From harmonic to Newman-Unti coordinates at the second post-Minkowskian order

In this paper, we present the complete transformations of a generic metric from (generalized) harmonic to Newman-Unti coordinates up to the second post-Minkowskian order $(G^2)$. This allows us to determine the asymptotic shear, the Bondi mass aspect, and the angular-momentum aspect at both orders.

💡 Research Summary

The paper by Mao and Zeng presents a comprehensive derivation of the coordinate transformation that maps a generic metric expressed in (generalized) harmonic coordinates to Newman‑Unti (NU) coordinates through second post‑Minkowskian order (i.e., up to terms of order G²). The motivation stems from the need to connect two complementary approaches to gravitational‑wave physics: the perturbative post‑Minkowskian (PM) expansion, which provides explicit waveforms generated by sources in the bulk of spacetime, and the asymptotic Bondi‑Sachs/Newman‑Unti (BS/NU) frameworks, which give clean definitions of conserved quantities (energy‑momentum, angular momentum, news, shear) at future null infinity.

The authors adopt a strategy that differs from earlier works (e.g., Refs.