Double-binary-pulsar test of Chern-Simons modified gravity

Chern-Simons modified gravity is a string theory and loop-quantum-gravity inspired effective theory that modifies general relativity by adding a parity-violating Pontryagin density to the Einstein-Hilbert action multiplied by a coupling scalar. We strongly constrain nondynamical Chern-Simons modified gravity with a timelike Chern-Simons scalar through observations of the double-binary-pulsar PSR J0737-3039A/B. We first calculate Chern-Simons corrections to the orbital evolution of binary systems. We find that the ratio of the correction to periastron precession to the general relativistic prediction scales quadratically with the semimajor axis and inversely with the square of the object's radius. Binary pulsar systems are thus ideal to test this theory, since periastron precession can be measured with subdegree accuracies and the semimajor axis is millions of times larger than the stellar radius. Using data from PSR J0737-3039A/B we dramatically constrain the nondynamical Chern-Simons coupling to MCS1/|θ̇|>33meV, approximately a hundred billion times better than current Solar System tests.