What is the cosmological factor that causes light from very distant galaxies to shift toward the red end of the spectrum?

Answer

The expansion of space, known as cosmological redshift

The expansion of space actively stretches the wavelengths of light traveling through it. This phenomenon, cosmological redshift, shifts light from distant sources toward the red end of the spectrum and beyond.

What is the cosmological factor that causes light from very distant galaxies to shift toward the red end of the spectrum?

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