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Area of Science:

  • Cosmology
  • Theoretical Physics
  • Gravitational Wave Astronomy

Background:

  • Scalar-tensor theories are candidates for dark energy models.
  • The gravitational wave event GW170817 constrained these theories due to the speed of gravitational waves.
  • Previous analyses deemed many scalar-tensor theories unviable based on GW170817.

Purpose of the Study:

  • To reassess the viability of scalar-tensor theories in light of GW170817.
  • To identify conditions under which scalar-tensor theories could be compatible with gravitational wave observations.
  • To investigate the role of scalar field dynamics and inhomogeneities.

Main Methods:

  • Analysis of cosmological background equations for scalar-tensor theories.
  • Investigating the relationship between anomalous gravitational wave speed and scalar field dynamics.
  • Inclusion of large-scale inhomogeneities to test the robustness of theoretical solutions.

Main Results:

  • A class of scalar-tensor theories, previously excluded, shows potential viability.
  • Anomalous gravitational wave speed is linked to the scalar equation of motion.
  • This compatibility breaks down when large-scale inhomogeneities are considered.

Conclusions:

  • Certain scalar-tensor theories, including generalized Galileons, might be compatible with GW170817 at the background level.
  • The presence of scalar field dynamics offers a potential loophole for these theories.
  • The inclusion of inhomogeneities ultimately invalidates this loophole, highlighting the importance of non-linear effects.