ultraviolet gradient instability, although the squared sound speed c_R^2 becomes negative within a finite infrared region in the Fourier space for a finite time interval. Numerical evolution, including superhorizon growth, yields a nearly scale-invariant, slightly red-tilted scalar spectrum over a broad region of parameter space. The tensor spectrum is blue, with a strongly suppressed tensor-to-scalar ratio. Finally, using the matter fluctuation δχ, whose asymptotic normalization is regular, we find that the background-dependent cubic couplings vanish in the far past at fixed nonzero momenta. The surviving matter--graviton and graviton self-interactions are Planck suppressed.
ER  -

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