Constraining effective quantum gravity with LISA

TL;DR

This paper explores how LISA can detect quantum gravity effects, specifically amplitude birefringence caused by a Chern-Simons term, by analyzing gravitational waves from distant black hole binaries.

Contribution

It provides a detailed calculation of LISA's potential to constrain Chern-Simons modified gravity through gravitational wave observations.

Findings

LISA can detect amplitude birefringence effects in gravitational waves.

LISA could set bounds on quantum gravity effects much stronger than current Solar System tests.

The effect appears as an anomalous precession of binary orbital angular momentum.

Abstract

All modern routes leading to a quantum theory of gravity -- i.e., perturbative quantum gravitational one-loop exact correction to the global chiral current in the standard model, string theory, and perhaps even loop quantum gravity -- require supplementing the Einstein-Hilbert action with a parity-violating Chern-Simons term. Such a term leads to amplitude-birefringent gravitational wave propagation: i.e., one (circular) polarization state amplified with propagation while the other is attenuated. The proposed Laser Interferometer Space Antenna (LISA) is capable of observing gravitational wave sources at cosmological distances, suggesting the possibility that LISA observations may place a strong bound on this manifestation of quantum gravity. Here we report on a calculation of the effect that spacetime amplitude birefringence has on the signal LISA is capable of observing from inspiraling supermassive black hole binaries at large redshift. We find that the birefringence manifests itself in the observations as an anomalous precession of the binary's orbital angular momentum as it evolves toward coalescence, whose magnitude depends on the integrated history of the Chern-Simons coupling over the worldline of radiation wavefront. We estimate that LISA could place bounds on Chern-Simons modified gravity that are several orders of magnitude stronger than the present Solar System constraints, thus providing a probe of the quantum structure of spacetime.