Photon Decoupling: Effects on Universe Expansion

[Adhruth Ganesh]

Why did the expansion of the universe cause an increase in the wavelength of the photons that existed during the time of Photon decoupling ?
Does this mean that expansion of universe stretches everything and stars that were present when the universe considerably expanded also got stretched?

 

[Orodruin]

Gravitationally (or otherwise) bound systems do not expand.

 

[Adhruth Ganesh]

Gravitationally (or otherwise) bound systems do not expand.

Fine. Thanks.