Spin Coefficients, Raychaudhuri's Equation and Optical Scalars
For a better understanding of the previous section, we will briefly review the meanings of relevant NP spin coefficients in depicting null congruences. The tensor form of Raychaudhuri's equation governing null flows reads
where is defined such that . The quantities in Raychaudhuri's equation are related with the spin coefficients via
where Eq(24) follows directly from and
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