What physical effect causes the spectral lines of distant galaxies to shift toward the red end of the spectrum?

Answer

The stretching of light waves as space expands between galaxies.

As the universe expands, the space itself stretches, which in turn stretches the light waves traveling through it. Longer wavelengths appear redder, resulting in the spectral shift known as redshift.

What physical effect causes the spectral lines of distant galaxies to shift toward the red end of the spectrum?

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