Speed of gravitational waves and the fate of scalar-tensor gravity

Abstract

The direct detection of gravitational waves (GWs) is an invaluable new tool to probe gravity and the nature of cosmic acceleration. A large class of scalar-tensor theories predict that GWs propagate with velocity different than the speed of light, a difference that can be O(1) for many models of dark energy. We determine the conditions behind the anomalous GW speed, namely that the scalar field spontaneously breaks Lorentz invariance and couples to the metric perturbations via the Weyl tensor. If these conditions are realized in nature, the delay between GW and electromagnetic (EM) signals from distant events will run beyond human timescales, making it impossible to measure the speed of GWs using neutron star mergers or other violent events. We present a robust strategy to exclude or confirm an anomalous speed of GWs using eclipsing binary systems, whose EM phase can be exquisitely determined. he white dwarf binary J0651+2844 is a known example of such system that can be used to probe deviations in the GW speed as small as cg/c − 1 2 · 10 −12 when LISA comes online. This test will either eliminate many contender models for cosmic acceleration or wreck a fundamental pillar of general relativity.PACS numbers: 04.30.Nk 04.50. Kd, 95.36.+x, Introduction and summary. The direct detection of gravitational radiation [1,2] has initiated a new era for astronomy, astrophysics and fundamental physics. The observed gravitational wave (GW) events and the ones to come will usher in novel ways to test the nature of gravity [3]. Here, we will argue that probing the speed of GWs will be a decisive test for gravity and dark energy models.

Inhomogeneous Extra Space as a Tool for the Top-Down Approach

“…Specific choice of theory could produce additional effects like GW propagation with a speed different from the speed of light [37]. Nevertheless, new methods will undoubtedly help to extract promising directions in gravitational physics.…”

Dark Energy After GW170817: Dead Ends and the Road Ahead

“…Alternative scenarios that explain DE dynamically require either additional degrees of freedom (beyond the massless spin-2 field of GR) or a low-energy violation of fundamental principles such as locality [9]. The extremely low energy scale for DE requires additional degrees of freedom to be hidden on small scales by a screening mechanism [10], which also suppresses their rate of emission as additional gravitational wave polarizations [11].New fields coupled to gravity can affect the propagation speed of the standard GW polarizations, as measured by GW170817 and its counterparts [12]. Anomalous GW speed can be used to test even screened theories, as signals from extra-galactic sources probe unscreened, cosmological scales.…”

Black holes in self-tuning cubic Horndeski cosmology

“…Such an approach has proven particularly popular in recent years for constructing novel descriptions of dark energy. The recent gravitational wave data [8][9][10][11][12][13] has dealt a strong blow to such models, proving that in the late Universe light and gravity propagate at identical speeds, and thus ruling out large regions of the parameter space of these theories. However, a simple subclass of Horndeski Lagrangians passes this test.…”