The phenomenon of redshift occurs due to the expansion of the universe and the Doppler effect. While electromagnetic waves do not lose energy when traveling through a vacuum, the wavelength of the waves can be stretched or compressed as they propagate through space.
When an object emitting light or any other electromagnetic wave is moving away from an observer, the wavelengths of the waves are stretched, resulting in a shift towards the red end of the spectrum. This is known as redshift. On the other hand, if the object is moving closer to the observer, the wavelengths are compressed, leading to a shift towards the blue end of the spectrum, known as blueshift.
The expansion of the universe causes the space between galaxies to stretch. As a result, light traveling through this expanding space experiences a stretching of its wavelength, causing a redshift. The greater the distance between the observer and the source of light, the more significant the redshift.
The phenomenon of cosmological redshift is observed in the light coming from distant galaxies and other celestial objects. It provides valuable information about the expansion rate of the universe and the distances to these objects. It is important to note that this redshift is not due to the loss of energy by the electromagnetic waves themselves, but rather a consequence of the expansion of space. The energy of the photons in the electromagnetic waves remains constant during their travel through a vacuum.