This smears the measurements so the actual data will extracted statistically. Data we now have, from microwave intensity surveys, show remarkable uniformity with variations exactly like I'd expect considering the finite size of the source. The instrument needed to measure phase, a giant interferometer, is improbably expensive, but not beyond what could be done on a space station. This lead me to consider the trajectory of 2 photons of the background radiation. Certainly the background radiation is thermally distributed, as you say, a blackbody spectrum.
Coherence has two components, spatial and temporal.
Spatial coherence comes about from a point source, while temporal coherence implies that the photons are emitted simultaneously so that they are 'in phase' with each other. Now we can investigate the coherence of individual photons, which we will interpret as the geometry and movement of the source. If the background source shows movement, then we are moving with respect to an absolute frame of reference. I don't have a picture of this type of universe. I'd like to build on the diagrammatic model I've been using lately, so here is a picture of the trajectories I am comparing.