quantumkiko said:
... My question is why do the photons have to increase their wavelengths? Can't the wavelengths stay the same while the universe expands?
The effect is not limited to CMB photons. It affects all photons traveling large distances
It is called the cosmological redshift.
The wavelength of any photon is increased, during its travel time, by the same factor that the universe's largescale distances expand during the time it has been traveling.
If the universe expands by a factor of TWO while the light is traveling, then when the light arrives here its wavelength will be TWICE what it was when the light was originally emitted and began its journey.
Now you ask some interesting questions. You ask how does the wavelength get stretched, in exactly the same amount as average intergalactic distances get stretched. By what mechanism?
And you ask where does the lost energy go?
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But nobody has a satisfactory answer to where does the energy go, believe it or not! In fact it is not absolutely clear that global energy conservation law can be proven mathematically in the context of General Relativity. Any simple version of the law would be violated by "dark energy", since dark energy has a constant density of 0.6 joules per cubic kilometer. If your sample volume doubles the amount of energy in it doubles and no one can say where the extra came from.
That is tough. It is unintuitive. Energy conservation can be proven in the context of flat (uncurved) spacetime where you have a localized isolated system. Like something in a big imaginary box. Simple naive extensions of this idea to the whole expanding universe don't seem to work. People make special complicated definitions in the attempt to get something partially satisfactory. And some people will say that the energy lost by redshifted light goes into "gravitational energy". But how it goes into that form, and how you could harness it to do work, is not straightforward.
The CMB light has been redshifted by a factor of 1100. It has lost 1099/1100 of its energy. It has lost about 99.9 percent of its energy by the time it reaches us. The original light could have been harnessed to do work. Now if the microwaves were harnessed, they could do only 1/10 percent as much work. What could be harnessed to recoup the lost work? Frankly I don't think anybody has a clear idea.
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The mechanism of redshift you asked about is simply Maxwell equations where space is expanding. Maxwell equations is just how electromag. waves propagate. The exansion is very gradual. Distances are now expanding at a rate of 1/140 percent every million years. That is very very slow expansion. But the waves are trying to travel thru this expanding space and distances between peaks are increasing. It gradually takes its toll. Each undulation is determined by the previous one. But by the time it is happening the previous one is just a tiny bit extended. The stretching builds up cumulatively. After 140 million years have gone by, the wavelength of the light is one percent longer.
We are used to visualizing waves traveling in space where distances are not expanding, so this is unintuitive. but you can try thinking of wave equations, or especially Maxwell (lightwave) equations, where distances are expanding.
Or you can explain it by thinking in terms of coordinate transformations, but I don't think that is any more intuitive or gives any more physical feel.
Good luck understanding this stuff!
