Why have the more distant galaxies had their light shifted from visible to infrared wavelengths? I have searched everywhere, but cannot find the answer.
Here is some background information which you may already know, but I want to make sure we are on the same page when the explanation starts.
BACKGROUND - Billions of years ago, the universe came into existence through an event called the "big bang" and started rapidly expanding from nothing into the universe we see today. It has been getting bigger ever since. So the first stars and galaxies that formed are now at the edge of the universe and are moving away from the younger ones, increasing in separation as time goes by. Waves coming from objects that are moving with relation to us have their wavelength shifted depending on their motion. Electromagnetic waves coming from approaching objects have their wavelengths shortened. They are lengthened for objects moving away from us. This is the Doppler Effect. This happens with sound waves too. You have probably heard the example of the pitch of a train horn sounding as it approaches and passes you. The pitch of the horn drops, because the wavelengths are longer as it is moving away. Think of the waves as being compressed as the train approaches and stretched as it moves away.
EXPLANATION - The same thing happens with the light waves and all the other waves emanating from the galaxy or object moving away from us. The visible light shifts towards longer wavelengths in the red direction. This phenomena is know as Redshift. In our universe, the farther away a galaxy is from the initial origin point, the big bang, the faster it is moving. What happens is just a shift towards the red direction of the Electromagnetic Spectrum. It doesn't become red. Instead, the entire spectrum shifts towards the red direction. The shorter wavelengths in the spectrum of the galaxy, the ultraviolet waves for example, shift towards the visible range. In the chart in the link below, redshift is in the up direction.
E-M Spectrum Chart link http://www.sir-ray.com/Electromagnetic%20Spectrum%20Chart.htm
I am glad to be of service. If you still have some question, ask away.