What question did this study set out to answer?

This research aims to explore the implications of a fifth-force sector in strong-field binary systems and to establish detection criteria.

February 16, 2026Open Access

Curvature-Activated Vacuum Coupling and Fifth-Force Dynamics in Strong-Field Binaries: Frequency-Domain Phasing, Detector Projections, and Weak-Field Consistency

SHSIKX HILTON

Key Points

This research aims to explore the implications of a fifth-force sector in strong-field binary systems and to establish detection criteria.
Developed a microphysical model for fifth-force dynamics derived from an action-level vacuum coupling.
Formulated equations of motion influenced by vacuum pressure and curvature effects.
Analyzed frequency-domain waveform modifications and their correlation with spin and other parameters.
Evaluated detector sensitivities for Advanced LIGO and future detectors, like Cosmic Explorer.
Identified that fifth-force effects are suppressed in extreme mass-ratio inspirals but are significant in comparable-mass mergers.
Demonstrated that the newly proposed model enhances the strong-field detection capacity near merger events.
Showed notable improvements in weak-field safety with curvature activation, reducing solar-system and binary-pulsar constraints.

Abstract

We formulate a microphysical “fifth-force” sector derived from an action-level couplingbetween compact objects and a vacuum response, constructed to preserve total stress-energyconservation while permitting genuine non-geodesic motion. The worldline action is modified bya curvature-dependent coupling to a scalar vacuum-pressure proxy P (x),Sm = −∫dτ m + α0(K) P (x),yielding a projected equation of motionM uν ∇ν uμ = −(gμν + uμuν )∇ν M with M = m + α0(K)P.In the two-body problem, equivalence-principle-respecting (universal) couplings produce a relative-acceleration correction that is automatically suppressed in extreme mass-ratio inspirals (EMRIs)by the symmetric mass ratio η ≪ 1, rendering them negligible for LISA; comparable-massmergers remove this suppression and provide a strong-field discovery channel. To ensure weak-field safety and strong-field activity we introduce curvature activation for α0(K), leveraging theK ∝ r−6 hierarchy. We insert a smooth-saturation activation model into the frequency-domaininspiral waveform and derive the resulting phase deformation, its effective PN behavior, and itscorrelations with spin and standard PN coefficients. We provide Fisher-style sensitivity scalingsfor Advanced LIGO and for third-generation detectors (Cosmic Explorer / Einstein Telescope),and show that curvature activation suppresses solar-system and binary-pulsar constraints bymany orders of magnitude while permitting O(10−2–10−1) strong-field activity near merger.

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