Null geodesic congruence
= Null geodesic congruence
A null geodesic congruence has an affinely parametrized null tangent $k^a$ satisfying $k^ak_a=0$ and $k^b\nabla_bk^a=0$.
= Null geodesic congruence
A null geodesic congruence has an affinely parametrized null tangent $k^a$ satisfying $k^ak_a=0$ and $k^b\nabla_bk^a=0$.