Tensor-scalar gravity and binary-pulsar experiments

TL;DR

This paper investigates how strong-field effects in tensor-scalar gravity, especially spontaneous scalarization, cause deviations from general relativity in binary pulsars, and shows these effects are tightly constrained by pulsar data.

Contribution

It introduces a scalar analog of ferromagnetism in tensor-scalar theories and quantifies their deviations from general relativity in strong gravitational fields.

Findings

Binary pulsar data strongly constrain nonperturbative scalar effects.

Scalarization causes significant deviations in pulsar timing.

Pulsar experiments are more sensitive than solar-system tests for certain scalar-tensor theories.

Abstract

Some recently discovered nonperturbative strong-field effects in tensor-scalar theories of gravitation are interpreted as a scalar analog of ferromagnetism: "spontaneous scalarization". This phenomenon leads to very significant deviations from general relativity in conditions involving strong gravitational fields, notably binary-pulsar experiments. Contrary to solar-system experiments, these deviations do not necessarily vanish when the weak-field scalar coupling tends to zero. We compute the scalar "form factors" measuring these deviations, and notably a parameter entering the pulsar timing observable gamma through scalar-field-induced variations of the inertia moment of the pulsar. An exploratory investigation of the confrontation between tensor-scalar theories and binary-pulsar experiments shows that nonperturbative scalar field effects are already very tightly constrained by published data on three binary-pulsar systems. We contrast the probing power of pulsar experiments with that of solar-system ones by plotting the regions they exclude in a generic two-dimensional plane of tensor-scalar theories.