If a supernova in a distant, rapidly receding galaxy should take time $T$ to fade, how long do observers see it take?
Answer
$1+z$ times longer than $T$
Due to the cosmological expansion stretching the space between the observer and the source, events observed in distant, rapidly receding galaxies appear temporally stretched or dilated by a factor of $1+z$, where $z$ is the redshift.
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