Why does gravitational redshift occur?

Gravitational redshift occurs because time slows down in a strong gravitational field. This causes the frequency of a wavelength of light to lower. Lower frequencies occur at the red end of the electromagnetic spectrum, so the light appears shifted to the red wavelengths.

Answer: Einstein's theory of general relativity predicts that the wavelength of electromagnetic radiation will lengthen as it climbs out of a gravitational well.

Explanation: Einstein's theory of general relativity predicts that the wavelength of electromagnetic radiation will lengthen as it climbs out of a gravitational well.