Abstract
We revisit the status of scalar-tensor theories with applications to dark energy in the aftermath of the gravitational wave signal GW170817 and its optical counterpart GRB170817A. At the level of the cosmological background, we identify a class of theories, previously declared unviable in this context, whose anomalous gravitational wave speed is proportional to the scalar equation of motion. As long as the scalar field is assumed not to couple directly to matter, this raises the possibility of compatibility with the gravitational wave data, for any cosmological sources, thanks to the scalar dynamics. This newly "rescued" class of theories includes examples of generalized quintic Galileons from Horndeski theories. Despite the promise of this leading order result, we show that the loophole ultimately fails when we include the effect of large scale inhomogeneities.
Highlights
The observation of the neutron star merger GW170817 at redshift z ∼ 0.01 and its optical counterpart GRB170817A [1,2,3,4,5] has provided spectacularly strong constraints on the relevance of modified gravity [6,7,8] to understanding the dynamics of the late Universe [9]
Generic interactions between the massless spin 2 and additional light fields can cause the gravitational wave to propagate through the cosmological background at speeds different from its electromagnetic counterpart, even when it passes through overdense regions where socalled screening mechanisms might be expected to operate [10,11,12]
In the light of the LIGO/Virgo observation, a careful analysis of this effect in a wide class of scalar tensor theories has led to a dramatic reduction in the landscape of modified gravity models that are relevant for dark energy and remain observationally viable at late times [13,14,15,16](see [17,18,19,20,21])
Summary
The observation of the neutron star merger GW170817 at redshift z ∼ 0.01 and its optical counterpart GRB170817A [1,2,3,4,5] has provided spectacularly strong constraints on the relevance of modified gravity [6,7,8] to understanding the dynamics of the late Universe [9]. It was recently noted that the frequency scales of the neutron star event lie close to the strong coupling scale associated with many dark energy models If they are known, ultra violet effects could impact the speed of the gravitational wave, and if they are not known, one is attempting to constrain a theory outside of its regime of validity [27]. The sight and sound of the neutron star merger detected on August 17, 2017 has constrained the speed of gravitational and electromagnetic waves through the cosmological medium at late times, to be identical to an accuracy of the order 10−15 These speeds ing the vanishing of the scalar equation of motion means that we should not treat φas independent of φ, X, H, H. We consider a wide class of scalar where tensor theories, including Horndeski [36, 37] and beyond S=
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