Why does the expansion of space lead to cosmic redshift? I have always thought of light as a point-like object in space, with the EM field strengths dependent on time.
However, stretching of space would not affect a point-like object.
Thus, I assume that a photon occupies a non-zero-sized space, it has different parts located at different places, and the expansion of space pulls the different parts of a photon apart, so that the photon becomes bigger.
We usually visualize EM waves as 2 sinuses, one for the electric, one for the magnetic field. If a single photon is spread out in space, it has different parts all over the place, this depiction is not correct, as it only tracks 1 part of the photon. It would be more correct, to add at least 1 spatial dimension to the depiction, and along the spatial axis, draw a section, or a fuzzy section, to show that the wave is spread out in space.
point-like in time:
E
^
|  .-.       .-
| /   \     /
|/     \   /
|       ._.
+-------------> time

band-like in space:
E
^
|  ..--..        ..--
| //    \\      //
|//      \\    //
|         ..__..
+-------------> x

QUESTION:
Am I correct in these assumptions?
If not, how can expansion of space affect the frequency of light?
 A: This is a completely classical effect, so there is no need to talk about photons. (And in any case, photons are not really pointlike, since they behave according to wave-particle duality.)
Cosmological expansion acts as though space itself is expanding. (The mathematical expression of this in general relativity doesn't actually express it that way, but it's a good enough conceptual framework for most purposes.) When space expands by a factor $a$, the wavelength of an electromagnetic wave expands by the same factor.
At a slightly more rigorous level, you can model two successive wavefronts by two null geodesics. The proper distance between these (i.e., the distance measured by an observer at rest relative to the Hubble flow) scales with $a$.
A: It is the relativistic doppler effect.Imagine a source emmiting waves with a costant frequency .Now we start moving the source relative to you. Let's take the period of time before the second  emission and after the first emission.Since the source is moving relative to you, its position in space will change . And when the time comes for the second emission the distance between the waves will be different than if the source wasnt moving relative to you.So you believe that the frequency of the source is different than its actual frequency.
Now when the space its expanding , its like the galaxy from where the light comes from is moving away from you!
