What observational effect occurs because the universe expands while light travels across vast distances?

Answer

Time dilation, causing distant processes to appear slower

If a signal takes a certain time to cross a distance, the expansion occurring during that transit time causes the observer to measure the process unfolding over a longer duration, effectively seeing distant galaxy processes in slow motion.

What observational effect occurs because the universe expands while light travels across vast distances?

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What Does Redshift Tell Us About Distant Galaxies? - Physics Frontier

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