Details
| Original language | English |
|---|---|
| Article number | 020 |
| Journal | Journal of Cosmology and Astroparticle Physics |
| Volume | 2025 |
| Issue number | 1 |
| Early online date | 8 Jan 2025 |
| Publication status | Published - 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.
Keywords
- Cosmological perturbation theory in GR and beyond, gravitational waves / theory, modified gravity, primordial gravitational waves (theory)
ASJC Scopus subject areas
- Physics and Astronomy(all)
- Astronomy and Astrophysics
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In: Journal of Cosmology and Astroparticle Physics, Vol. 2025, No. 1, 020, 2025.
Research output: Contribution to journal › Article › Research › peer review
}
TY - JOUR
T1 - Enhanced induced gravitational waves in Horndeski gravity
AU - Domènech, Guillem
AU - Ganz, Alexander
N1 - Publisher Copyright: © 2025 The Author(s)
PY - 2025
Y1 - 2025
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.
AB - 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.
KW - Cosmological perturbation theory in GR and beyond
KW - gravitational waves / theory
KW - modified gravity
KW - primordial gravitational waves (theory)
UR - http://www.scopus.com/inward/record.url?scp=85214787098&partnerID=8YFLogxK
U2 - 10.48550/arXiv.2406.19950
DO - 10.48550/arXiv.2406.19950
M3 - Article
AN - SCOPUS:85214787098
VL - 2025
JO - Journal of Cosmology and Astroparticle Physics
JF - Journal of Cosmology and Astroparticle Physics
SN - 1475-7516
IS - 1
M1 - 020
ER -