Why does the simple linear form of Hubble’s Law fail for high-redshift objects?

Answer

The universe's expansion rate has not been constant over the entire light travel time.

The linear approximation assumes a constant expansion rate, but for high redshifts, the expansion rate has changed over cosmic history due to factors like matter and dark energy, invalidating the simple linear formula.

Why does the simple linear form of Hubble’s Law fail for high-redshift objects?
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