Past exam of the mathematics course of the University of Cambridge 2021 iii Paper 309 2 b Solution 2026-09-28
In the weak-field, slow-source approximation, the leading luminosity is the quadrupole formulain units , whereOnly the symmetric trace-free coefficient contributes. Indeed,and contraction with traceless removes the first term. ThereforeThe power crossing the large sphere at time is consequentlyThe monopole is conserved total mass, the dipole is center-of-mass motion, and every term is orthogonal to the trace-free quadrupole at this leading order.
The quadrupole formula in units isHere is constant, so its trace subtraction has no third derivative. If , thenThus andRestoring units multiplies this by .