Enhanced induced gravitational waves in Horndeski gravity

Publikation: Beitrag in FachzeitschriftArtikelForschungPeer-Review

Autorschaft

  • Guillem Domènech
  • Alexander Ganz

Organisationseinheiten

Externe Organisationen

  • Max-Planck-Institut für Gravitationsphysik (Albert-Einstein-Institut)
Forschungs-netzwerk anzeigen

Details

OriginalspracheEnglisch
Aufsatznummer020
FachzeitschriftJournal of Cosmology and Astroparticle Physics
Jahrgang2025
Ausgabenummer1
Frühes Online-Datum8 Jan. 2025
PublikationsstatusVeröffentlicht - 2025

Abstract

We study secondary gravitational wave production in Horndeski gravity when the scalar field dominates the very early universe. We find that higher derivative interactions easily dominate the source term on subhorizon scales and significantly enhance the amplitude of induced GWs. For the first time, we analytically derive the Horndeski-induced GW spectrum for a general class of power-law solutions. The main effects of modifications of gravity are stronger resonances and growth of tensor fluctuations on small scales. The maximum attainable amplitude of the induced GW spectrum is bounded by the possible backreaction of higher derivatives on curvature fluctuations, thereby shutting down the source term to induced GWs. We argue that the maximum attainable amplitude depends linearly on the primordial curvature spectrum (ΩGW ∝ P ζ), as opposed to the standard case where it depends quadratically. Resonances may further enhance the maximum amplitude by a factor (k/H t )2 or (k/H t ) respectively for sharp and broad peaks (including a scale-invariant) primordial spectrum, where H t is the comoving horizon at the time when standard gravity is recovered. Remarkably, in the scale-invariant case, the Horndeski-induced GW spectrum grows as k 3. This opens up the interesting possibility that induced GWs might be observable despite no enhancement of the primordial curvature spectrum. Our formalism can be generalized to a wider class of solutions and to more general scalar-tensor theories, such as DHOST and spatially covariant gravity. In the appendices, we discuss the gauge issue and disformal transformations of induced GWs.

ASJC Scopus Sachgebiete

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Enhanced induced gravitational waves in Horndeski gravity. / Domènech, Guillem; Ganz, Alexander.
in: Journal of Cosmology and Astroparticle Physics, Jahrgang 2025, Nr. 1, 020, 2025.

Publikation: Beitrag in FachzeitschriftArtikelForschungPeer-Review

Domènech G, Ganz A. Enhanced induced gravitational waves in Horndeski gravity. Journal of Cosmology and Astroparticle Physics. 2025;2025(1):020. Epub 2025 Jan 8. doi: 10.48550/arXiv.2406.19950, 10.1088/1475-7516/2025/01/020
Domènech, Guillem ; Ganz, Alexander. / Enhanced induced gravitational waves in Horndeski gravity. in: Journal of Cosmology and Astroparticle Physics. 2025 ; Jahrgang 2025, Nr. 1.
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N2 - We study secondary gravitational wave production in Horndeski gravity when the scalar field dominates the very early universe. We find that higher derivative interactions easily dominate the source term on subhorizon scales and significantly enhance the amplitude of induced GWs. For the first time, we analytically derive the Horndeski-induced GW spectrum for a general class of power-law solutions. The main effects of modifications of gravity are stronger resonances and growth of tensor fluctuations on small scales. The maximum attainable amplitude of the induced GW spectrum is bounded by the possible backreaction of higher derivatives on curvature fluctuations, thereby shutting down the source term to induced GWs. We argue that the maximum attainable amplitude depends linearly on the primordial curvature spectrum (ΩGW ∝ P ζ), as opposed to the standard case where it depends quadratically. Resonances may further enhance the maximum amplitude by a factor (k/H t )2 or (k/H t ) respectively for sharp and broad peaks (including a scale-invariant) primordial spectrum, where H t is the comoving horizon at the time when standard gravity is recovered. Remarkably, in the scale-invariant case, the Horndeski-induced GW spectrum grows as k 3. This opens up the interesting possibility that induced GWs might be observable despite no enhancement of the primordial curvature spectrum. Our formalism can be generalized to a wider class of solutions and to more general scalar-tensor theories, such as DHOST and spatially covariant gravity. In the appendices, we discuss the gauge issue and disformal transformations of induced GWs.

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