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  • Letter
  • Open Access

Luminal scalar-tensor theories for a not so dark dark energy

S. Mironov1,2,3,*, A. Shtennikova1,2,†, and M. Valencia-Villegas2,1,‡

  • *Contact author: sa.mironov_1@physics.msu.ru
  • †Contact author: shtennikova@inr.ru
  • ‡Contact author: mvalenciavillegas@itmp.msu.ru

Phys. Rev. D 111, L101501 – Published 7 May, 2025

DOI: https://doi.org/10.1103/PhysRevD.111.L101501

Abstract

In general the speed of gravitational waves (GWs) in scalar-tensor modifications of Einstein’s gravity is different from the speed of light. Nevertheless, it has been measured that their speeds are nearly the same. For the most general scalar-tensor theories classified to date that do propagate a graviton—DHOST, including Horndeski and beyond Horndeski (BH) theories—we show that, remarkably, up to five self-consistent couplings of the scalar of dark energy (DE) to the photon are enough to make their GWs luminal in a wide set of cases. We find at least one luminal BH theory for which the GW decay into DE is suppressed in any cosmological background.

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