Luminal Scalar-Tensor theories for a not so dark Dark Energy

TL;DR

This paper demonstrates that certain Scalar-Tensor theories, including Horndeski and Beyond Horndeski, can be modified to produce luminal gravitational wave speeds, aligning with observations, and suppressing GW decay into Dark Energy.

Contribution

It introduces a class of Luminal Scalar-Tensor theories that reconcile GW speed constraints with Dark Energy models, expanding the theoretical landscape.

Findings

At least one Luminal Beyond Horndeski theory suppresses GW decay into Dark Energy.

Five scalar couplings suffice to make GWs luminal in many cases.

Theories can match GW speed constraints without excluding Dark Energy interactions.

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, {\it including Horndeski and Beyond Horndeski (BH) theories} -- we show that, remarkably, up to 5 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 Beyond Horndeski theory for which the GW decay into Dark Energy is suppressed in any cosmological background.