Mirage of Luminal Modified Gravitational-Wave Propagation

Area of Science:

• Cosmology
• Gravitational Physics
• Modified Gravity Theories

Background:

• General relativity (GR) is the standard model for gravity.
• Modified gravity theories explore deviations from GR, particularly concerning gravitational wave (GW) propagation.
• Understanding GWs and their relation to electromagnetic waves (EMWs) is crucial for testing gravity theories.

Purpose of the Study:

• To investigate GW propagation in luminal modified gravity theories using conformal invariance.
• To analyze the relationship between GW luminosity distance and EMW luminosity distance.
• To generalize findings to non-luminal and time-dependent GW speed scenarios.

Main Methods:

• Conformal invariance of gravitational waves.
• Effective field theory (EFT) for modified gravity.
• Analysis of matter coupling to Jordan and Einstein frames.

Main Results:

• For luminal modified gravity, GW propagation and luminosity distance match GR.
• The GW-EMW luminosity distance relation is modified for electromagnetism coupled to the Jordan frame metric.
• The modified relation holds for non-luminal theories and time-dependent GW speeds.

Conclusions:

• Conformal invariance provides a powerful tool to simplify GW analysis in modified gravity.
• Deviations in GW-EMW distance relations offer potential observational probes of modified gravity.
• The study unifies analyses for luminal, non-luminal, and time-varying GW speed theories.