Conformal preservation of null geodesic paths ($\tilde{g}={\Omega }^{2}g$)

The difference of the two Levi-Civita connections is ${\tilde{\Gamma }}^a{}_{bc}-{\Gamma }^a{}_{bc}={\delta }_b^a{\partial }_c\log \Omega +{\delta }_c^a{\partial }_b\log \Omega -g_{bc}{g}^{ad}{\partial }_d\log \Omega$. Along a null geodesic with tangent $k$, the last term vanishes and the remaining acceleration is $2k(\log \Omega )k$. It is parallel to the tangent, so changing the affine parameter removes it. Thus a positive conformal factor preserves unparametrized null geodesics, although it generally does not preserve timelike or spacelike geodesics.

If $\tilde{g}={\Lambda }^{2}g$, a null geodesic with affine parameter $\tau$ for $g$ has affine parameter $\tilde{\tau }$ for $\tilde{g}$ when $d\tilde{\tau }/d\tau =C{\Lambda }^2$ along the curve. Indeed the conformal change of the Levi-Civita connection gives ${\tilde{\nabla }}_{V}V=2V(\log |\Lambda |)V$; rescaling $V$ by $d\tau /d\tilde{\tau }$ cancels this tangential acceleration. Constant $C>0$ preserves the direction of parametrization. Agreement of unparametrized null geodesics does not imply agreement of their affine parameters or completeness.