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The Speed of Gravity and the Fate of Dark Energy

Jeremy Sakstein, Bhuvnesh Jain

TL;DR

The paper analyzes how GW170817/GRB170817A constrains dark-energy models, focusing on GLPV beyond-Horndeski theories and disformal couplings. It connects cosmological perturbation parameters ($M^2$, $α_M$, $α_K$, $α_B$, $α_T$, $α_H$) with astrophysical screening via $Υ_1$ and $Υ_2$, and uses the bound $|c_T^2-1|\lesssim 10^{-15}$ to eliminate broad classes of models, notably quartic Galileons. It shows that after GW170817, $α_T\approx 0$ reduces degeneracies to constraints on $α_B$ and $α_H$, excludes quartic Galileons as drivers of dark energy, and tightly limits disformal photon couplings with $\mathcal{M}_γ \gtrsim 10$ MeV. Looking forward, evolving dark energy signals from DESI keep the search open, but the surviving theories form a tightly constrained foundation for future model-building and interpretation, guiding the landscape of viable dark-energy physics.

Abstract

On August 17$^{\rm th}$ 2017, observatories worldwide made a landmark detection: gravitational waves and light from a binary neutron star merger. This event revolutionized our understanding of astrophysics, cosmology, and gravitation. In this proceeding of the 2025 International Congress of Basic Science, we describe how it transformed our view of cosmic acceleration (dark energy). The near-simultaneous arrival of light and gravitational waves shows that their speeds agree to within one part in $10^{15}$, excluding large classes of modified gravity theories and interactions between dark energy and matter.

The Speed of Gravity and the Fate of Dark Energy

TL;DR

The paper analyzes how GW170817/GRB170817A constrains dark-energy models, focusing on GLPV beyond-Horndeski theories and disformal couplings. It connects cosmological perturbation parameters (, , , , , ) with astrophysical screening via and , and uses the bound to eliminate broad classes of models, notably quartic Galileons. It shows that after GW170817, reduces degeneracies to constraints on and , excludes quartic Galileons as drivers of dark energy, and tightly limits disformal photon couplings with MeV. Looking forward, evolving dark energy signals from DESI keep the search open, but the surviving theories form a tightly constrained foundation for future model-building and interpretation, guiding the landscape of viable dark-energy physics.

Abstract

On August 17 2017, observatories worldwide made a landmark detection: gravitational waves and light from a binary neutron star merger. This event revolutionized our understanding of astrophysics, cosmology, and gravitation. In this proceeding of the 2025 International Congress of Basic Science, we describe how it transformed our view of cosmic acceleration (dark energy). The near-simultaneous arrival of light and gravitational waves shows that their speeds agree to within one part in , excluding large classes of modified gravity theories and interactions between dark energy and matter.

Paper Structure

This paper contains 11 sections, 23 equations, 2 figures.

Table of Contents

  1. Introduction
  2. GLPV Theories
  3. Action
  4. The Vainshtein Mechanism
  5. Dark Energy and the Speed of Gravity in GLPV Theories
  6. Vainshtein Breaking
  7. GW170817/GRB170817A and their Implications for GLPV Theories
  8. The building blocks of quartic GLPV dark energy theories can be constrained directly using astrophysical bodies
  9. Quartic Galileon Theories Cannot Explain Dark Energy
  10. The Disformal Coupling to Photons is Strongly Constrained
  11. Outlook and Conclusions

Figures (2)

  • Figure 1: Bounds on the building blocks of dark energy $\alpha_B$ and $\alpha_H$ derived by translating bounds on $\Upsilon_1$ from dwarf stars Sakstein:2015zoa (blue regions), and the bounds on $\Upsilon_1$ and $\Upsilon_2$ from galaxy clusters Sakstein:2016ggl (green regions) via equations \ref{['eq:U1NEW']} and \ref{['eq:U2NEW']}. Figure taken from Sakstein:2017xjx.
  • Figure 2: Constraints in the $c_4$–$\Omega_\phi$ plane from the near equality of the speed of gravitons and photons (orange) and the lack of a supermassive black hole offset in M87 (red). Shaded regions are excluded for $c_0=1$; dashed/dotted curves show equivalent limits for $c_0=3$. Figure taken from Sakstein:2017xjx.