= Luminosity variability size bound
{title2=$R'\lesssim cD\Delta t_{\rm obs}/(1+z)$}
Coherent intrinsic <luminosity> variation on observed time $\Delta t_{\rm obs}$ bounds the characteristic emitting radius by a light-crossing argument. <relativistic beaming> shortens the observed interval by <Doppler factor> $D$, while <cosmological redshift> lengthens it by $1+z$. This bound assumes that the variation samples the emitting region causally; geometry, independent subregions and propagation-driven variation can change its interpretation.
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