How Symmetry constrains Gravitational Radiation : Christodolou's Memory and it's Descendants.

Gravitational Scattering of Massive objects such as black holes or Neutron Stars is a computational nightmare. Due to the complexity of Einstein's equations, the analytic formula for gravitational flux emitted during such collisions are hard to come by. However, in  the early 90s, Braginsky, Thorne and Christodolou derived an exact formula for gravitational radiation in the low frequency limit. This result is known as the Gravitational memory. 

Over the last decade or so, we have now understood that the memory formula is not an accident, but is a consequence of an extremely rich symmetry structure of General Relativity. This observation has led to a discovery of a new class of low frequency observables.  In this talk, we will review some of these developments, which has been aided by our increased understanding of the infra-red structure of Quantum Gravity S-matrix