In any region of space there is a background of cosmic radiation - such as that from the big bang.
This radiation may be detected by an aerial and then amplified - resulting in a detected signal. If the radiaion were constant,then the signal would be consant also - an assumption!
What appens if there is an absense of signal for a short period? Obviosly it is detected as ”noise”.
The question is therefore: “What is the probability that this noise is caused by flucuations in the number of photons present”. In particular: “What is the probability that in a small region of space, there are no photons existing for a specific period?”
The answer will relate the size of the region, the time for which no radiation is present and how often such a situation exists for!
Together these will give a spectral distribution and amplitude for such “Radiation Gap Noise”!