What physical effect caused by the expansion of space shifts the ancient, high-energy light from the young universe into lower energy ranges?

Answer

Cosmological redshift

As space expands over cosmic timescales, it stretches the wavelengths of photons traveling through it, causing this stretching effect known as cosmological redshift.

What physical effect caused by the expansion of space shifts the ancient, high-energy light from the young universe into lower energy ranges?
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