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arXiv · 2509.05027

Unique gravitational wave signatures of GLPV scalar-tensor theories

Guillem Domènech·Alexander Ganz·Mohammad Ali Gorji·Masahide Yamaguchi
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Abstract

We study gravitational waves induced by scalar primordial fluctuations in Gleyzes-Langlois-Piazza-Vernizzi (GLPV), beyond Horndeski, scalar-tensor theories. We uncover, at the level of the action, a new scalar-scalar-tensor interaction, unique to GLPV models disconnected from Horndeski via disformal transformation. The new interaction, arising in the unitary-degenerate (U-DHOST) sector of GLPV, leads to third derivatives in the source for scalar-induced tensor modes, which are absent in Horndeski-related theories. Such new higher-derivative terms lead to a further enhanced production of induced gravitational waves. We predict that for a scale-invariant primordial spectrum, the induced gravitational wave spectral density has a characteristic frequency dependence proportional to $f^5$. Such a fast-rising spectrum offers a potential unique signature of modified gravity in the early universe.

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Guillem Domènech, Alexander Ganz, Mohammad Ali Gorji, Masahide Yamaguchi. 2025-09-05. Unique gravitational wave signatures of GLPV scalar-tensor theories. https://doi.org/10.1088/1475-7516%2F2026%2F03%2F070

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