It has been argued in some publications that the age of the universe represents a lower limit on observable frequencies (in their paper, corresponding to a value of $10^{-17}$ Hz). The authors do not explain exactly why this is, and my assumption is that this is because frequencies lower than $10^{-17}$ Hz would have periods longer than the age of the universe, making it impossible to observe periodic behavior.
My question is whether the above argument holds for the modulation/offset frequencies in Leeson's equation; i.e., offset frequencies with values less than $10^{-17}$ Hz would have periods longer than the age of the universe, making it impossible to observe the periodic behavior of said offset frequencies.
