OK , I can think about both.:-)

I think I know what byronm is asking about.

When we say Expansion of Space does it mean that you strech the existing space or you add some space in between (2 objects, an object and an observer)?
I asked this question some time ago here and nobody answered.
Now I know why. Nobody knows he he.
There is a debate running (source: arxiv) on Expansion of Space as obviously Space is presented as nothing in SRT (not sure about GRT :-), so, what is expanding actually?

To byronm:
Doppler redshift occurs when you have two objects moving with different velocities as Grant says, but with that we say that the objects have kinetic energy mv^2/2.
SRT and GRT do not allow v>c and if you would interpret the redshift as Doppler you will end up with the observation of v>c, which is contradictory.
It will ultimately imply that "something" kicked a galaxy or group of galaxy so hard that they achieved a velocity of near c, c or even more then c. No source of energy of that scale is known.
Except for Bing Bang.
The very first version (help with source needed, Gamow?) of Bing Bang stated just that.
In the beginning was there just a big ball of neutrons that exploded (?).
(OK, working hypothesis, we will not go in debth with that)
In that way the matter aquired the kinetic energy and hence redshift, which in the beginning was Doppler redshift. Later, the situation worsened as we could observe objects with z>1. With that, the idea of Doppler redshift as a cosmological redshift was burried deep, very deep. With that went the big ball of neutrons, too.
New equation(relativistic) was found to acoount for a redshift and the Expansion of Space was introduced as the cause of it.
But, as you can read on arxiv, 50 years later it's still not clear what is expanding.
The problem lies in that, if you apply some properties to the space you will contradict GRT,
promoting Space to Aether.
Someone will say, Space does not expand only distances between objects increase(?)(whatever that means)
OK, if I go with that, how do you achieve movement of galaxies?
Kinetic energy? No? (of course not)
Space is bearing the galaxies as in "Matter tells space how to curve, space tells matter how to move"? How nothing can communicate with anything? Are you saying that GRT is effectively an Aether theory ?(I don't have any problems with that).
I can imagine that gravity communicates to space via gravitational field and gravitons but I'm not aware that GRT and extensions say anything of another way around. If you introduce zero-point energy et al. , what are carriers of that energy? Is it Space or something filling the space?
Another thing.
Light leaving gravitational body appear to be redshifted to a distant observer.
But so does the light leaving "antigravitational body" i.g. bobble of expanding space which has curvature with oposite sign of that of the gravity?! Light is redshifted in both cases!


What's wrong with this thought experiment in the era of Dark Energy:
A bobble of space is expanding with accelerated rate (due to Dark Energy).
When we observe test photon leaving an object on the oposite side of the bobble (we are not gravitationaly bound, gravitational and Doppler redshift are negligable) we will see
that photon gets redshifted up to the centre of the bobble. After that, Dark Energy is actually pushing the photon towards us and the photon aquire blue shift. (c =const)
Netto shift should be : zero.

Similarely, is this correct?:
Beam of light strikes a mirror placed on a massive body.
All other causes for redshift are negligable except gravitational. We can observe the frequency of the light at the time of emission f_e and at the time of apsorption f_a.
We observe that f_e = f_a.