Energy equals frequency ( E = hf )
Area Increases, Energy must divide.

Light energy wave vibrations expand spherically away from their point of origin (The Star), also the waves or vibrations are expanding away from each other. There is more and more area to cover with the same unit amount of Energy.

click for large picture of expanding waves

What this means is light vibrations along the strings propagating through space will be divided to fill the ever increasing area. This is accomplished by a reduction in frequency or an increase in wavelength and is the reason for what is otherwise known as the Red shift (except of course some actual random movement). The further away, the greater the division or separation of particle vectors, the more red shift.

The Universe isn't expanding, although the fusion reaction in the sun and other stars does allow the release and expansion of billions and billions of Flux particles from confinement in nuclei back into their normal state, this excess amount will generate raw material for the formation of other matter elsewhere in the Universe and give space an ever changing star field by a type of cosmic convection. Stars radiate spherically so, their energy, matter or light is distributed evenly throughout the cosmos... and since the light from all stars also converges or can be seen from any point in the Universe this means we are a small part of everything in the Universe.
The Universe extends to an inconceivable infinity. However far you go into Space there's more Space; filled with Planets, Stars, and Galaxies with no place left to expand into... Think about it... the Universe can't expand... it's already there.

Professor Stephen Hawking has blundered... This picture is from his book "a briefer history of time" page 29 it's easy to see the Waves getting further apart from each other as time increase.

Big-Bang...not this time.