[Paper Review] Improved Bounds on Violation of the Strong Equivalence Principle

Zaven Arzoumanian|arXiv (Cornell University)|Dec 7, 2002

Pulsars and Gravitational Waves Research3 citations

TL;DR

This study presents improved constraints on violations of the Strong Equivalence Principle (SEP) using 20 years of timing data from the binary pulsar B0655+64. By measuring the absence of orbital decay, the analysis sets a 2σ upper limit of |Ṗb/Pb| < 1.0×10⁻¹⁰ yr⁻¹, leading to a lower bound of ωBD > 320 for Brans-Dicke theory, significantly tightening previous limits on dipolar gravitational radiation and SEP violation.

ABSTRACT

I describe a unique, 20-year-long timing program for the binary pulsar B0655+64, the stalwart control experiment for measurements of gravitational radiation damping in relativistic neutron-star binaries. Observed limits on evolution of the B0655+64 orbit provide new bounds on the existence of dipolar gravitational radiation, and hence on violation of the Strong Equivalence Principle.

Motivation & Objective

• To test the Strong Equivalence Principle (SEP) by searching for dipolar gravitational radiation in a relativistic binary pulsar system.
• To improve existing bounds on alternative gravity theories, particularly Brans-Dicke theory, by leveraging long-term timing stability of PSR B0655+64.
• To constrain the existence of dipole radiation, which would signal a violation of SEP and imply a non-zero scalar field coupling in alternative gravity models.
• To assess the potential of relativistic neutron star–white dwarf binaries as future probes of fundamental gravity, especially as new systems are discovered.

Proposed method

• Conducted a 20-year timing program on PSR B0655+64, measuring pulse arrival times with high precision using radio telescopes.
• Applied the Tempo software package and the ell1 binary timing model to fit rotational, astrometric, and orbital parameters to the full dataset.
• Used the observed orbital period derivative (Ṗb/Pb) to constrain contributions from quadrupole and dipole gravitational radiation, as well as potential time variation of G and relative acceleration effects.
• Accounted for the Shklovskii effect and Galactic acceleration using a new proper motion measurement (μ = 6.8 ± 1.1 mas yr⁻¹) to correct for systematic timing errors.
• Calculated theoretical predictions for dipole radiation using the sensitivity parameter s, assuming s = 0.2 for the neutron star and negligible sensitivity for the white dwarf.
• Compared observed limits on orbital evolution with theoretical predictions from Brans-Dicke and other scalar-tensor theories to derive bounds on ωBD and κD G⁻².

Experimental results

Research questions

• RQ1 What is the upper limit on the rate of orbital decay in PSR B0655+64, and how does it constrain dipolar gravitational radiation?
• RQ2 To what extent does the observed stability of the B0655+64 orbit constrain deviations from the Strong Equivalence Principle?
• RQ3 How do the timing measurements improve existing bounds on the Brans-Dicke parameter ωBD compared to previous pulsar and solar system experiments?
• RQ4 Can the absence of orbital decay in this NS-WD binary system rule out significant dipole radiation, and what does this imply for alternative gravity theories?
• RQ5 What is the potential of future relativistic NS-WD binaries with shorter periods to provide even tighter constraints on SEP violation?

Key findings

• The observed orbital period derivative is consistent with zero, yielding a 2σ upper limit of |Ṗb/Pb| < 1.0×10⁻¹⁰ yr⁻¹ on the rate of orbital decay.
• The absence of detectable orbital evolution implies a bound on the dipole radiation strength: κD G⁻² < 0.006 (s/0.2)⁻² at 2σ confidence.
• For Brans-Dicke theory, this results in a lower limit of ωBD > 320 (s/0.2)² at 2σ, corresponding to ωBD > 320 when s = 0.2.
• The constraint on ωBD is approximately 7 times tighter than the previous best limit from binary pulsars and rivals solar system bounds.
• The study confirms that PSR B0655+64 remains a robust and sensitive laboratory for testing SEP, despite its relatively long orbital period.
• Future discoveries of closer, shorter-period NS-WD binaries are expected to yield even stronger constraints on dipole radiation and SEP violation.