Gravitational Bremsstrahlung from Reverse Unitarity

TL;DR

The total radiated momentum carried by gravitational waves during the scattering of two spinless black holes at the lowest order in Newton's constant, O(G^{3}), and all orders in velocity is computed.

Abstract

We compute the total radiated momentum carried by gravitational waves during the scattering of two spinless black holes at the lowest order in Newton's constant, $\mathcal O(G^3)$, and all orders in velocity. By analytic continuation into the bound state regime, we obtain the ${\cal O}(G^3)$ energy loss in elliptic orbits. This provides an essential step towards the complete understanding of the third-post-Minkowskian binary dynamics. We employ the formalism of Kosower, Maybee, and O'Connell (KMOC) which relates classical observables to quantum scattering amplitudes and derive the relevant integrands using generalized unitarity. The subsequent phase-space integrations are performed via the reverse unitarity method familiar from collider physics, using differential equations to obtain the exact velocity dependence from near-static boundary conditions.

Figures (3)

• Figure 1: Generalized unitarity cuts relevant for the radiated momentum. Shaded blobs denote tree-level amplitudes, visible legs are on shell, and we exclude any phase-space integrals.
• Figure 2: Cubic diagrams relevant for the radiated momentum.
• Figure 3: Master integrals relevant for the radiated momentum. The dashed line indicates the cut; double lines, cut or uncut, are linearized propagators, and a dot ($\bullet$) indicates a squared propagator, corresponding to the $n{=}2$ case of Eq. \ref{['eq:deltaToProp']}.